Add PII redaction POC for secure LLM prompting.

Implements detect-redact-sanitize pipeline with regex, domain rules, and ONNX NER before the LLM boundary, plus NUnit tests and Xenovex push documentation. Co-authored-by: Cursor <cursoragent@cursor.com>
2026-07-07 13:05:07 +05:30
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+## .NET
+bin/
+obj/
+*.user
+*.suo
+.vs/
+
+## ONNX models (place downloaded models here locally)
+models/*.onnx
+models/vocab.txt
+models/ner-labels.txt
+models/*.json
+!models/.gitkeep
+
+## IDE
+.idea/
+*.swp
+
+## Local experiments (not part of the solution)
+scratch/
+
+## Test artifacts
+**/nunit_random_seed.tmp
--- a/PiiRedaction.slnx
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+<Solution>
+  <Folder Name="/src/">
+    <Project Path="src/PiiRedaction.ConsoleApp/PiiRedaction.ConsoleApp.csproj" />
+    <Project Path="src/PiiRedaction.Core/PiiRedaction.Core.csproj" />
+    <Project Path="src/PiiRedaction.Infrastructure/PiiRedaction.Infrastructure.csproj" />
+  </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/PiiRedaction.Core.Tests/PiiRedaction.Core.Tests.csproj" />
+    <Project Path="tests/PiiRedaction.Infrastructure.Tests/PiiRedaction.Infrastructure.Tests.csproj" />
+  </Folder>
+</Solution>
--- a/README.md
+++ b/README.md
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+# PII Redaction POC
+
+A proof-of-concept .NET solution that redacts personally identifiable information (PII) from user prompts **before** sending them to a large language model (LLM). The design demonstrates enterprise-grade separation of concerns using SOLID principles, dependency injection, and the `Microsoft.Extensions.AI` abstractions.
+
+## Purpose
+
+Financial and customer-service prompts often contain regulated data (names, government IDs, account numbers). This POC shows how to:
+
+1. Accept a console prompt
+2. Detect PII using **Regex**, **ONNX NER**, and **domain rules**
+3. Replace values with stable placeholders
+4. Send only the **sanitized** prompt to an LLM (mocked for now)
+
+## Architecture
+
+For solution design, data-flow diagrams, trust boundaries, and project responsibilities, see **[docs/architecture.md](docs/architecture.md)**.
+
+## Why Three Detection Strategies?
+
+| Strategy | Used For | Rationale |
+|----------|----------|-----------|
+| **Regex** | Email, phone, PAN, Aadhaar, credit card | Deterministic, format-bound identifiers with stable rules that are easy to audit and test |
+| **ONNX NER** | Person names | Contextual entities without rigid formats; names vary widely in surface form |
+| **Domain rules** | Loan number, customer ID, account number | Business-specific identifiers defined by internal systems, not inferable from generic models alone |
+
+## Why the LLM Receives Only Sanitized Text
+
+The placeholder map (`<PERSON_1>` → original value) is kept **in-process** for audit or downstream de-tokenization. Only the sanitized prompt crosses the LLM boundary. This reduces data-exposure risk and supports compliance requirements for regulated workloads.
+
+## Project Structure
+
+```
+src/
+├── PiiRedaction.ConsoleApp/     # Presentation: input/output, DI bootstrap
+├── PiiRedaction.Core/           # Business logic: detection, redaction, models
+└── PiiRedaction.Infrastructure/ # Technical adapters: ONNX Runtime, mock LLM
+models/                          # Optional ONNX model files (gitignored)
+```
+
+| Project | Responsibility |
+|---------|----------------|
+| `PiiRedaction.ConsoleApp` | Read prompt, call sanitizer, display results, call LLM service |
+| `PiiRedaction.Core` | PII detection abstractions, redaction, sanitization orchestration |
+| `PiiRedaction.Infrastructure` | ONNX model runner, `IChatClient` mock implementation |
+
+## Prerequisites
+
+- [.NET SDK](https://dotnet.microsoft.com/download) 10.x (or compatible SDK for `net10.0`)
+- **ONNX NER model** for person-name detection (see [ONNX Model Setup](#onnx-model-setup))
+- Python 3.10+ (only for the model download script)
+
+> **Note:** This environment targets `net10.0` because .NET 10 SDK is installed. The architecture is identical to the planned .NET 9 layout; change `TargetFramework` in `.csproj` files if you use .NET 9 SDK.
+
+## Build and Run
+
+From the repository root:
+
+```bash
+dotnet restore
+dotnet build
+dotnet run --project src/PiiRedaction.ConsoleApp
+```
+
+By default the console app runs **11 curated sample prompts** covering NER/person names, regex identifiers, domain IDs, combined scenarios, and a clean no-PII ticket.
+
+List available samples:
+
+```bash
+dotnet run --project src/PiiRedaction.ConsoleApp -- --list
+```
+
+Run a single sample by index or name:
+
+```bash
+dotnet run --project src/PiiRedaction.ConsoleApp -- --sample 2
+dotnet run --project src/PiiRedaction.ConsoleApp -- --name MrTitlePerson
+```
+
+Interactive mode (enter your own prompt):
+
+```bash
+dotnet run --project src/PiiRedaction.ConsoleApp -- --interactive
+```
+
+### Console sample catalog
+
+Samples are defined in [`SamplePromptCatalog.cs`](src/PiiRedaction.ConsoleApp/Samples/SamplePromptCatalog.cs).
+
+| # | Name | Category | NER / Person example |
+|---|------|----------|----------------------|
+| 0 | FullFinancialWithCustomer | NER + Regex + Domain | `Customer Ravi Kumar` + email, phone, loan, PAN |
+| 1 | CustomerNameOnly | NER | `Customer Anita Sharma` |
+| 2 | MrTitlePerson | NER | `Mr. John Smith` |
+| 3 | MrsTitlePerson | NER | `Mrs. Lakshmi Reddy` |
+| 4 | DrTitlePerson | NER | `Dr. Jane Doe` |
+| 5 | TwoCustomersInOnePrompt | NER | `Customer Ravi Kumar` and `Customer Priya Nair` |
+| 6 | PersonWithDomainIds | NER + Domain | `Customer Meera Iyer` + CID / ACC |
+| 7 | PersonWithEmailNoPhone | NER + Regex | `Customer Arjun Mehta` + email |
+| 8 | AllRegexTypes | Regex | email, phone, PAN, Aadhaar, card |
+| 9 | AllDomainIds | Domain | LN, CID, ACC |
+| 10 | NoPiiCleanTicket | Negative | no redaction |
+
+Person names are detected via **ONNX NER** using `dslim/bert-base-NER` (or a compatible token-classification export). A real model is **required** for person-name detection; there is no regex or heuristic fallback.
+
+## NuGet Packages
+
+```bash
+# Infrastructure
+dotnet add src/PiiRedaction.Infrastructure package Microsoft.ML.OnnxRuntime
+dotnet add src/PiiRedaction.Infrastructure package Microsoft.ML.Tokenizers
+dotnet add src/PiiRedaction.Infrastructure package Microsoft.Extensions.AI.Abstractions
+dotnet add src/PiiRedaction.Infrastructure package Microsoft.Extensions.AI
+dotnet add src/PiiRedaction.Infrastructure package Microsoft.Extensions.Logging.Abstractions
+dotnet add src/PiiRedaction.Infrastructure package Microsoft.Extensions.Options
+
+# Core
+dotnet add src/PiiRedaction.Core package Microsoft.Extensions.Options
+
+# ConsoleApp
+dotnet add src/PiiRedaction.ConsoleApp package Microsoft.Extensions.Hosting
+dotnet add src/PiiRedaction.ConsoleApp package Microsoft.Extensions.DependencyInjection
+dotnet add src/PiiRedaction.ConsoleApp package Microsoft.Extensions.Configuration.Json
+dotnet add src/PiiRedaction.ConsoleApp package Microsoft.Extensions.Configuration.EnvironmentVariables
+```
+
+## Configuration
+
+[`appsettings.json`](src/PiiRedaction.ConsoleApp/appsettings.json):
+
+```json
+{
+  "PiiRedaction": {
+    "OnnxModelPath": "models/ner-model.onnx"
+  }
+}
+```
+
+| Setting | Description |
+|---------|-------------|
+| `OnnxModelPath` | Path to ONNX NER model (relative to working directory or discovered by walking up from the current directory) |
+
+## ONNX Model Setup
+
+Person-name detection requires a token-classification ONNX model and companion tokenizer files in the `models/` directory:
+
+| File | Purpose |
+|------|---------|
+| `models/ner-model.onnx` | Exported NER model |
+| `models/vocab.txt` | BERT WordPiece vocabulary |
+| `models/ner-labels.txt` | One BIO label per line (`O`, `B-PER`, `I-PER`, etc.) |
+
+### Download script
+
+From the repository root:
+
+```powershell
+.\scripts\download-ner-model.ps1
+```
+
+Or with Python directly:
+
+```bash
+python scripts/download-ner-model.py
+```
+
+The script exports [`dslim/bert-base-NER`](https://huggingface.co/dslim/bert-base-NER) via Hugging Face Optimum when Python is available. Otherwise it downloads the pre-exported ONNX assets from Hugging Face directly.
+
+### Inference pipeline
+
+`OnnxNerModelRunner` performs the full pipeline:
+
+- BERT WordPiece tokenization (`Microsoft.ML.Tokenizers`)
+- ONNX Runtime inference (`input_ids`, `attention_mask`, optional `token_type_ids`)
+- BIO label decoding (`B-PER` / `I-PER` → `PiiEntityType.Person`)
+- Character-span alignment back to the source text
+
+When the model or tokenizer files are missing, person detection returns no results.
+
+## Swapping Mock LLM for Azure OpenAI
+
+The application depends on `ILlmPromptService` (Core) and `IChatClient` (Microsoft.Extensions.AI). To use Azure OpenAI later, replace the mock registration in [`ServiceCollectionExtensions.cs`](src/PiiRedaction.ConsoleApp/DependencyInjection/ServiceCollectionExtensions.cs):
+
+```csharp
+// Remove:
+// services.AddSingleton<IChatClient, MockChatClient>();
+
+// Add (example — package and API may vary by provider SDK version):
+// services.AddAzureOpenAIChatClient(
+//     new Uri(configuration["AzureOpenAI:Endpoint"]!),
+//     configuration["AzureOpenAI:ApiKey"]!,
+//     configuration["AzureOpenAI:DeploymentName"]!);
+
+services.AddSingleton<ILlmPromptService, MockLlmPromptService>(); // unchanged
+```
+
+`MockLlmPromptService` already uses `IChatClient`, so it works with any registered chat client implementation.
+
+## Sample Execution Output
+
+```
+=== PII Redaction POC ===
+
+Original Prompt:
+Customer Ravi Kumar with email ravi.kumar@gmail.com and phone 9876543210 has LoanNumber LN-456789 and PAN ABCDE1234F. Please summarize this customer issue.
+
+Detected PII:
+  [PERSON       ] Ravi Kumar             (Ner)
+  [EMAIL        ] ravi.kumar@gmail.com   (Regex)
+  [PHONE        ] 9876543210             (Regex)
+  [LOAN_NUMBER  ] LN-456789              (Domain)
+  [PAN          ] ABCDE1234F             (Regex)
+
+Sanitized Prompt:
+Customer <PERSON_1> with email <EMAIL_1> and phone <PHONE_1> has LoanNumber <LOAN_NUMBER_1> and PAN <PAN_1>. Please summarize this customer issue.
+
+Internal Placeholder Map (not sent to LLM):
+  <EMAIL_1>        -> ravi.kumar@gmail.com
+  <LOAN_NUMBER_1>  -> LN-456789
+  <PAN_1>          -> ABCDE1234F
+  <PERSON_1>       -> Ravi Kumar
+  <PHONE_1>        -> 9876543210
+
+Mock LLM Response:
+[Mock LLM Response] Received sanitized prompt (146 chars). No original PII was transmitted.
+```
+
+## SOLID Principles Applied
+
+| Principle | Application |
+|-----------|-------------|
+| **Single Responsibility** | Each detector, redactor, and runner has one job; `Program.cs` only orchestrates |
+| **Open/Closed** | Add new `IPiiDetector` implementations without changing merge logic |
+| **Liskov Substitution** | All detectors are interchangeable via `IPiiDetector` |
+| **Interface Segregation** | Separate interfaces for detection, redaction, sanitization, and LLM |
+| **Dependency Inversion** | Core defines abstractions; Infrastructure implements them |
+
+## Testing
+
+The solution includes an **NUnit** test suite across two projects:
+
+| Project | Focus |
+|---------|-------|
+| `tests/PiiRedaction.Core.Tests` | Detectors, redactor, sanitizer, golden pipeline scenarios (fake NER), real-model integration tests |
+| `tests/PiiRedaction.Infrastructure.Tests` | Mock LLM, ONNX runner unit tests, real-model NER runner tests |
+
+### Run tests
+
+```bash
+dotnet test
+dotnet test --filter "FullyQualifiedName~GoldenPromptTests"
+dotnet test --filter "Category=RealModel"
+dotnet test --logger "console;verbosity=detailed"
+```
+
+Fast CI runs without the ONNX model: fake-based tests always execute; tests marked **`Category=RealModel`** are skipped when `models/ner-model.onnx` is absent. Download the model first:
+
+```powershell
+.\scripts\download-ner-model.ps1
+```
+
+### Test architecture
+
+- **`PromptScenarioCatalog`** — five focused end-to-end scenarios (canonical demo, multi-regex, duplicate people, overlap stress, no-PII negative)
+- **`ProductionPipelineFactory`** — builds the same Domain → Regex → OnnxNer composite stack as production DI; `CreateWithRealModel(runner)` wires a real `OnnxNerModelRunner`
+- **`FakeOnnxNerModelRunner`** — unit-test double for NER; golden tests inject person spans per scenario
+- **`GoldenPromptTests`** — end-to-end sanitization proof across the catalog (fake NER)
+- **`RealNerModelFixture`** — shared fixture that loads `models/ner-model.onnx` once per class; skips when model missing
+- **`RealNerModelRunnerTests`** — direct ONNX inference with span accuracy checks
+- **`RealNerPipelineTests`** — full pipeline with real NER (canonical, multi-person, clean-ticket negative)
+- **`OnnxNerModelRunnerTests`** — unit tests for missing/invalid model paths (no download required)
+- **`CompositePiiDetectorTests`** — overlap merge and source-priority rules
+- **`LlmBoundaryTests`** — verifies raw PII never appears in outbound LLM messages
+
+Assertions use **FluentAssertions** for readable failures on long prompt strings.
+
+## Future Enhancements
+
+- ASP.NET Core API host with request/response middleware
+- Persistent audit log of redaction events (without storing raw PII)
+- Secure vault for reversible tokenization
+- Real Azure OpenAI / OpenAI provider registration
--- a/docs/architecture.md
+++ b/docs/architecture.md
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+# PII Redaction POC — Solution Architecture
+
+## Purpose
+
+This document describes the architectural design of the **PII Redaction POC**, a .NET proof-of-concept that intercepts user prompts containing regulated personally identifiable information (PII), redacts sensitive values into stable placeholders, and transmits **only sanitized text** across the LLM trust boundary. The solution is structured for enterprise adoption: clear layer separation, interface-driven composition, dependency injection, and swappable infrastructure adapters (ONNX NER, `Microsoft.Extensions.AI` chat clients).
+
+The POC validates a compliance-oriented pattern suitable for financial and customer-service workloads where raw PII must not leave the application process when invoking external language models.
+
+---
+
+## Canonical Example
+
+The console application ships with a **sample catalog** (11 prompts). The canonical demo is sample `FullFinancialWithCustomer`. The table below shows the exact strings produced by the production pipeline when the ONNX NER model is loaded (run `scripts/download-ner-model.ps1` first).
+
+| Stage | Value |
+|-------|-------|
+| **Input** | `Customer Ravi Kumar with email ravi.kumar@gmail.com and phone 9876543210 has LoanNumber LN-456789 and PAN ABCDE1234F. Please summarize this customer issue.` |
+| **Sanitized Output** | `Customer <PERSON_1> with email <EMAIL_1> and phone <PHONE_1> has LoanNumber <LOAN_NUMBER_1> and PAN <PAN_1>. Please summarize this customer issue.` |
+| **Mock LLM Response** | `[Mock LLM Response] Received sanitized prompt (146 chars). No original PII was transmitted.` |
+
+Detected entities for this prompt:
+
+| Type | Value | Detection Source |
+|------|-------|------------------|
+| PERSON | Ravi Kumar | Ner |
+| EMAIL | ravi.kumar@gmail.com | Regex |
+| PHONE | 9876543210 | Regex |
+| LOAN_NUMBER | LN-456789 | Domain |
+| PAN | ABCDE1234F | Regex |
+
+The internal placeholder map (`<PERSON_1>` → `Ravi Kumar`, etc.) is retained in-process and is **not** included in the outbound LLM request.
+
+---
+
+## Console Sample Catalog
+
+Running `dotnet run --project src/PiiRedaction.ConsoleApp` executes all samples sequentially. Use `--list`, `--sample N`, or `--name SampleName` to filter.
+
+### NER / person-name samples
+
+These prompts exercise `OnnxNerPiiDetector` and `OnnxNerModelRunner`. Person names require the ONNX model (`models/ner-model.onnx` plus `vocab.txt` and `ner-labels.txt`). Without the model, person spans are not detected.
+
+| Sample | Input (excerpt) | Detected person | Sanitized (excerpt) |
+|--------|-----------------|-----------------|---------------------|
+| **CustomerNameOnly** | Customer Anita Sharma reported unauthorized… | Anita Sharma | Customer `<PERSON_1>` reported unauthorized… |
+| **MrTitlePerson** | Mr. John Smith called about a duplicate debit… | John Smith | `<PERSON_1>` called about a duplicate debit… |
+| **MrsTitlePerson** | Mrs. Lakshmi Reddy requested a callback regarding LN-112233. | Lakshmi Reddy | `<PERSON_1>` requested a callback regarding `<LOAN_NUMBER_1>`. |
+| **DrTitlePerson** | Dr. Jane Doe escalated a complaint… | Jane Doe | `<PERSON_1>` escalated a complaint… |
+| **TwoCustomersInOnePrompt** | Customer Ravi Kumar and Customer Priya Nair… | Ravi Kumar, Priya Nair | Customer `<PERSON_1>` and Customer `<PERSON_2>`… |
+| **PersonWithDomainIds** | Customer Meera Iyer holds CID-7070… | Meera Iyer | Customer `<PERSON_1>` holds `<CUSTOMER_ID_1>`… |
+| **PersonWithEmailNoPhone** | Customer Arjun Mehta wrote from arjun.mehta@company.in… | Arjun Mehta | Customer `<PERSON_1>` wrote from `<EMAIL_1>`… |
+
+### Other sample categories
+
+| Category | Sample | Purpose |
+|----------|--------|---------|
+| NER + Regex + Domain | FullFinancialWithCustomer | End-to-end financial prompt (canonical) |
+| Regex only | AllRegexTypes | Email, phone, PAN, Aadhaar, credit card |
+| Domain only | AllDomainIds | Loan number, customer ID, account number |
+| Negative | NoPiiCleanTicket | Passthrough with no detected PII |
+
+Sample definitions live in [`SamplePromptCatalog.cs`](../src/PiiRedaction.ConsoleApp/Samples/SamplePromptCatalog.cs).
+
+---
+
+## High-Level Data Flow
+
+The diagram below traces the canonical example from console input through Core sanitization to the Infrastructure LLM adapter. Data labels reflect the canonical strings at each stage.
+
+```mermaid
+flowchart TB
+    subgraph consoleApp [PiiRedaction.ConsoleApp]
+        program["Program.cs"]
+        di["ServiceRegistration"]
+    end
+
+    subgraph core [PiiRedaction.Core]
+        sanitizer["PromptSanitizer"]
+        composite["CompositePiiDetector"]
+        regexDet["RegexPiiDetector"]
+        domainDet["DomainRulePiiDetector"]
+        onnxDet["OnnxNerPiiDetector"]
+        redactor["PlaceholderPiiRedactor"]
+    end
+
+    subgraph infra [PiiRedaction.Infrastructure]
+        onnxRunner["OnnxNerModelRunner"]
+        mockLlm["MockLlmPromptService"]
+        mockChat["MockChatClient"]
+    end
+
+    rawPrompt["Raw prompt with PII"]
+    sanitizedPrompt["Sanitized prompt with placeholders"]
+    llmResponse["Mock LLM acknowledgment"]
+
+    program -->|"Customer Ravi Kumar ... PAN ABCDE1234F"| sanitizer
+    sanitizer --> composite
+    composite --> domainDet
+    composite --> regexDet
+    composite --> onnxDet
+    onnxDet --> onnxRunner
+    sanitizer --> redactor
+    redactor -->|"Customer PERSON_1 ... PAN PAN_1"| sanitizedPrompt
+    program -->|"SanitizedPrompt only"| mockLlm
+    mockLlm --> mockChat
+    mockChat --> llmResponse
+
+    rawPrompt -.-> program
+    di -.-> sanitizer
+    di -.-> mockLlm
+```
+
+---
+
+## Detection to Redaction Detail
+
+`PromptSanitizer` orchestrates a two-phase pipeline: **detect** then **redact**. `CompositePiiDetector` aggregates spans from all registered detectors, resolves overlaps by registration order and source priority, and returns a merged entity list. `PlaceholderPiiRedactor` replaces spans right-to-left to preserve indices, assigns stable per-type counters, and builds the in-process placeholder map.
+
+```mermaid
+flowchart LR
+  inputText["Original prompt text"]
+
+  subgraph detectPhase [Detection Phase]
+    domainDet["DomainRulePiiDetector"]
+    regexDet["RegexPiiDetector"]
+    onnxDet["OnnxNerPiiDetector"]
+    composite["CompositePiiDetector"]
+    merge["Overlap merge and source priority"]
+    entityList["PiiEntity list"]
+  end
+
+  subgraph redactPhase [Redaction Phase]
+    redactor["PlaceholderPiiRedactor"]
+    replace["Right-to-left span replacement"]
+    placeholderMap["Placeholder map in-process"]
+    sanitizedText["Sanitized text"]
+  end
+
+  inputText --> domainDet
+  inputText --> regexDet
+  inputText --> onnxDet
+  domainDet --> composite
+  regexDet --> composite
+  onnxDet --> composite
+  composite --> merge
+  merge --> entityList
+  entityList --> redactor
+  inputText --> redactor
+  redactor --> replace
+  replace --> sanitizedText
+  replace --> placeholderMap
+```
+
+**Overlap resolution rules** (applied by `CompositePiiDetector`):
+
+1. Detectors run in registration order: **Domain → Regex → ONNX NER**.
+2. On overlapping spans, the first registered detector wins.
+3. Tie-breaking uses source priority: Domain (3) > Regex (2) > NER (1).
+
+**Placeholder assignment** (applied by `PlaceholderPiiRedactor`):
+
+- Format: `<{TYPE}_{n}>` (e.g. `<EMAIL_1>`, `<PERSON_1>`).
+- Duplicate values of the same type reuse the same placeholder.
+- Replacement proceeds from highest `StartIndex` to lowest to avoid index drift.
+
+---
+
+## Runtime Sequence
+
+```mermaid
+sequenceDiagram
+    participant User
+    participant Program as Program.cs
+    participant DI as ServiceProvider
+    participant Sanitizer as PromptSanitizer
+    participant Detector as CompositePiiDetector
+    participant Redactor as PlaceholderPiiRedactor
+    participant LlmSvc as MockLlmPromptService
+    participant Chat as MockChatClient
+
+    User->>Program: Start application
+    Program->>DI: Resolve IPromptSanitizer, ILlmPromptService
+    DI-->>Program: Sanitizer, LlmService
+
+    alt Interactive mode
+        User->>Program: Enter prompt via console
+    else Default mode
+        Program->>Program: Load canonical sample prompt
+    end
+
+    Program->>Sanitizer: Sanitize(SanitizationRequest)
+    Sanitizer->>Detector: Detect(originalPrompt)
+    Detector-->>Sanitizer: IReadOnlyList PiiEntity
+    Sanitizer->>Redactor: Redact(originalPrompt, entities)
+    Redactor-->>Sanitizer: RedactionResult
+    Sanitizer-->>Program: SanitizationResult
+
+    Program->>Program: Display detected entities
+    Program->>Program: Display sanitized prompt
+    Program->>Program: Display placeholder map in-process
+
+    Program->>LlmSvc: SendPromptAsync(sanitizedPrompt)
+    Note over Program,LlmSvc: Placeholder map never passed
+    LlmSvc->>Chat: GetResponseAsync(user message)
+    Chat-->>LlmSvc: Assistant response
+    LlmSvc-->>Program: Mock LLM response string
+    Program->>User: Write LLM response to console
+```
+
+---
+
+## Trust Boundary
+
+The LLM boundary is the point at which data leaves the application process via `ILlmPromptService` / `IChatClient`. Only the sanitized prompt crosses this boundary. Original PII values, detection metadata, and the placeholder-to-value map remain in-process.
+
+```mermaid
+flowchart TB
+    subgraph inProcess [In-Process Trust Zone]
+        originalPrompt["Original prompt with raw PII"]
+        detectedEntities["Detected PiiEntity list"]
+        placeholderMap["Placeholder map"]
+        sanitizationResult["SanitizationResult"]
+        consoleDisplay["Console audit output"]
+    end
+
+    subgraph llmBoundary [LLM Trust Boundary]
+        sanitizedOnly["Sanitized prompt text only"]
+    end
+
+    subgraph externalLlm [External LLM Provider]
+        chatClient["IChatClient implementation"]
+        modelInference["Model inference"]
+    end
+
+    originalPrompt --> sanitizationResult
+    detectedEntities --> sanitizationResult
+    placeholderMap --> sanitizationResult
+    sanitizationResult --> consoleDisplay
+    sanitizationResult -->|"SendPromptAsync"| sanitizedOnly
+    sanitizedOnly --> chatClient
+    chatClient --> modelInference
+
+    originalPrompt -.-x|"Never transmitted"| chatClient
+    placeholderMap -.-x|"Never transmitted"| chatClient
+    detectedEntities -.-x|"Never transmitted"| chatClient
+```
+
+In the POC, `MockChatClient` simulates the external provider without network I/O. Replacing it with Azure OpenAI or another `IChatClient` implementation does not change the trust model: `MockLlmPromptService` (or a future production adapter) continues to accept only the sanitized string.
+
+---
+
+## Project Responsibilities
+
+| Project | Layer | Responsibility |
+|---------|-------|----------------|
+| `PiiRedaction.ConsoleApp` | Presentation | Application entry point; reads prompt (sample or interactive); bootstraps `IHost` and DI via `AddPiiRedactionServices`; orchestrates sanitization and LLM invocation; renders audit output (detected entities, sanitized text, placeholder map). |
+| `PiiRedaction.Core` | Domain / Application | Defines abstractions (`IPiiDetector`, `IPiiRedactor`, `IPromptSanitizer`, `ILlmPromptService`); implements detection strategies (`RegexPiiDetector`, `DomainRulePiiDetector`, `OnnxNerPiiDetector`, `CompositePiiDetector`); implements `PlaceholderPiiRedactor` and `PromptSanitizer`; owns domain models (`PiiEntity`, `SanitizationResult`, `RedactionResult`) and configuration (`PiiRedactionOptions`). Has no dependency on ONNX Runtime or LLM SDKs. |
+| `PiiRedaction.Infrastructure` | Infrastructure | Implements technical adapters: `OnnxNerModelRunner` (ONNX Runtime inference), `MockChatClient` and `MockLlmPromptService` (`Microsoft.Extensions.AI`); depends on Core abstractions and is swappable without changing domain logic. |
+| `tests/PiiRedaction.Core.Tests` | Test | Unit and integration tests for detectors, redactor, sanitizer, overlap rules, golden prompt scenarios (`PromptScenarioCatalog`), and LLM boundary assertions. |
+| `tests/PiiRedaction.Infrastructure.Tests` | Test | Tests for mock LLM behavior and ONNX runner load semantics. |
+
+**Dependency direction:** `ConsoleApp` → `Infrastructure` → `Core`. Core references no outer layers, preserving the Dependency Inversion Principle and enabling future hosts (ASP.NET Core API, worker services) to reuse the same Core and Infrastructure assemblies.
+
+---
+
+## Key Abstractions and Extension Points
+
+| Abstraction | Defined In | Default Implementation | Extension |
+|-------------|------------|------------------------|-----------|
+| `IPiiDetector` | Core | `CompositePiiDetector` wrapping Domain, Regex, ONNX | Add new detector; register in composite order |
+| `IPiiRedactor` | Core | `PlaceholderPiiRedactor` | Replace with hashing, vault-backed tokens, etc. |
+| `IPromptSanitizer` | Core | `PromptSanitizer` | Unlikely to change; orchestrates detect + redact |
+| `ILlmPromptService` | Core | `MockLlmPromptService` | Production adapter with telemetry, retry, policy |
+| `IChatClient` | Microsoft.Extensions.AI | `MockChatClient` | Azure OpenAI, OpenAI, or other provider SDK |
+| `IOnnxNerModelRunner` | Core | `OnnxNerModelRunner` | BERT WordPiece tokenization, ONNX inference, BIO label decoding |
+
+---
+
+## Configuration Surface
+
+Runtime behavior is controlled via `appsettings.json` under the `PiiRedaction` section:
+
+| Setting | Effect |
+|---------|--------|
+| `OnnxModelPath` | Path to ONNX NER model (`models/ner-model.onnx` by default). Companion files `vocab.txt` and `ner-labels.txt` must live in the same directory. |
+
+Download the model assets with `scripts/download-ner-model.ps1` (exports `dslim/bert-base-NER`).
+
+---
+
+## Related Documentation
+
+- [README](../README.md) — build, run, configuration, and testing instructions
+- [ServiceCollectionExtensions.cs](../src/PiiRedaction.ConsoleApp/DependencyInjection/ServiceCollectionExtensions.cs) — DI registration and detector ordering
+- [PromptScenarioCatalog.cs](../tests/PiiRedaction.Core.Tests/TestSupport/PromptScenarioCatalog.cs) — focused golden pipeline scenarios including the canonical example
--- a/docs/git-xenovex-setup.md
+++ b/docs/git-xenovex-setup.md
@@ -0,0 +1,62 @@
+# Git setup — Xenovex (xts.xenovex.com)
+
+This repository is ready for push to your Xenovex Git server after you create a remote repository.
+
+## Prerequisites
+
+- Git 2.x (installed at `C:\Program Files\Git\bin\git.exe`)
+- Access to https://xts.xenovex.com/explore/repos
+- .NET 10 SDK for build/test
+
+## 1. Create the remote repository
+
+1. Sign in to **https://xts.xenovex.com**
+2. Open **Explore repos** (or **New repository**)
+3. Create a new empty repository, e.g. `llm-pii-poc`
+4. Copy the **HTTPS** or **SSH** clone URL (example shapes):
+   - `https://xts.xenovex.com/<org-or-user>/llm-pii-poc.git`
+   - `git@xts.xenovex.com:<org-or-user>/llm-pii-poc.git`
+
+Do **not** initialize the remote with a README if you are pushing an existing local history.
+
+## 2. Add remote and push (from repository root)
+
+```powershell
+cd C:\Users\bilal.n\Projects\llm-pii-poc
+
+# Use full path if git is not on PATH
+$git = "C:\Program Files\Git\bin\git.exe"
+
+& $git remote add origin <YOUR_CLONE_URL>
+& $git branch -M main
+& $git push -u origin main
+```
+
+If the remote already has commits (e.g. auto-generated README), either use an empty remote or:
+
+```powershell
+& $git pull origin main --rebase
+& $git push -u origin main
+```
+
+## 3. What is committed vs excluded
+
+| Included | Excluded (`.gitignore`) |
+|----------|-------------------------|
+| Source (`src/`), tests, scripts, docs | `bin/`, `obj/`, `.vs/` |
+| `README.md`, `PiiRedaction.slnx` | `models/*.onnx`, `vocab.txt`, `ner-labels.txt` (~431MB model) |
+| `models/.gitkeep` (empty models folder) | `scratch/` |
+
+After clone, download the NER model locally:
+
+```powershell
+.\scripts\download-ner-model.ps1
+```
+
+## 4. Verify after clone
+
+```powershell
+dotnet build
+dotnet test
+dotnet run --project src/PiiRedaction.ConsoleApp -- --sample 0
+```
--- a/models/.gitkeep
+++ b/models/.gitkeep
--- a/scripts/download-ner-model.ps1
+++ b/scripts/download-ner-model.ps1
@@ -0,0 +1,101 @@
+# Downloads dslim/bert-base-NER ONNX assets to models/ for the PII Redaction POC.
+param(
+    [string]$Python = "python"
+)
+
+$ErrorActionPreference = "Stop"
+$repoRoot = Split-Path -Parent $PSScriptRoot
+$modelsDir = Join-Path $repoRoot "models"
+$scriptPath = Join-Path $PSScriptRoot "download-ner-model.py"
+$baseUrl = "https://huggingface.co/dslim/bert-base-NER/resolve/main/onnx"
+
+function Ensure-ModelsDirectory {
+    New-Item -ItemType Directory -Force -Path $modelsDir | Out-Null
+}
+
+function Download-HuggingFaceAsset {
+    param(
+        [string]$RelativePath,
+        [string]$Destination
+    )
+
+    $url = "$baseUrl/$RelativePath"
+    Write-Host "Downloading $url"
+    Invoke-WebRequest -Uri $url -OutFile $Destination -UseBasicParsing
+}
+
+function Export-LabelsFromConfig {
+    param([string]$ConfigPath, [string]$LabelsPath)
+
+    $config = Get-Content $ConfigPath -Raw | ConvertFrom-Json
+    $labelMap = @{}
+    foreach ($property in $config.id2label.PSObject.Properties) {
+        $labelMap[[int]$property.Name] = [string]$property.Value
+    }
+
+    $labels = for ($index = 0; $index -lt $labelMap.Count; $index++) {
+        $labelMap[$index]
+    }
+
+    $labels | Set-Content -Path $LabelsPath -Encoding utf8
+}
+
+function Download-WithPowerShell {
+    Ensure-ModelsDirectory
+
+    $modelPath = Join-Path $modelsDir "ner-model.onnx"
+    $vocabPath = Join-Path $modelsDir "vocab.txt"
+    $configPath = Join-Path $modelsDir "config.json"
+    $labelsPath = Join-Path $modelsDir "ner-labels.txt"
+
+    Download-HuggingFaceAsset -RelativePath "model.onnx" -Destination $modelPath
+    Download-HuggingFaceAsset -RelativePath "vocab.txt" -Destination $vocabPath
+    Download-HuggingFaceAsset -RelativePath "config.json" -Destination $configPath
+    Export-LabelsFromConfig -ConfigPath $configPath -LabelsPath $labelsPath
+    Remove-Item $configPath -Force
+
+    Write-Host ""
+    Write-Host "NER model assets saved:"
+    Write-Host "  $modelPath"
+    Write-Host "  $vocabPath"
+    Write-Host "  $labelsPath"
+    Write-Host ""
+    Write-Host "Run from repository root:"
+    Write-Host "  dotnet run --project src/PiiRedaction.ConsoleApp -- --name CustomerNameOnly"
+}
+
+function Download-WithPython {
+    if (-not (Get-Command $Python -ErrorAction SilentlyContinue)) {
+        return $false
+    }
+
+    $pythonCommand = Get-Command $Python
+    if ($pythonCommand.Source -like "*WindowsApps*") {
+        return $false
+    }
+
+    Write-Host "Using Python: $($pythonCommand.Source)"
+    Write-Host "Repository root: $repoRoot"
+    Write-Host ""
+
+    Push-Location $repoRoot
+    try {
+        & $Python $scriptPath
+        if ($LASTEXITCODE -ne 0) {
+            throw "Model download script failed with exit code $LASTEXITCODE."
+        }
+    }
+    finally {
+        Pop-Location
+    }
+
+    return $true
+}
+
+Write-Host "Repository root: $repoRoot"
+
+if (-not (Download-WithPython)) {
+    Write-Host "Python export unavailable; downloading pre-exported ONNX assets from Hugging Face..."
+    Write-Host ""
+    Download-WithPowerShell
+}
--- a/scripts/download-ner-model.py
+++ b/scripts/download-ner-model.py
@@ -0,0 +1,83 @@
+#!/usr/bin/env python3
+"""Download and export dslim/bert-base-NER to ONNX for the PII Redaction POC."""
+
+from __future__ import annotations
+
+import json
+import shutil
+import subprocess
+import sys
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+MODELS_DIR = REPO_ROOT / "models"
+MODEL_ID = "dslim/bert-base-NER"
+REQUIRED_PACKAGES = ("transformers", "optimum[onnxruntime]", "onnx", "torch")
+
+
+def ensure_dependencies() -> None:
+    try:
+        import optimum.onnxruntime  # noqa: F401
+        import transformers  # noqa: F401
+    except ImportError:
+        print("Installing Python dependencies (this may take a few minutes)...")
+        subprocess.check_call(
+            [sys.executable, "-m", "pip", "install", *REQUIRED_PACKAGES],
+            stdout=sys.stdout,
+            stderr=sys.stderr,
+        )
+
+
+def export_model() -> None:
+    from optimum.onnxruntime import ORTModelForTokenClassification
+    from transformers import AutoTokenizer
+
+    MODELS_DIR.mkdir(parents=True, exist_ok=True)
+    temp_dir = MODELS_DIR / "_export_temp"
+    if temp_dir.exists():
+        shutil.rmtree(temp_dir)
+    temp_dir.mkdir()
+
+    print(f"Exporting {MODEL_ID} to ONNX...")
+    model = ORTModelForTokenClassification.from_pretrained(MODEL_ID, export=True)
+    tokenizer = AutoTokenizer.from_pretrained(MODEL_ID)
+
+    model.save_pretrained(temp_dir)
+    tokenizer.save_pretrained(temp_dir)
+
+    onnx_files = sorted(temp_dir.glob("*.onnx"))
+    if not onnx_files:
+        raise FileNotFoundError("Export completed but no .onnx file was produced.")
+
+    target_onnx = MODELS_DIR / "ner-model.onnx"
+    shutil.copy(onnx_files[0], target_onnx)
+    shutil.copy(temp_dir / "vocab.txt", MODELS_DIR / "vocab.txt")
+
+    config_path = temp_dir / "config.json"
+    with config_path.open(encoding="utf-8") as config_file:
+        config = json.load(config_file)
+
+    id2label = config.get("id2label", {})
+    labels = [id2label[str(index)] for index in range(len(id2label))]
+    (MODELS_DIR / "ner-labels.txt").write_text("\n".join(labels), encoding="utf-8")
+
+    shutil.rmtree(temp_dir)
+
+    print()
+    print("NER model assets saved:")
+    print(f"  {target_onnx}")
+    print(f"  {MODELS_DIR / 'vocab.txt'}")
+    print(f"  {MODELS_DIR / 'ner-labels.txt'}")
+    print()
+    print("Run from repository root:")
+    print("  dotnet run --project src/PiiRedaction.ConsoleApp -- --name CustomerNameOnly")
+
+
+def main() -> int:
+    ensure_dependencies()
+    export_model()
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
--- a/src/PiiRedaction.ConsoleApp/DependencyInjection/ServiceCollectionExtensions.cs
+++ b/src/PiiRedaction.ConsoleApp/DependencyInjection/ServiceCollectionExtensions.cs
@@ -0,0 +1,41 @@
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Configuration;
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Redaction;
+using PiiRedaction.Core.Sanitization;
+using PiiRedaction.Infrastructure.Llm;
+using PiiRedaction.Infrastructure.Onnx;
+
+namespace PiiRedaction.ConsoleApp.DependencyInjection;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection AddPiiRedactionServices(this IServiceCollection services, IConfiguration configuration)
+    {
+        services.Configure<PiiRedactionOptions>(configuration.GetSection(PiiRedactionOptions.SectionName));
+
+        services.AddSingleton<DomainRulePiiDetector>();
+        services.AddSingleton<RegexPiiDetector>();
+        services.AddSingleton<OnnxNerPiiDetector>();
+
+        services.AddSingleton<IPiiDetector>(provider => new CompositePiiDetector(
+        [
+            provider.GetRequiredService<DomainRulePiiDetector>(),
+            provider.GetRequiredService<RegexPiiDetector>(),
+            provider.GetRequiredService<OnnxNerPiiDetector>()
+        ]));
+
+        services.AddSingleton<IPiiRedactor, PlaceholderPiiRedactor>();
+        services.AddSingleton<IPromptSanitizer, PromptSanitizer>();
+
+        services.AddSingleton<OnnxNerModelRunner>();
+        services.AddSingleton<IOnnxNerModelRunner>(provider => provider.GetRequiredService<OnnxNerModelRunner>());
+        services.AddSingleton<IChatClient, MockChatClient>();
+        services.AddSingleton<ILlmPromptService, MockLlmPromptService>();
+
+        return services;
+    }
+}
--- a/src/PiiRedaction.ConsoleApp/PiiRedaction.ConsoleApp.csproj
+++ b/src/PiiRedaction.ConsoleApp/PiiRedaction.ConsoleApp.csproj
@@ -0,0 +1,28 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <ItemGroup>
+    <ProjectReference Include="..\PiiRedaction.Core\PiiRedaction.Core.csproj" />
+    <ProjectReference Include="..\PiiRedaction.Infrastructure\PiiRedaction.Infrastructure.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="10.0.9" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="10.0.9" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="10.0.9" />
+    <PackageReference Include="Microsoft.Extensions.Hosting" Version="10.0.9" />
+  </ItemGroup>
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Update="appsettings.json">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
+
+</Project>
--- a/src/PiiRedaction.ConsoleApp/Program.cs
+++ b/src/PiiRedaction.ConsoleApp/Program.cs
@@ -0,0 +1,88 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using PiiRedaction.ConsoleApp.DependencyInjection;
+using PiiRedaction.ConsoleApp.Samples;
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+var interactive = args.Contains("--interactive", StringComparer.OrdinalIgnoreCase);
+var listSamples = args.Contains("--list", StringComparer.OrdinalIgnoreCase);
+
+var host = Host.CreateDefaultBuilder(args)
+    .ConfigureServices((context, services) =>
+    {
+        services.AddPiiRedactionServices(context.Configuration);
+    })
+    .Build();
+
+if (listSamples)
+{
+    PromptDemoRunner.ListSamples();
+    return;
+}
+
+var sanitizer = host.Services.GetRequiredService<IPromptSanitizer>();
+var llmService = host.Services.GetRequiredService<ILlmPromptService>();
+var runner = new PromptDemoRunner(sanitizer, llmService);
+
+Console.WriteLine("=== PII Redaction POC ===");
+Console.WriteLine();
+
+if (interactive)
+{
+    await RunInteractiveAsync(sanitizer, llmService);
+}
+else
+{
+    var samples = ResolveSamples(args);
+    await RunSamplesAsync(runner, samples);
+}
+
+await host.StopAsync();
+
+static IReadOnlyList<SamplePromptDefinition> ResolveSamples(string[] args)
+{
+    var nameIndex = Array.FindIndex(args, arg => arg.Equals("--name", StringComparison.OrdinalIgnoreCase));
+    if (nameIndex >= 0 && nameIndex + 1 < args.Length)
+    {
+        var sample = SamplePromptCatalog.FindByName(args[nameIndex + 1])
+            ?? throw new ArgumentException($"Unknown sample name: {args[nameIndex + 1]}");
+
+        return [sample];
+    }
+
+    var sampleIndex = Array.FindIndex(args, arg => arg.Equals("--sample", StringComparison.OrdinalIgnoreCase));
+    if (sampleIndex >= 0 && sampleIndex + 1 < args.Length)
+    {
+        if (!int.TryParse(args[sampleIndex + 1], out var index) ||
+            index < 0 ||
+            index >= SamplePromptCatalog.All.Count)
+        {
+            throw new ArgumentException($"Sample index must be between 0 and {SamplePromptCatalog.All.Count - 1}.");
+        }
+
+        return [SamplePromptCatalog.All[index]];
+    }
+
+    return SamplePromptCatalog.All;
+}
+
+static async Task RunSamplesAsync(PromptDemoRunner runner, IReadOnlyList<SamplePromptDefinition> samples)
+{
+    for (var i = 0; i < samples.Count; i++)
+    {
+        await runner.RunAsync(samples[i], i + 1, samples.Count);
+    }
+}
+
+static async Task RunInteractiveAsync(IPromptSanitizer sanitizer, ILlmPromptService llmService)
+{
+    Console.WriteLine("Enter a prompt (press Enter on an empty line to finish):");
+    var prompt = Console.ReadLine() ?? string.Empty;
+
+    var runner = new PromptDemoRunner(sanitizer, llmService);
+    await runner.RunAsync(
+        new SamplePromptDefinition("Interactive", "Custom", "User-provided prompt.", prompt),
+        1,
+        1);
+}
--- a/src/PiiRedaction.ConsoleApp/Samples/PromptDemoRunner.cs
+++ b/src/PiiRedaction.ConsoleApp/Samples/PromptDemoRunner.cs
@@ -0,0 +1,124 @@
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.ConsoleApp.Samples;
+
+public sealed class PromptDemoRunner
+{
+    private readonly IPromptSanitizer _sanitizer;
+    private readonly ILlmPromptService _llmService;
+
+    public PromptDemoRunner(IPromptSanitizer sanitizer, ILlmPromptService llmService)
+    {
+        _sanitizer = sanitizer;
+        _llmService = llmService;
+    }
+
+    public async Task RunAsync(SamplePromptDefinition sample, int index, int total)
+    {
+        Console.WriteLine(new string('=', 72));
+        Console.WriteLine($"Sample {index}/{total}: {sample.Name} [{sample.Category}]");
+        Console.WriteLine(sample.Description);
+        Console.WriteLine(new string('-', 72));
+        Console.WriteLine();
+        Console.WriteLine("Original Prompt:");
+        Console.WriteLine(sample.Prompt);
+        Console.WriteLine();
+
+        var result = _sanitizer.Sanitize(new SanitizationRequest(sample.Prompt));
+
+        DisplayDetectedEntities(result.DetectedEntities);
+
+        Console.WriteLine("Sanitized Prompt:");
+        Console.WriteLine(result.SanitizedPrompt);
+        Console.WriteLine();
+
+        DisplayPlaceholderMap(result.Redaction.PlaceholderMap);
+
+        var llmResponse = await _llmService.SendPromptAsync(result.SanitizedPrompt);
+
+        Console.WriteLine("Mock LLM Response:");
+        Console.WriteLine(llmResponse);
+        Console.WriteLine();
+    }
+
+    public static void ListSamples()
+    {
+        Console.WriteLine("Available samples:");
+        Console.WriteLine();
+
+        for (var i = 0; i < SamplePromptCatalog.All.Count; i++)
+        {
+            var sample = SamplePromptCatalog.All[i];
+            Console.WriteLine($"  [{i}] {sample.Name,-28} [{sample.Category}] {sample.Description}");
+        }
+
+        Console.WriteLine();
+        Console.WriteLine("Usage:");
+        Console.WriteLine("  dotnet run --project src/PiiRedaction.ConsoleApp              # run all samples");
+        Console.WriteLine("  dotnet run --project src/PiiRedaction.ConsoleApp -- --sample 2");
+        Console.WriteLine("  dotnet run --project src/PiiRedaction.ConsoleApp -- --name MrTitlePerson");
+        Console.WriteLine("  dotnet run --project src/PiiRedaction.ConsoleApp -- --list");
+        Console.WriteLine("  dotnet run --project src/PiiRedaction.ConsoleApp -- --interactive");
+    }
+
+    private static void DisplayDetectedEntities(IReadOnlyList<PiiEntity> entities)
+    {
+        Console.WriteLine("Detected PII:");
+
+        if (entities.Count == 0)
+        {
+            Console.WriteLine("  (none)");
+            Console.WriteLine();
+            return;
+        }
+
+        foreach (var entity in entities.OrderBy(e => e.StartIndex))
+        {
+            var sourceLabel = entity.Source switch
+            {
+                PiiDetectionSource.Regex => "Regex",
+                PiiDetectionSource.Domain => "Domain",
+                PiiDetectionSource.Ner => "Ner",
+                _ => entity.Source.ToString()
+            };
+
+            Console.WriteLine($"  [{ToDisplayType(entity.Type),-13}] {entity.Value,-28} ({sourceLabel})");
+        }
+
+        Console.WriteLine();
+    }
+
+    private static void DisplayPlaceholderMap(IReadOnlyDictionary<string, string> placeholderMap)
+    {
+        Console.WriteLine("Internal Placeholder Map (not sent to LLM):");
+
+        if (placeholderMap.Count == 0)
+        {
+            Console.WriteLine("  (none)");
+            Console.WriteLine();
+            return;
+        }
+
+        foreach (var (placeholder, originalValue) in placeholderMap.OrderBy(pair => pair.Key, StringComparer.Ordinal))
+        {
+            Console.WriteLine($"  {placeholder,-16} -> {originalValue}");
+        }
+
+        Console.WriteLine();
+    }
+
+    private static string ToDisplayType(PiiEntityType type) => type switch
+    {
+        PiiEntityType.Person => "PERSON",
+        PiiEntityType.Email => "EMAIL",
+        PiiEntityType.Phone => "PHONE",
+        PiiEntityType.Pan => "PAN",
+        PiiEntityType.Aadhaar => "AADHAAR",
+        PiiEntityType.CreditCard => "CREDIT_CARD",
+        PiiEntityType.LoanNumber => "LOAN_NUMBER",
+        PiiEntityType.CustomerId => "CUSTOMER_ID",
+        PiiEntityType.AccountNumber => "ACCOUNT_NUMBER",
+        _ => type.ToString().ToUpperInvariant()
+    };
+}
--- a/src/PiiRedaction.ConsoleApp/Samples/SamplePromptCatalog.cs
+++ b/src/PiiRedaction.ConsoleApp/Samples/SamplePromptCatalog.cs
@@ -0,0 +1,80 @@
+namespace PiiRedaction.ConsoleApp.Samples;
+
+/// <summary>
+/// Curated demonstration prompts for the console POC.
+/// NER samples require the ONNX model (see scripts/download-ner-model.ps1).
+/// </summary>
+public static class SamplePromptCatalog
+{
+    public static IReadOnlyList<SamplePromptDefinition> All { get; } =
+    [
+        new(
+            "FullFinancialWithCustomer",
+            "NER + Regex + Domain",
+            "Canonical demo: person name plus email, phone, loan number, and PAN.",
+            "Customer Ravi Kumar with email ravi.kumar@gmail.com and phone 9876543210 has LoanNumber LN-456789 and PAN ABCDE1234F. Please summarize this customer issue."),
+
+        new(
+            "CustomerNameOnly",
+            "NER",
+            "Person name detected via ONNX NER after 'Customer' keyword.",
+            "Customer Anita Sharma reported unauthorized transactions on her savings account."),
+
+        new(
+            "MrTitlePerson",
+            "NER",
+            "Person detected via ONNX NER (title prefix Mr.).",
+            "Mr. John Smith called about a duplicate debit on 15 March."),
+
+        new(
+            "MrsTitlePerson",
+            "NER",
+            "Person detected via ONNX NER (title prefix Mrs.).",
+            "Mrs. Lakshmi Reddy requested a callback regarding LN-112233."),
+
+        new(
+            "DrTitlePerson",
+            "NER",
+            "Person detected via ONNX NER (title prefix Dr).",
+            "Dr. Jane Doe escalated a complaint about delayed loan disbursement."),
+
+        new(
+            "TwoCustomersInOnePrompt",
+            "NER",
+            "Two distinct person names in the same prompt.",
+            "Customer Ravi Kumar and Customer Priya Nair disputed the same charge."),
+
+        new(
+            "PersonWithDomainIds",
+            "NER + Domain",
+            "Person name combined with business identifiers.",
+            "Customer Meera Iyer holds CID-7070 and account ACC-606060 for verification."),
+
+        new(
+            "PersonWithEmailNoPhone",
+            "NER + Regex",
+            "Person and email without phone number.",
+            "Customer Arjun Mehta wrote from arjun.mehta@company.in about KYC renewal."),
+
+        new(
+            "AllRegexTypes",
+            "Regex",
+            "Email, phone, PAN, Aadhaar, and credit card in one prompt.",
+            "Email a@b.co phone 9001234567 PAN ABCDE1234F aadhaar 1234 5678 9012 card 4111-1111-1111-1111."),
+
+        new(
+            "AllDomainIds",
+            "Domain",
+            "Loan number, customer ID, and account number together.",
+            "Please verify LN-100200 for CustomerId CID-3000 on AccountNumber ACC-400500."),
+
+        new(
+            "NoPiiCleanTicket",
+            "Negative",
+            "No PII — prompt passes through unchanged.",
+            "What is the status of ticket TKT-99887 and when will the API maintenance end?")
+    ];
+
+    public static SamplePromptDefinition? FindByName(string name) =>
+        All.FirstOrDefault(sample => sample.Name.Equals(name, StringComparison.OrdinalIgnoreCase));
+}
--- a/src/PiiRedaction.ConsoleApp/Samples/SamplePromptDefinition.cs
+++ b/src/PiiRedaction.ConsoleApp/Samples/SamplePromptDefinition.cs
@@ -0,0 +1,7 @@
+namespace PiiRedaction.ConsoleApp.Samples;
+
+public sealed record SamplePromptDefinition(
+    string Name,
+    string Category,
+    string Description,
+    string Prompt);
--- a/src/PiiRedaction.ConsoleApp/appsettings.json
+++ b/src/PiiRedaction.ConsoleApp/appsettings.json
@@ -0,0 +1,5 @@
+{
+  "PiiRedaction": {
+    "OnnxModelPath": "models/ner-model.onnx"
+  }
+}
--- a/src/PiiRedaction.Core/Abstractions/ILlmPromptService.cs
+++ b/src/PiiRedaction.Core/Abstractions/ILlmPromptService.cs
@@ -0,0 +1,6 @@
+namespace PiiRedaction.Core.Abstractions;
+
+public interface ILlmPromptService
+{
+    Task<string> SendPromptAsync(string sanitizedPrompt, CancellationToken cancellationToken = default);
+}
--- a/src/PiiRedaction.Core/Abstractions/IPiiDetector.cs
+++ b/src/PiiRedaction.Core/Abstractions/IPiiDetector.cs
@@ -0,0 +1,8 @@
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Abstractions;
+
+public interface IPiiDetector
+{
+    IReadOnlyList<PiiEntity> Detect(string text);
+}
--- a/src/PiiRedaction.Core/Abstractions/IPiiRedactor.cs
+++ b/src/PiiRedaction.Core/Abstractions/IPiiRedactor.cs
@@ -0,0 +1,8 @@
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Abstractions;
+
+public interface IPiiRedactor
+{
+    RedactionResult Redact(string text, IReadOnlyList<PiiEntity> entities);
+}
--- a/src/PiiRedaction.Core/Abstractions/IPromptSanitizer.cs
+++ b/src/PiiRedaction.Core/Abstractions/IPromptSanitizer.cs
@@ -0,0 +1,8 @@
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Abstractions;
+
+public interface IPromptSanitizer
+{
+    SanitizationResult Sanitize(SanitizationRequest request);
+}
--- a/src/PiiRedaction.Core/Configuration/PiiRedactionOptions.cs
+++ b/src/PiiRedaction.Core/Configuration/PiiRedactionOptions.cs
@@ -0,0 +1,8 @@
+namespace PiiRedaction.Core.Configuration;
+
+public sealed class PiiRedactionOptions
+{
+    public const string SectionName = "PiiRedaction";
+
+    public string OnnxModelPath { get; set; } = "models/ner-model.onnx";
+}
--- a/src/PiiRedaction.Core/Detection/CompositePiiDetector.cs
+++ b/src/PiiRedaction.Core/Detection/CompositePiiDetector.cs
@@ -0,0 +1,55 @@
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Detection;
+
+/// <summary>
+/// Aggregates multiple PII detectors and merges overlapping spans.
+/// Detectors are applied in registration order; earlier detectors win on overlap.
+/// </summary>
+public sealed class CompositePiiDetector : IPiiDetector
+{
+    private readonly IReadOnlyList<IPiiDetector> _detectors;
+
+    public CompositePiiDetector(IEnumerable<IPiiDetector> detectors)
+    {
+        _detectors = detectors.ToList();
+    }
+
+    public IReadOnlyList<PiiEntity> Detect(string text)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(text);
+
+        var candidates = _detectors
+            .SelectMany(detector => detector.Detect(text))
+            .OrderBy(entity => entity.StartIndex)
+            .ThenByDescending(entity => entity.Length)
+            .ThenByDescending(entity => GetSourcePriority(entity.Source))
+            .ToList();
+
+        var merged = new List<PiiEntity>();
+
+        foreach (var candidate in candidates)
+        {
+            if (merged.Any(existing => Overlaps(existing, candidate)))
+            {
+                continue;
+            }
+
+            merged.Add(candidate);
+        }
+
+        return merged.OrderBy(entity => entity.StartIndex).ToList();
+    }
+
+    private static bool Overlaps(PiiEntity left, PiiEntity right) =>
+        left.StartIndex < right.EndIndex && right.StartIndex < left.EndIndex;
+
+    private static int GetSourcePriority(PiiDetectionSource source) => source switch
+    {
+        PiiDetectionSource.Domain => 3,
+        PiiDetectionSource.Regex => 2,
+        PiiDetectionSource.Ner => 1,
+        _ => 0
+    };
+}
--- a/src/PiiRedaction.Core/Detection/DomainRulePiiDetector.cs
+++ b/src/PiiRedaction.Core/Detection/DomainRulePiiDetector.cs
@@ -0,0 +1,56 @@
+using System.Text.RegularExpressions;
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Detection;
+
+/// <summary>
+/// Detects organization-specific identifiers using business domain rules.
+/// Domain rules are needed because identifiers like loan numbers and customer IDs
+/// are defined by internal systems and cannot be inferred reliably by generic NER or public regex alone.
+/// </summary>
+public sealed partial class DomainRulePiiDetector : IPiiDetector
+{
+    private static readonly (PiiEntityType Type, Regex Pattern)[] Patterns =
+    [
+        (PiiEntityType.LoanNumber, LoanNumberPattern()),
+        (PiiEntityType.CustomerId, CustomerIdPattern()),
+        (PiiEntityType.AccountNumber, AccountNumberPattern())
+    ];
+
+    public IReadOnlyList<PiiEntity> Detect(string text)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(text);
+
+        var entities = new List<PiiEntity>();
+
+        foreach (var (type, pattern) in Patterns)
+        {
+            foreach (Match match in pattern.Matches(text))
+            {
+                if (!match.Success)
+                {
+                    continue;
+                }
+
+                entities.Add(new PiiEntity(
+                    type,
+                    match.Value,
+                    match.Index,
+                    match.Length,
+                    PiiDetectionSource.Domain));
+            }
+        }
+
+        return entities;
+    }
+
+    [GeneratedRegex(@"(?i)\bLN-\d{6,}\b", RegexOptions.Compiled)]
+    private static partial Regex LoanNumberPattern();
+
+    [GeneratedRegex(@"(?i)\bCID-\d{4,}\b", RegexOptions.Compiled)]
+    private static partial Regex CustomerIdPattern();
+
+    [GeneratedRegex(@"(?i)\bACC-\d{6,}\b", RegexOptions.Compiled)]
+    private static partial Regex AccountNumberPattern();
+}
--- a/src/PiiRedaction.Core/Detection/IOnnxNerModelRunner.cs
+++ b/src/PiiRedaction.Core/Detection/IOnnxNerModelRunner.cs
@@ -0,0 +1,11 @@
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Detection;
+
+public interface IOnnxNerModelRunner
+{
+    bool IsModelAvailable { get; }
+
+    IReadOnlyList<PiiEntity> PredictEntities(string text);
+}
--- a/src/PiiRedaction.Core/Detection/OnnxNerPiiDetector.cs
+++ b/src/PiiRedaction.Core/Detection/OnnxNerPiiDetector.cs
@@ -0,0 +1,31 @@
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Detection;
+
+/// <summary>
+/// Detects contextual entities such as person names using ONNX-based NER.
+/// NER is used for entities that lack rigid formats and vary in surface form across prompts.
+/// Requires a loaded ONNX model; returns no person entities when the model is unavailable.
+/// </summary>
+public sealed class OnnxNerPiiDetector : IPiiDetector
+{
+    private readonly IOnnxNerModelRunner _modelRunner;
+
+    public OnnxNerPiiDetector(IOnnxNerModelRunner modelRunner)
+    {
+        _modelRunner = modelRunner;
+    }
+
+    public IReadOnlyList<PiiEntity> Detect(string text)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(text);
+
+        if (!_modelRunner.IsModelAvailable)
+        {
+            return [];
+        }
+
+        return _modelRunner.PredictEntities(text);
+    }
+}
--- a/src/PiiRedaction.Core/Detection/RegexPiiDetector.cs
+++ b/src/PiiRedaction.Core/Detection/RegexPiiDetector.cs
@@ -0,0 +1,64 @@
+using System.Text.RegularExpressions;
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Detection;
+
+/// <summary>
+/// Detects PII using deterministic regular-expression patterns.
+/// Regex is used for format-bound identifiers (email, PAN, phone, Aadhaar, credit card)
+/// where rules are stable, auditable, and produce predictable matches without model inference.
+/// </summary>
+public sealed partial class RegexPiiDetector : IPiiDetector
+{
+    private static readonly (PiiEntityType Type, Regex Pattern)[] Patterns =
+    [
+        (PiiEntityType.Email, EmailPattern()),
+        (PiiEntityType.Phone, PhonePattern()),
+        (PiiEntityType.Aadhaar, AadhaarPattern()),
+        (PiiEntityType.Pan, PanPattern()),
+        (PiiEntityType.CreditCard, CreditCardPattern())
+    ];
+
+    public IReadOnlyList<PiiEntity> Detect(string text)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(text);
+
+        var entities = new List<PiiEntity>();
+
+        foreach (var (type, pattern) in Patterns)
+        {
+            foreach (Match match in pattern.Matches(text))
+            {
+                if (!match.Success)
+                {
+                    continue;
+                }
+
+                entities.Add(new PiiEntity(
+                    type,
+                    match.Value,
+                    match.Index,
+                    match.Length,
+                    PiiDetectionSource.Regex));
+            }
+        }
+
+        return entities;
+    }
+
+    [GeneratedRegex(@"[\w.+-]+@[\w.-]+\.\w+", RegexOptions.Compiled)]
+    private static partial Regex EmailPattern();
+
+    [GeneratedRegex(@"(?<!\d)\d{10}(?!\d)", RegexOptions.Compiled)]
+    private static partial Regex PhonePattern();
+
+    [GeneratedRegex(@"\b\d{4}\s?\d{4}\s?\d{4}\b", RegexOptions.Compiled)]
+    private static partial Regex AadhaarPattern();
+
+    [GeneratedRegex(@"\b[A-Z]{5}\d{4}[A-Z]\b", RegexOptions.Compiled)]
+    private static partial Regex PanPattern();
+
+    [GeneratedRegex(@"\b(?:\d{4}[-\s]?){3}\d{4}\b", RegexOptions.Compiled)]
+    private static partial Regex CreditCardPattern();
+}
--- a/src/PiiRedaction.Core/Models/PiiDetectionSource.cs
+++ b/src/PiiRedaction.Core/Models/PiiDetectionSource.cs
@@ -0,0 +1,8 @@
+namespace PiiRedaction.Core.Models;
+
+public enum PiiDetectionSource
+{
+    Regex,
+    Domain,
+    Ner
+}
--- a/src/PiiRedaction.Core/Models/PiiEntity.cs
+++ b/src/PiiRedaction.Core/Models/PiiEntity.cs
@@ -0,0 +1,12 @@
+namespace PiiRedaction.Core.Models;
+
+public sealed record PiiEntity(
+    PiiEntityType Type,
+    string Value,
+    int StartIndex,
+    int Length,
+    PiiDetectionSource Source,
+    double? Confidence = null)
+{
+    public int EndIndex => StartIndex + Length;
+}
--- a/src/PiiRedaction.Core/Models/PiiEntityType.cs
+++ b/src/PiiRedaction.Core/Models/PiiEntityType.cs
@@ -0,0 +1,14 @@
+namespace PiiRedaction.Core.Models;
+
+public enum PiiEntityType
+{
+    Person,
+    Email,
+    Phone,
+    Pan,
+    Aadhaar,
+    CreditCard,
+    LoanNumber,
+    CustomerId,
+    AccountNumber
+}
--- a/src/PiiRedaction.Core/Models/RedactionResult.cs
+++ b/src/PiiRedaction.Core/Models/RedactionResult.cs
@@ -0,0 +1,5 @@
+namespace PiiRedaction.Core.Models;
+
+public sealed record RedactionResult(
+    string SanitizedText,
+    IReadOnlyDictionary<string, string> PlaceholderMap);
--- a/src/PiiRedaction.Core/Models/SanitizationRequest.cs
+++ b/src/PiiRedaction.Core/Models/SanitizationRequest.cs
@@ -0,0 +1,3 @@
+namespace PiiRedaction.Core.Models;
+
+public sealed record SanitizationRequest(string OriginalPrompt);
--- a/src/PiiRedaction.Core/Models/SanitizationResult.cs
+++ b/src/PiiRedaction.Core/Models/SanitizationResult.cs
@@ -0,0 +1,7 @@
+namespace PiiRedaction.Core.Models;
+
+public sealed record SanitizationResult(
+    string OriginalPrompt,
+    string SanitizedPrompt,
+    IReadOnlyList<PiiEntity> DetectedEntities,
+    RedactionResult Redaction);
--- a/src/PiiRedaction.Core/PiiRedaction.Core.csproj
+++ b/src/PiiRedaction.Core/PiiRedaction.Core.csproj
@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.Options" Version="10.0.9" />
+  </ItemGroup>
+
+</Project>
--- a/src/PiiRedaction.Core/Redaction/PlaceholderPiiRedactor.cs
+++ b/src/PiiRedaction.Core/Redaction/PlaceholderPiiRedactor.cs
@@ -0,0 +1,58 @@
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Redaction;
+
+public sealed class PlaceholderPiiRedactor : IPiiRedactor
+{
+    public RedactionResult Redact(string text, IReadOnlyList<PiiEntity> entities)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(text);
+        ArgumentNullException.ThrowIfNull(entities);
+
+        var placeholderByValue = new Dictionary<string, string>(StringComparer.Ordinal);
+        var counters = new Dictionary<PiiEntityType, int>();
+        var orderedEntities = entities.OrderByDescending(entity => entity.StartIndex).ToList();
+        var sanitized = text;
+
+        foreach (var entity in orderedEntities)
+        {
+            var mapKey = CreateValueKey(entity);
+            if (!placeholderByValue.TryGetValue(mapKey, out var placeholder))
+            {
+                counters.TryGetValue(entity.Type, out var count);
+                count++;
+                counters[entity.Type] = count;
+                placeholder = $"<{ToPlaceholderPrefix(entity.Type)}_{count}>";
+                placeholderByValue[mapKey] = placeholder;
+            }
+
+            sanitized = string.Concat(
+                sanitized.AsSpan(0, entity.StartIndex),
+                placeholder,
+                sanitized.AsSpan(entity.EndIndex));
+        }
+
+        var placeholderMap = placeholderByValue
+            .ToDictionary(pair => pair.Value, pair => pair.Key.Split('|', 2)[1], StringComparer.Ordinal);
+
+        return new RedactionResult(sanitized, placeholderMap);
+    }
+
+    private static string CreateValueKey(PiiEntity entity) =>
+        $"{entity.Type}|{entity.Value}";
+
+    private static string ToPlaceholderPrefix(PiiEntityType type) => type switch
+    {
+        PiiEntityType.Person => "PERSON",
+        PiiEntityType.Email => "EMAIL",
+        PiiEntityType.Phone => "PHONE",
+        PiiEntityType.Pan => "PAN",
+        PiiEntityType.Aadhaar => "AADHAAR",
+        PiiEntityType.CreditCard => "CREDIT_CARD",
+        PiiEntityType.LoanNumber => "LOAN_NUMBER",
+        PiiEntityType.CustomerId => "CUSTOMER_ID",
+        PiiEntityType.AccountNumber => "ACCOUNT_NUMBER",
+        _ => type.ToString().ToUpperInvariant()
+    };
+}
--- a/src/PiiRedaction.Core/Sanitization/PromptSanitizer.cs
+++ b/src/PiiRedaction.Core/Sanitization/PromptSanitizer.cs
@@ -0,0 +1,31 @@
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Sanitization;
+
+public sealed class PromptSanitizer : IPromptSanitizer
+{
+    private readonly IPiiDetector _detector;
+    private readonly IPiiRedactor _redactor;
+
+    public PromptSanitizer(IPiiDetector detector, IPiiRedactor redactor)
+    {
+        _detector = detector;
+        _redactor = redactor;
+    }
+
+    public SanitizationResult Sanitize(SanitizationRequest request)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+        ArgumentException.ThrowIfNullOrWhiteSpace(request.OriginalPrompt);
+
+        var entities = _detector.Detect(request.OriginalPrompt);
+        var redaction = _redactor.Redact(request.OriginalPrompt, entities);
+
+        return new SanitizationResult(
+            request.OriginalPrompt,
+            redaction.SanitizedText,
+            entities,
+            redaction);
+    }
+}
--- a/src/PiiRedaction.Infrastructure/Llm/MockChatClient.cs
+++ b/src/PiiRedaction.Infrastructure/Llm/MockChatClient.cs
@@ -0,0 +1,41 @@
+using System.Runtime.CompilerServices;
+using Microsoft.Extensions.AI;
+
+namespace PiiRedaction.Infrastructure.Llm;
+
+/// <summary>
+/// Mock chat client for POC demonstrations. Returns deterministic responses without external API calls.
+/// </summary>
+public sealed class MockChatClient : IChatClient
+{
+    public Task<ChatResponse> GetResponseAsync(
+        IEnumerable<ChatMessage> messages,
+        ChatOptions? options = null,
+        CancellationToken cancellationToken = default)
+    {
+        var userMessage = messages.LastOrDefault(message => message.Role == ChatRole.User);
+        var userText = userMessage?.Text ?? string.Empty;
+
+        var response = new ChatResponse(new ChatMessage(
+            ChatRole.Assistant,
+            $"[Mock LLM Response] Received sanitized prompt ({userText.Length} chars). No original PII was transmitted."));
+
+        return Task.FromResult(response);
+    }
+
+    public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
+        IEnumerable<ChatMessage> messages,
+        ChatOptions? options = null,
+        [EnumeratorCancellation] CancellationToken cancellationToken = default)
+    {
+        var response = await GetResponseAsync(messages, options, cancellationToken).ConfigureAwait(false);
+        var text = response.Messages.LastOrDefault()?.Text ?? string.Empty;
+        yield return new ChatResponseUpdate(ChatRole.Assistant, text);
+    }
+
+    public object? GetService(Type serviceType, object? serviceKey = null) => null;
+
+    public void Dispose()
+    {
+    }
+}
--- a/src/PiiRedaction.Infrastructure/Llm/MockLlmPromptService.cs
+++ b/src/PiiRedaction.Infrastructure/Llm/MockLlmPromptService.cs
@@ -0,0 +1,35 @@
+using Microsoft.Extensions.AI;
+using PiiRedaction.Core.Abstractions;
+
+namespace PiiRedaction.Infrastructure.Llm;
+
+/// <summary>
+/// Sends only sanitized prompts to the LLM boundary.
+/// Original PII values and placeholder mappings remain in-process and are never transmitted.
+/// </summary>
+public sealed class MockLlmPromptService : ILlmPromptService
+{
+    private readonly IChatClient _chatClient;
+
+    public MockLlmPromptService(IChatClient chatClient)
+    {
+        _chatClient = chatClient;
+    }
+
+    public async Task<string> SendPromptAsync(string sanitizedPrompt, CancellationToken cancellationToken = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(sanitizedPrompt);
+
+        var messages = new[]
+        {
+            new ChatMessage(ChatRole.User, sanitizedPrompt)
+        };
+
+        var response = await _chatClient
+            .GetResponseAsync(messages, cancellationToken: cancellationToken)
+            .ConfigureAwait(false);
+
+        return response.Messages.LastOrDefault(message => message.Role == ChatRole.Assistant)?.Text
+            ?? string.Empty;
+    }
+}
--- a/src/PiiRedaction.Infrastructure/Onnx/OnnxNerModelRunner.cs
+++ b/src/PiiRedaction.Infrastructure/Onnx/OnnxNerModelRunner.cs
@@ -0,0 +1,325 @@
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using Microsoft.ML.OnnxRuntime;
+using Microsoft.ML.OnnxRuntime.Tensors;
+using Microsoft.ML.Tokenizers;
+using PiiRedaction.Core.Configuration;
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Infrastructure.Onnx;
+
+/// <summary>
+/// Wraps ONNX Runtime inference for NER models.
+/// Tokenization and tensor preparation are isolated here so detectors remain model-agnostic.
+/// </summary>
+public sealed class OnnxNerModelRunner : IOnnxNerModelRunner, IDisposable
+{
+    private const int MaxSequenceLength = 128;
+
+    private readonly ILogger<OnnxNerModelRunner> _logger;
+    private readonly string _modelPath;
+    private readonly BertTokenizer? _tokenizer;
+    private readonly string[] _labels;
+    private InferenceSession? _session;
+
+    public OnnxNerModelRunner(IOptions<PiiRedactionOptions> options, ILogger<OnnxNerModelRunner> logger)
+    {
+        _logger = logger;
+        _modelPath = ResolveAssetPath(options.Value.OnnxModelPath);
+        var modelDirectory = Path.GetDirectoryName(_modelPath) ?? Environment.CurrentDirectory;
+        _labels = LoadLabels(modelDirectory);
+        _tokenizer = TryLoadTokenizer(modelDirectory);
+        _session = TryCreateSession();
+    }
+
+    public bool IsModelAvailable => _session is not null && _tokenizer is not null && _labels.Length > 0;
+
+    public IReadOnlyList<PiiEntity> PredictEntities(string text)
+    {
+        if (!IsModelAvailable || _session is null || _tokenizer is null)
+        {
+            return [];
+        }
+
+        var encodedTokens = _tokenizer.EncodeToTokens(text, out _, considerPreTokenization: true, considerNormalization: true);
+        var wordTokens = encodedTokens.Take(Math.Max(0, MaxSequenceLength - 2)).ToList();
+        if (wordTokens.Count == 0)
+        {
+            return [];
+        }
+
+        var sequenceLength = wordTokens.Count + 2;
+        var inputIds = new long[sequenceLength];
+        var attentionMask = new long[sequenceLength];
+        var tokenTypeIds = new long[sequenceLength];
+        var offsets = new (int Start, int End)[sequenceLength];
+        var tokenIds = new int[sequenceLength];
+
+        inputIds[0] = _tokenizer.ClassificationTokenId;
+        attentionMask[0] = 1;
+        tokenIds[0] = _tokenizer.ClassificationTokenId;
+        offsets[0] = (0, 0);
+
+        for (var i = 0; i < wordTokens.Count; i++)
+        {
+            var token = wordTokens[i];
+            var index = i + 1;
+            inputIds[index] = token.Id;
+            attentionMask[index] = 1;
+            tokenIds[index] = token.Id;
+            offsets[index] = ToCharOffsets(token.Offset, text.Length);
+        }
+
+        inputIds[sequenceLength - 1] = _tokenizer.SeparatorTokenId;
+        attentionMask[sequenceLength - 1] = 1;
+        tokenIds[sequenceLength - 1] = _tokenizer.SeparatorTokenId;
+        offsets[sequenceLength - 1] = (0, 0);
+
+        var predictedLabelIds = RunInference(inputIds, attentionMask, tokenTypeIds, sequenceLength);
+        return DecodePersonEntities(text, predictedLabelIds, offsets, tokenIds, sequenceLength);
+    }
+
+    private int[] RunInference(long[] inputIds, long[] attentionMask, long[] tokenTypeIds, int sequenceLength)
+    {
+        var inputIdsTensor = CreateTensor(inputIds, sequenceLength);
+        var attentionMaskTensor = CreateTensor(attentionMask, sequenceLength);
+        var inputs = new List<NamedOnnxValue>
+        {
+            NamedOnnxValue.CreateFromTensor(_session!.InputMetadata.Keys.First(key => key.Contains("input_ids", StringComparison.OrdinalIgnoreCase)), inputIdsTensor),
+            NamedOnnxValue.CreateFromTensor(_session.InputMetadata.Keys.First(key => key.Contains("attention_mask", StringComparison.OrdinalIgnoreCase)), attentionMaskTensor)
+        };
+
+        var tokenTypeInputName = _session.InputMetadata.Keys.FirstOrDefault(key => key.Contains("token_type", StringComparison.OrdinalIgnoreCase));
+        if (tokenTypeInputName is not null)
+        {
+            inputs.Add(NamedOnnxValue.CreateFromTensor(tokenTypeInputName, CreateTensor(tokenTypeIds, sequenceLength)));
+        }
+
+        using var results = _session.Run(inputs);
+        var outputName = _session.OutputMetadata.Keys.FirstOrDefault(key =>
+            key.Contains("logits", StringComparison.OrdinalIgnoreCase))
+            ?? results.First().Name;
+        var logits = results.First(result => result.Name == outputName).AsTensor<float>();
+        var numLabels = _labels.Length;
+        var predictions = new int[sequenceLength];
+
+        for (var tokenIndex = 0; tokenIndex < sequenceLength; tokenIndex++)
+        {
+            var bestLabel = 0;
+            var bestScore = float.MinValue;
+
+            for (var labelIndex = 0; labelIndex < numLabels; labelIndex++)
+            {
+                var score = logits[0, tokenIndex, labelIndex];
+                if (score > bestScore)
+                {
+                    bestScore = score;
+                    bestLabel = labelIndex;
+                }
+            }
+
+            predictions[tokenIndex] = bestLabel;
+        }
+
+        return predictions;
+    }
+
+    private static DenseTensor<long> CreateTensor(long[] values, int sequenceLength)
+    {
+        var tensor = new DenseTensor<long>([1, sequenceLength]);
+        for (var i = 0; i < sequenceLength; i++)
+        {
+            tensor[0, i] = values[i];
+        }
+
+        return tensor;
+    }
+
+    private IReadOnlyList<PiiEntity> DecodePersonEntities(
+        string text,
+        int[] predictedLabelIds,
+        (int Start, int End)[] offsets,
+        int[] tokenIds,
+        int sequenceLength)
+    {
+        var entities = new List<PiiEntity>();
+        int? entityStart = null;
+        int? entityEnd = null;
+
+        void FlushEntity()
+        {
+            if (!entityStart.HasValue || !entityEnd.HasValue || entityEnd.Value <= entityStart.Value)
+            {
+                entityStart = null;
+                entityEnd = null;
+                return;
+            }
+
+            var value = text[entityStart.Value..entityEnd.Value];
+            if (!string.IsNullOrWhiteSpace(value))
+            {
+                entities.Add(new PiiEntity(
+                    PiiEntityType.Person,
+                    value,
+                    entityStart.Value,
+                    entityEnd.Value - entityStart.Value,
+                    PiiDetectionSource.Ner));
+            }
+
+            entityStart = null;
+            entityEnd = null;
+        }
+
+        for (var i = 0; i < sequenceLength; i++)
+        {
+            if (IsSpecialToken(tokenIds[i]))
+            {
+                FlushEntity();
+                continue;
+            }
+
+            var label = _labels[predictedLabelIds[i]];
+            var (start, end) = offsets[i];
+            var hasOffset = end > start;
+
+            if (!IsPersonLabel(label))
+            {
+                FlushEntity();
+                continue;
+            }
+
+            if (label.StartsWith("B-", StringComparison.Ordinal))
+            {
+                FlushEntity();
+                if (hasOffset)
+                {
+                    entityStart = start;
+                    entityEnd = end;
+                }
+            }
+            else if (label.StartsWith("I-", StringComparison.Ordinal))
+            {
+                if (!entityStart.HasValue && hasOffset)
+                {
+                    entityStart = start;
+                    entityEnd = end;
+                }
+                else if (hasOffset)
+                {
+                    entityEnd = Math.Max(entityEnd ?? end, end);
+                }
+            }
+        }
+
+        FlushEntity();
+        return entities;
+    }
+
+    private bool IsSpecialToken(int tokenId) =>
+        tokenId == _tokenizer!.ClassificationTokenId ||
+        tokenId == _tokenizer.SeparatorTokenId ||
+        tokenId == _tokenizer.PaddingTokenId;
+
+    private static (int Start, int End) ToCharOffsets(Range offset, int textLength)
+    {
+        var (start, length) = offset.GetOffsetAndLength(textLength);
+        return (start, start + length);
+    }
+
+    private static bool IsPersonLabel(string label) =>
+        label is "B-PER" or "I-PER" or "B-PERSON" or "I-PERSON"
+        || (label.EndsWith("-PER", StringComparison.Ordinal) &&
+            (label.StartsWith("B-", StringComparison.Ordinal) || label.StartsWith("I-", StringComparison.Ordinal)));
+
+    private InferenceSession? TryCreateSession()
+    {
+        if (!File.Exists(_modelPath))
+        {
+            _logger.LogWarning(
+                "ONNX NER model not found at {ModelPath}. Person-name detection will return no results.",
+                _modelPath);
+            return null;
+        }
+
+        if (_tokenizer is null || _labels.Length == 0)
+        {
+            _logger.LogWarning(
+                "Tokenizer vocabulary or label map missing for ONNX NER model at {ModelPath}.",
+                _modelPath);
+            return null;
+        }
+
+        try
+        {
+            var session = new InferenceSession(_modelPath);
+            _logger.LogInformation("ONNX NER model loaded from {ModelPath}.", _modelPath);
+            return session;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Failed to load ONNX NER model from {ModelPath}.", _modelPath);
+            return null;
+        }
+    }
+
+    private BertTokenizer? TryLoadTokenizer(string modelDirectory)
+    {
+        var vocabPath = ResolveAssetPath(Path.Combine(modelDirectory, "vocab.txt"));
+        if (!File.Exists(vocabPath))
+        {
+            _logger.LogWarning("Tokenizer vocabulary not found at {VocabPath}.", vocabPath);
+            return null;
+        }
+
+        try
+        {
+            return BertTokenizer.Create(vocabPath, new BertOptions
+            {
+                LowerCaseBeforeTokenization = false
+            });
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Failed to load BERT tokenizer from {VocabPath}.", vocabPath);
+            return null;
+        }
+    }
+
+    private static string[] LoadLabels(string modelDirectory)
+    {
+        var labelsPath = ResolveAssetPath(Path.Combine(modelDirectory, "ner-labels.txt"));
+        if (!File.Exists(labelsPath))
+        {
+            return [];
+        }
+
+        return File.ReadAllLines(labelsPath)
+            .Where(line => !string.IsNullOrWhiteSpace(line))
+            .ToArray();
+    }
+
+    internal static string ResolveAssetPath(string configuredPath)
+    {
+        if (Path.IsPathRooted(configuredPath) && File.Exists(configuredPath))
+        {
+            return configuredPath;
+        }
+
+        var directory = new DirectoryInfo(Environment.CurrentDirectory);
+        while (directory is not null)
+        {
+            var candidate = Path.GetFullPath(Path.Combine(directory.FullName, configuredPath));
+            if (File.Exists(candidate))
+            {
+                return candidate;
+            }
+
+            directory = directory.Parent;
+        }
+
+        return Path.GetFullPath(Path.Combine(Environment.CurrentDirectory, configuredPath));
+    }
+
+    public void Dispose() => _session?.Dispose();
+}
--- a/src/PiiRedaction.Infrastructure/PiiRedaction.Infrastructure.csproj
+++ b/src/PiiRedaction.Infrastructure/PiiRedaction.Infrastructure.csproj
@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <ItemGroup>
+    <ProjectReference Include="..\PiiRedaction.Core\PiiRedaction.Core.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.AI" Version="10.7.0" />
+    <PackageReference Include="Microsoft.Extensions.AI.Abstractions" Version="10.7.0" />
+    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="10.0.9" />
+    <PackageReference Include="Microsoft.Extensions.Options" Version="10.0.9" />
+    <PackageReference Include="Microsoft.ML.OnnxRuntime" Version="1.27.0" />
+    <PackageReference Include="Microsoft.ML.Tokenizers" Version="2.0.0" />
+  </ItemGroup>
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>
--- a/tests/PiiRedaction.Core.Tests/Detection/CompositePiiDetectorTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Detection/CompositePiiDetectorTests.cs
@@ -0,0 +1,127 @@
+using FluentAssertions;
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.Detection;
+
+[TestFixture]
+public sealed class CompositePiiDetectorTests
+{
+    [Test]
+    public void Detect_AdjacentSpans_KeepsBothEntities()
+    {
+        var composite = new CompositePiiDetector([new FixedDetector(
+            new PiiEntity(PiiEntityType.Email, "a@b.co", 0, 6, PiiDetectionSource.Regex),
+            new PiiEntity(PiiEntityType.Phone, "9876543210", 6, 10, PiiDetectionSource.Regex))]);
+
+        var entities = composite.Detect("abcdef9876543210padding");
+
+        entities.Should().HaveCount(2);
+    }
+
+    [Test]
+    public void Detect_NestedSpan_KeepsLongerSpan()
+    {
+        var composite = new CompositePiiDetector([new FixedDetector(
+            new PiiEntity(PiiEntityType.Aadhaar, "123456789012", 0, 12, PiiDetectionSource.Regex),
+            new PiiEntity(PiiEntityType.Phone, "4567890123", 2, 10, PiiDetectionSource.Regex))]);
+
+        var entities = composite.Detect("123456789012");
+
+        entities.Should().ContainSingle(entity => entity.Type == PiiEntityType.Aadhaar);
+    }
+
+    [Test]
+    public void Detect_OverlappingSameStart_LongerSpanWins()
+    {
+        var composite = new CompositePiiDetector([new FixedDetector(
+            new PiiEntity(PiiEntityType.LoanNumber, "LN-456789", 0, 9, PiiDetectionSource.Domain),
+            new PiiEntity(PiiEntityType.Phone, "456789", 3, 6, PiiDetectionSource.Regex))]);
+
+        var entities = composite.Detect("LN-456789");
+
+        entities.Should().ContainSingle(entity => entity.Type == PiiEntityType.LoanNumber);
+    }
+
+    [Test]
+    public void Detect_OverlappingDifferentPriority_DomainBeatsRegex()
+    {
+        var composite = new CompositePiiDetector([new FixedDetector(
+            new PiiEntity(PiiEntityType.LoanNumber, "LN-456789", 0, 9, PiiDetectionSource.Domain),
+            new PiiEntity(PiiEntityType.Phone, "456789", 0, 6, PiiDetectionSource.Regex))]);
+
+        var entities = composite.Detect("LN-456789");
+
+        entities.Should().ContainSingle(entity =>
+            entity.Type == PiiEntityType.LoanNumber &&
+            entity.Source == PiiDetectionSource.Domain);
+    }
+
+    [Test]
+    public void Detect_OverlappingPriority_RegexBeatsNer()
+    {
+        var composite = new CompositePiiDetector([new FixedDetector(
+            new PiiEntity(PiiEntityType.Email, "a@b.co", 0, 6, PiiDetectionSource.Regex),
+            new PiiEntity(PiiEntityType.Person, "a@b", 0, 3, PiiDetectionSource.Ner))]);
+
+        var entities = composite.Detect("a@b.co");
+
+        entities.Should().ContainSingle(entity => entity.Source == PiiDetectionSource.Regex);
+    }
+
+    [Test]
+    public void Detect_OverlappingPriority_DomainBeatsNer()
+    {
+        var composite = new CompositePiiDetector([new FixedDetector(
+            new PiiEntity(PiiEntityType.LoanNumber, "LN-456789", 0, 9, PiiDetectionSource.Domain),
+            new PiiEntity(PiiEntityType.Person, "LN-456", 0, 6, PiiDetectionSource.Ner))]);
+
+        var entities = composite.Detect("LN-456789");
+
+        entities.Should().ContainSingle(entity => entity.Source == PiiDetectionSource.Domain);
+    }
+
+    [Test]
+    public void Detect_DuplicateOverlappingSpan_KeepsFirstAccepted()
+    {
+        var composite = new CompositePiiDetector([new FixedDetector(
+            new PiiEntity(PiiEntityType.Phone, "9876543210", 0, 10, PiiDetectionSource.Regex),
+            new PiiEntity(PiiEntityType.Phone, "9876543210", 0, 10, PiiDetectionSource.Regex))]);
+
+        var entities = composite.Detect("9876543210");
+
+        entities.Should().HaveCount(1);
+    }
+
+    [Test]
+    public void Detect_AadhaarWithEmbeddedPhone_PrefersAadhaarSpan()
+    {
+        var detector = new CompositePiiDetector(
+        [
+            new DomainRulePiiDetector(),
+            new RegexPiiDetector()
+        ]);
+
+        var entities = detector.Detect("Aadhaar 987654321012 phone 9876543210.");
+
+        entities.Should().Contain(entity => entity.Type == PiiEntityType.Aadhaar && entity.Value == "987654321012");
+        entities.Should().Contain(entity => entity.Type == PiiEntityType.Phone && entity.Value == "9876543210");
+        entities.Count(entity => entity.Type == PiiEntityType.Phone).Should().Be(1);
+    }
+
+    [TestCase(null)]
+    [TestCase("")]
+    [TestCase("   ")]
+    public void Detect_InvalidInput_ThrowsArgumentException(string? text)
+    {
+        var composite = new CompositePiiDetector([new RegexPiiDetector()]);
+        var action = () => composite.Detect(text!);
+        action.Should().Throw<ArgumentException>();
+    }
+
+    private sealed class FixedDetector(params PiiEntity[] entities) : IPiiDetector
+    {
+        public IReadOnlyList<PiiEntity> Detect(string text) => entities;
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/Detection/DomainRulePiiDetectorTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Detection/DomainRulePiiDetectorTests.cs
@@ -0,0 +1,70 @@
+using FluentAssertions;
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.Detection;
+
+[TestFixture]
+public sealed class DomainRulePiiDetectorTests
+{
+    private readonly DomainRulePiiDetector _detector = new();
+
+    [Test]
+    public void Detect_PositiveLoanNumber_ReturnsValueOnly()
+    {
+        var entities = _detector.Detect("Loan LN-456789 active.");
+
+        entities.Should().ContainSingle(entity =>
+            entity.Type == PiiEntityType.LoanNumber &&
+            entity.Value == "LN-456789" &&
+            entity.Source == PiiDetectionSource.Domain);
+    }
+
+    [Test]
+    public void Detect_PositiveCustomerId_ReturnsEntity()
+    {
+        var entities = _detector.Detect("CustomerId CID-1234 found.");
+
+        entities.Should().ContainSingle(entity =>
+            entity.Type == PiiEntityType.CustomerId &&
+            entity.Value == "CID-1234");
+    }
+
+    [Test]
+    public void Detect_PositiveAccountNumber_ReturnsEntity()
+    {
+        var entities = _detector.Detect("Account ACC-123456 open.");
+
+        entities.Should().ContainSingle(entity =>
+            entity.Type == PiiEntityType.AccountNumber &&
+            entity.Value == "ACC-123456");
+    }
+
+    [TestCase("LN-12345")]
+    [TestCase("XLN-456789")]
+    [TestCase("CID-123")]
+    [TestCase("ACC-12345")]
+    public void Detect_InvalidDomainIds_ReturnsEmpty(string text)
+    {
+        _detector.Detect(text).Should().BeEmpty();
+    }
+
+    [Test]
+    public void Detect_LoanNumberLabel_PreservesLabelInSurroundingText()
+    {
+        const string text = "LoanNumber LN-456789 end.";
+        var entity = _detector.Detect(text).Single();
+
+        entity.Value.Should().Be("LN-456789");
+        entity.StartIndex.Should().Be("LoanNumber ".Length);
+    }
+
+    [TestCase(null)]
+    [TestCase("")]
+    [TestCase("   ")]
+    public void Detect_InvalidInput_ThrowsArgumentException(string? text)
+    {
+        var action = () => _detector.Detect(text!);
+        action.Should().Throw<ArgumentException>();
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/Detection/OnnxNerPiiDetectorTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Detection/OnnxNerPiiDetectorTests.cs
@@ -0,0 +1,54 @@
+using FluentAssertions;
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Models;
+using PiiRedaction.Core.Tests.TestSupport;
+
+namespace PiiRedaction.Core.Tests.Detection;
+
+[TestFixture]
+public sealed class OnnxNerPiiDetectorTests
+{
+    [Test]
+    public void Detect_ModelAvailable_ReturnsRunnerEntities()
+    {
+        var runner = new FakeOnnxNerModelRunner
+        {
+            IsModelAvailable = true,
+            EntitiesToReturn =
+            [
+                new PiiEntity(PiiEntityType.Person, "Onnx Person", 0, 11, PiiDetectionSource.Ner)
+            ]
+        };
+
+        var detector = new OnnxNerPiiDetector(runner);
+        var entities = detector.Detect("Any text");
+
+        entities.Should().ContainSingle(entity => entity.Value == "Onnx Person");
+        runner.LastPredictedText.Should().Be("Any text");
+    }
+
+    [Test]
+    public void Detect_ModelUnavailable_ReturnsEmpty()
+    {
+        var detector = CreateDetector(modelAvailable: false);
+        var entities = detector.Detect("Customer Ravi Kumar with email test@x.com.");
+
+        entities.Should().BeEmpty();
+    }
+
+    [TestCase(null)]
+    [TestCase("")]
+    [TestCase("   ")]
+    public void Detect_InvalidInput_ThrowsArgumentException(string? text)
+    {
+        var detector = CreateDetector(modelAvailable: false);
+        var action = () => detector.Detect(text!);
+        action.Should().Throw<ArgumentException>();
+    }
+
+    private static OnnxNerPiiDetector CreateDetector(bool modelAvailable)
+    {
+        var runner = new FakeOnnxNerModelRunner { IsModelAvailable = modelAvailable };
+        return new OnnxNerPiiDetector(runner);
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/Detection/RegexPiiDetectorTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Detection/RegexPiiDetectorTests.cs
@@ -0,0 +1,92 @@
+using FluentAssertions;
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.Detection;
+
+[TestFixture]
+public sealed class RegexPiiDetectorTests
+{
+    private readonly RegexPiiDetector _detector = new();
+
+    [Test]
+    public void Detect_PositiveEmail_ReturnsEntity()
+    {
+        const string text = "Contact ravi.kumar@gmail.com now.";
+        var entities = _detector.Detect(text);
+
+        entities.Should().ContainSingle(entity =>
+            entity.Type == PiiEntityType.Email &&
+            entity.Value == "ravi.kumar@gmail.com" &&
+            entity.Source == PiiDetectionSource.Regex);
+    }
+
+    [Test]
+    public void Detect_PositivePhone_ReturnsTenDigitEntity()
+    {
+        var entities = _detector.Detect("Call 9876543210 today.");
+
+        entities.Should().ContainSingle(entity =>
+            entity.Type == PiiEntityType.Phone &&
+            entity.Value == "9876543210" &&
+            entity.Source == PiiDetectionSource.Regex);
+    }
+
+    [TestCase("Aadhaar 1234 5678 9012 linked.", "1234 5678 9012")]
+    [TestCase("Aadhaar 123456789012 linked.", "123456789012")]
+    public void Detect_PositiveAadhaar_ReturnsEntity(string text, string expectedValue)
+    {
+        var entities = _detector.Detect(text);
+
+        entities.Should().ContainSingle(entity =>
+            entity.Type == PiiEntityType.Aadhaar &&
+            entity.Value == expectedValue);
+    }
+
+    [Test]
+    public void Detect_PositivePan_ReturnsUppercaseEntity()
+    {
+        var entities = _detector.Detect("PAN ABCDE1234F verified.");
+
+        entities.Should().ContainSingle(entity => entity.Type == PiiEntityType.Pan);
+        entities[0].Value.Should().MatchRegex("^[A-Z]{5}\\d{4}[A-Z]$");
+    }
+
+    [Test]
+    public void Detect_PositiveCreditCard_ReturnsEntity()
+    {
+        var entities = _detector.Detect("Card 4111-1111-1111-1111 used.");
+
+        entities.Should().ContainSingle(entity => entity.Type == PiiEntityType.CreditCard);
+    }
+
+    [TestCase("not-an-email")]
+    [TestCase("@missing.com")]
+    [TestCase("pan abcde1234f")]
+    [TestCase("Number 987654321")]
+    public void Detect_NegativePatterns_ReturnsNoMatch(string text)
+    {
+        _detector.Detect(text).Should().BeEmpty();
+    }
+
+    [Test]
+    public void Detect_EmailSpan_HasCorrectIndices()
+    {
+        const string text = "Email ravi@test.com end.";
+        var entities = _detector.Detect(text);
+
+        var email = entities.Single(entity => entity.Type == PiiEntityType.Email);
+        email.StartIndex.Should().Be(6);
+        email.Length.Should().Be("ravi@test.com".Length);
+        text[email.StartIndex..email.EndIndex].Should().Be("ravi@test.com");
+    }
+
+    [TestCase(null)]
+    [TestCase("")]
+    [TestCase("   ")]
+    public void Detect_InvalidInput_ThrowsArgumentException(string? text)
+    {
+        var action = () => _detector.Detect(text!);
+        action.Should().Throw<ArgumentException>();
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/Integration/GoldenPromptTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Integration/GoldenPromptTests.cs
@@ -0,0 +1,25 @@
+using FluentAssertions;
+using PiiRedaction.Core.Models;
+using PiiRedaction.Core.Tests.TestSupport;
+
+namespace PiiRedaction.Core.Tests.Integration;
+
+[TestFixture]
+public sealed class GoldenPromptTests
+{
+    [TestCaseSource(typeof(PromptScenarioCatalog), nameof(PromptScenarioCatalog.AllScenarios))]
+    public void Sanitize_PromptScenario_ProducesExpectedOutput(PromptScenario scenario)
+    {
+        var sanitizer = ProductionPipelineFactory.CreateForScenario(scenario);
+        var result = sanitizer.Sanitize(new SanitizationRequest(scenario.Prompt));
+
+        result.SanitizedPrompt.Should().Be(scenario.ExpectedSanitized, because: scenario.Name);
+        result.DetectedEntities.Select(entity => entity.Type)
+            .Should().BeEquivalentTo(scenario.ExpectedTypes, because: scenario.Name);
+
+        foreach (var forbidden in scenario.MustNotContainInSanitized)
+        {
+            result.SanitizedPrompt.Should().NotContain(forbidden, because: scenario.Name);
+        }
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/Integration/LlmBoundaryTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Integration/LlmBoundaryTests.cs
@@ -0,0 +1,52 @@
+using FluentAssertions;
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Models;
+using PiiRedaction.Core.Tests.TestSupport;
+using PiiRedaction.Infrastructure.Llm;
+
+namespace PiiRedaction.Core.Tests.Integration;
+
+[TestFixture]
+public sealed class LlmBoundaryTests
+{
+    [Test]
+    public async Task SendPromptAsync_DoesNotTransmitOriginalPii()
+    {
+        var capturingClient = new CapturingChatClient();
+        ILlmPromptService service = new MockLlmPromptService(capturingClient);
+
+        const string sanitized = "Customer <PERSON_1> with email <EMAIL_1>.";
+        await service.SendPromptAsync(sanitized);
+
+        capturingClient.LastUserMessage.Should().Be(sanitized);
+        capturingClient.LastUserMessage.Should().NotContain("ravi.kumar@gmail.com");
+        capturingClient.LastUserMessage.Should().NotContain("Ravi Kumar");
+    }
+
+    [Test]
+    public async Task SendPromptAsync_AfterSanitization_OnlyPlaceholdersReachLlm()
+    {
+        var capturingClient = new CapturingChatClient();
+        var sanitizer = ProductionPipelineFactory.Create().Sanitizer;
+        ILlmPromptService service = new MockLlmPromptService(capturingClient);
+
+        const string prompt = "Email ravi.kumar@gmail.com please.";
+        var result = sanitizer.Sanitize(new SanitizationRequest(prompt));
+
+        result.SanitizedPrompt.Should().Contain("<EMAIL_1>");
+        await service.SendPromptAsync(result.SanitizedPrompt);
+
+        capturingClient.LastUserMessage.Should().Contain("<EMAIL_1>");
+        capturingClient.LastUserMessage.Should().NotContain("ravi.kumar@gmail.com");
+    }
+
+    [TestCase(null)]
+    [TestCase("")]
+    [TestCase("   ")]
+    public void SendPromptAsync_InvalidSanitizedPrompt_ThrowsArgumentException(string? prompt)
+    {
+        ILlmPromptService service = new MockLlmPromptService(new MockChatClient());
+        var action = async () => await service.SendPromptAsync(prompt!);
+        action.Should().ThrowAsync<ArgumentException>();
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/Integration/RealNerPipelineTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Integration/RealNerPipelineTests.cs
@@ -0,0 +1,127 @@
+using FluentAssertions;
+
+using PiiRedaction.Core.Abstractions;
+
+using PiiRedaction.Core.Models;
+
+using PiiRedaction.Core.Tests.TestSupport;
+
+using PiiRedaction.Tests.Shared;
+
+
+
+namespace PiiRedaction.Core.Tests.Integration;
+
+
+
+/// <summary>
+
+/// End-to-end pipeline proof using the real ONNX NER model (no fakes).
+
+/// </summary>
+
+[TestFixture]
+
+[Category("RealModel")]
+
+public sealed class RealNerPipelineTests : RealNerModelFixture
+
+{
+
+    private IPromptSanitizer _sanitizer = null!;
+
+
+
+    [OneTimeSetUp]
+
+    public void OneTimeSetUpPipeline()
+
+    {
+
+        _sanitizer = ProductionPipelineFactory.CreateWithRealModel(Runner);
+
+    }
+
+
+
+    [Test]
+
+    public void Sanitize_FullFinancialWithCustomer_RedactsAllPiiTypes()
+
+    {
+
+        const string prompt =
+
+            "Customer Ravi Kumar with email ravi.kumar@gmail.com and phone 9876543210 has LoanNumber LN-456789 and PAN ABCDE1234F. Please summarize this customer issue.";
+
+
+
+        var result = _sanitizer.Sanitize(new SanitizationRequest(prompt));
+
+
+
+        result.SanitizedPrompt.Should().Be(
+
+            "Customer <PERSON_1> with email <EMAIL_1> and phone <PHONE_1> has LoanNumber <LOAN_NUMBER_1> and PAN <PAN_1>. Please summarize this customer issue.");
+
+
+
+        result.SanitizedPrompt.Should().NotContainAny("Ravi Kumar", "ravi.kumar@gmail.com", "9876543210", "LN-456789", "ABCDE1234F");
+
+        result.DetectedEntities.Should().Contain(entity =>
+
+            entity.Type == PiiEntityType.Person && entity.Source == PiiDetectionSource.Ner);
+
+    }
+
+
+
+    [Test]
+
+    public void Sanitize_TwoCustomersInOnePrompt_RedactsBothPeople()
+
+    {
+
+        const string prompt = "Customer Ravi Kumar and Customer Priya Nair disputed the same charge.";
+
+
+
+        var result = _sanitizer.Sanitize(new SanitizationRequest(prompt));
+
+
+
+        result.SanitizedPrompt.Should().Contain("<PERSON_1>");
+
+        result.SanitizedPrompt.Should().Contain("<PERSON_2>");
+
+        result.SanitizedPrompt.Should().NotContainAny("Ravi Kumar", "Priya Nair");
+
+        result.DetectedEntities.Count(entity => entity.Type == PiiEntityType.Person).Should().BeGreaterThanOrEqualTo(2);
+
+    }
+
+
+
+    [Test]
+
+    public void Sanitize_NoPiiCleanTicket_PassesThroughUnchanged()
+
+    {
+
+        const string prompt = "What is the status of ticket TKT-99887 and when will the API maintenance end?";
+
+
+
+        var result = _sanitizer.Sanitize(new SanitizationRequest(prompt));
+
+
+
+        result.SanitizedPrompt.Should().Be(prompt);
+
+        result.DetectedEntities.Should().BeEmpty();
+
+    }
+
+}
+
+
--- a/tests/PiiRedaction.Core.Tests/PiiRedaction.Core.Tests.csproj
+++ b/tests/PiiRedaction.Core.Tests/PiiRedaction.Core.Tests.csproj
@@ -0,0 +1,35 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="FluentAssertions" Version="8.10.0" />
+    <PackageReference Include="Microsoft.Extensions.AI.Abstractions" Version="10.7.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.0" />
+    <PackageReference Include="NUnit" Version="4.3.2" />
+    <PackageReference Include="NUnit.Analyzers" Version="4.7.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="5.0.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="NUnit.Framework" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="..\TestSupport.Shared\RealNerModelPaths.cs" Link="TestSupport.Shared\RealNerModelPaths.cs" />
+    <Compile Include="..\TestSupport.Shared\RealNerModelFixture.cs" Link="TestSupport.Shared\RealNerModelFixture.cs" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\PiiRedaction.Core\PiiRedaction.Core.csproj" />
+    <ProjectReference Include="..\..\src\PiiRedaction.Infrastructure\PiiRedaction.Infrastructure.csproj" />
+  </ItemGroup>
+
+</Project>
--- a/tests/PiiRedaction.Core.Tests/Redaction/PlaceholderPiiRedactorTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Redaction/PlaceholderPiiRedactorTests.cs
@@ -0,0 +1,134 @@
+using FluentAssertions;
+using PiiRedaction.Core.Models;
+using PiiRedaction.Core.Redaction;
+
+namespace PiiRedaction.Core.Tests.Redaction;
+
+[TestFixture]
+public sealed class PlaceholderPiiRedactorTests
+{
+    private readonly PlaceholderPiiRedactor _redactor = new();
+
+    [Test]
+    public void Redact_NoEntities_ReturnsOriginalTextAndEmptyMap()
+    {
+        const string text = "No PII here.";
+        var result = _redactor.Redact(text, []);
+
+        result.SanitizedText.Should().Be(text);
+        result.PlaceholderMap.Should().BeEmpty();
+    }
+
+    [Test]
+    public void Redact_SingleEntity_ReplacesWithTypedPlaceholder()
+    {
+        const string text = "Email ravi@test.com end.";
+        var entities = new[] { Entity(PiiEntityType.Email, "ravi@test.com", 6) };
+
+        var result = _redactor.Redact(text, entities);
+
+        result.SanitizedText.Should().Be("Email <EMAIL_1> end.");
+        result.PlaceholderMap["<EMAIL_1>"].Should().Be("ravi@test.com");
+    }
+
+    [Test]
+    public void Redact_DuplicateSameTypeAndValue_ReusesPlaceholder()
+    {
+        const string text = "a@b.co and a@b.co";
+        var entities = new[]
+        {
+            Entity(PiiEntityType.Email, "a@b.co", 0),
+            Entity(PiiEntityType.Email, "a@b.co", 11)
+        };
+
+        var result = _redactor.Redact(text, entities);
+
+        result.SanitizedText.Should().Be("<EMAIL_1> and <EMAIL_1>");
+        result.PlaceholderMap.Should().HaveCount(1);
+    }
+
+    [Test]
+    public void Redact_MultipleTypes_AssignsIndependentCounters()
+    {
+        const string text = "9876543210 ravi@test.com";
+        var entities = new[]
+        {
+            Entity(PiiEntityType.Phone, "9876543210", 0),
+            Entity(PiiEntityType.Email, "ravi@test.com", 11)
+        };
+
+        var result = _redactor.Redact(text, entities);
+
+        result.SanitizedText.Should().Be("<PHONE_1> <EMAIL_1>");
+    }
+
+    [Test]
+    public void Redact_MultipleSameTypeDifferentValues_IncrementsCounter()
+    {
+        const string text = "Customer Ravi Kumar and Customer Priya Nair";
+        var entities = new[]
+        {
+            Entity(PiiEntityType.Person, "Ravi Kumar", 9),
+            Entity(PiiEntityType.Person, "Priya Nair", 33)
+        };
+
+        var result = _redactor.Redact(text, entities);
+
+        result.SanitizedText.Should().Be("Customer <PERSON_2> and Customer <PERSON_1>");
+        result.PlaceholderMap.Should().HaveCount(2);
+    }
+
+    [Test]
+    public void Redact_EntityTypes_UseTypedPlaceholderPrefixes()
+    {
+        (PiiEntityType Type, string Prefix)[] cases =
+        [
+            (PiiEntityType.Person, "PERSON"),
+            (PiiEntityType.Email, "EMAIL"),
+            (PiiEntityType.LoanNumber, "LOAN_NUMBER")
+        ];
+
+        foreach (var (type, prefix) in cases)
+        {
+            const string value = "VALUE";
+            var text = $"start {value} end";
+            var result = _redactor.Redact(text, [Entity(type, value, 6)]);
+            result.SanitizedText.Should().Contain($"<{prefix}_1>", because: type.ToString());
+        }
+    }
+
+    [Test]
+    public void Redact_RightToLeftReplacement_PreservesCorrectOutput()
+    {
+        const string text = "AA BB CC";
+        var entities = new[]
+        {
+            Entity(PiiEntityType.Phone, "AA", 0),
+            Entity(PiiEntityType.Email, "BB", 3),
+            Entity(PiiEntityType.Pan, "CC", 6)
+        };
+
+        var result = _redactor.Redact(text, entities);
+
+        result.SanitizedText.Should().Be("<PHONE_1> <EMAIL_1> <PAN_1>");
+    }
+
+    [TestCase(null)]
+    [TestCase("")]
+    [TestCase("   ")]
+    public void Redact_InvalidText_ThrowsArgumentException(string? text)
+    {
+        var action = () => _redactor.Redact(text!, []);
+        action.Should().Throw<ArgumentException>();
+    }
+
+    [Test]
+    public void Redact_NullEntities_ThrowsArgumentNullException()
+    {
+        var action = () => _redactor.Redact("text", null!);
+        action.Should().Throw<ArgumentNullException>();
+    }
+
+    private static PiiEntity Entity(PiiEntityType type, string value, int start) =>
+        new(type, value, start, value.Length, PiiDetectionSource.Regex);
+}
--- a/tests/PiiRedaction.Core.Tests/Sanitization/PromptSanitizerTests.cs
+++ b/tests/PiiRedaction.Core.Tests/Sanitization/PromptSanitizerTests.cs
@@ -0,0 +1,26 @@
+using FluentAssertions;
+using PiiRedaction.Core.Sanitization;
+using PiiRedaction.Core.Tests.TestSupport;
+
+namespace PiiRedaction.Core.Tests.Sanitization;
+
+[TestFixture]
+public sealed class PromptSanitizerTests
+{
+    [Test]
+    public void Sanitize_NullRequest_ThrowsArgumentNullException()
+    {
+        var sanitizer = ProductionPipelineFactory.Create().Sanitizer;
+        var action = () => sanitizer.Sanitize(null!);
+        action.Should().Throw<ArgumentNullException>();
+    }
+
+    [TestCase("")]
+    [TestCase("   ")]
+    public void Sanitize_WhitespacePrompt_ThrowsArgumentException(string prompt)
+    {
+        var sanitizer = ProductionPipelineFactory.Create().Sanitizer;
+        var action = () => sanitizer.Sanitize(new(prompt));
+        action.Should().Throw<ArgumentException>();
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/TestSupport/CapturingChatClient.cs
+++ b/tests/PiiRedaction.Core.Tests/TestSupport/CapturingChatClient.cs
@@ -0,0 +1,38 @@
+using System.Runtime.CompilerServices;
+using Microsoft.Extensions.AI;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public sealed class CapturingChatClient : IChatClient
+{
+    public string? LastUserMessage { get; private set; }
+
+    public Task<ChatResponse> GetResponseAsync(
+        IEnumerable<ChatMessage> messages,
+        ChatOptions? options = null,
+        CancellationToken cancellationToken = default)
+    {
+        LastUserMessage = messages.LastOrDefault(message => message.Role == ChatRole.User)?.Text;
+
+        var response = new ChatResponse(new ChatMessage(
+            ChatRole.Assistant,
+            $"Captured {LastUserMessage?.Length ?? 0} chars."));
+
+        return Task.FromResult(response);
+    }
+
+    public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
+        IEnumerable<ChatMessage> messages,
+        ChatOptions? options = null,
+        [EnumeratorCancellation] CancellationToken cancellationToken = default)
+    {
+        var response = await GetResponseAsync(messages, options, cancellationToken).ConfigureAwait(false);
+        yield return new ChatResponseUpdate(ChatRole.Assistant, response.Messages.Last().Text);
+    }
+
+    public object? GetService(Type serviceType, object? serviceKey = null) => null;
+
+    public void Dispose()
+    {
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/TestSupport/FakeOnnxNerModelRunner.cs
+++ b/tests/PiiRedaction.Core.Tests/TestSupport/FakeOnnxNerModelRunner.cs
@@ -0,0 +1,19 @@
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public sealed class FakeOnnxNerModelRunner : IOnnxNerModelRunner
+{
+    public bool IsModelAvailable { get; set; }
+
+    public IReadOnlyList<PiiEntity> EntitiesToReturn { get; set; } = [];
+
+    public string? LastPredictedText { get; private set; }
+
+    public IReadOnlyList<PiiEntity> PredictEntities(string text)
+    {
+        LastPredictedText = text;
+        return EntitiesToReturn;
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/TestSupport/NerEntityBuilder.cs
+++ b/tests/PiiRedaction.Core.Tests/TestSupport/NerEntityBuilder.cs
@@ -0,0 +1,33 @@
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public static class NerEntityBuilder
+{
+    public static IReadOnlyList<PiiEntity> BuildFromScenario(PromptScenario scenario)
+    {
+        var entities = new List<PiiEntity>();
+
+        foreach (var (type, value) in scenario.ExpectedTypes.Zip(scenario.MustNotContainInSanitized))
+        {
+            if (type != PiiEntityType.Person)
+            {
+                continue;
+            }
+
+            var searchStart = 0;
+            while ((searchStart = scenario.Prompt.IndexOf(value, searchStart, StringComparison.Ordinal)) >= 0)
+            {
+                entities.Add(new PiiEntity(
+                    PiiEntityType.Person,
+                    value,
+                    searchStart,
+                    value.Length,
+                    PiiDetectionSource.Ner));
+                searchStart += value.Length;
+            }
+        }
+
+        return entities;
+    }
+}
--- a/tests/PiiRedaction.Core.Tests/TestSupport/ProductionPipelineFactory.cs
+++ b/tests/PiiRedaction.Core.Tests/TestSupport/ProductionPipelineFactory.cs
@@ -0,0 +1,49 @@
+using PiiRedaction.Core.Abstractions;
+using PiiRedaction.Core.Detection;
+using PiiRedaction.Core.Redaction;
+using PiiRedaction.Core.Sanitization;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public static class ProductionPipelineFactory
+{
+    public static IPromptSanitizer CreateWithRealModel(IOnnxNerModelRunner runner) =>
+        new PromptSanitizer(CreateCompositeDetector(runner), new PlaceholderPiiRedactor());
+
+    public static IPiiDetector CreateCompositeDetector(IOnnxNerModelRunner runner) =>
+        new CompositePiiDetector(
+        [
+            new DomainRulePiiDetector(),
+            new RegexPiiDetector(),
+            new OnnxNerPiiDetector(runner)
+        ]);
+
+    public static (IPromptSanitizer Sanitizer, FakeOnnxNerModelRunner NerRunner) Create(
+        bool modelAvailable = false,
+        IReadOnlyList<Models.PiiEntity>? nerEntities = null)
+    {
+        var nerRunner = CreateNerRunner(modelAvailable, nerEntities);
+        var sanitizer = new PromptSanitizer(CreateCompositeDetector(nerRunner), new PlaceholderPiiRedactor());
+        return (sanitizer, nerRunner);
+    }
+
+    public static IPromptSanitizer CreateForScenario(PromptScenario scenario)
+    {
+        var nerEntities = NerEntityBuilder.BuildFromScenario(scenario);
+        return Create(modelAvailable: nerEntities.Count > 0, nerEntities: nerEntities).Sanitizer;
+    }
+
+    public static IPiiDetector CreateCompositeDetector(
+        bool modelAvailable = false,
+        IReadOnlyList<Models.PiiEntity>? nerEntities = null) =>
+        CreateCompositeDetector(CreateNerRunner(modelAvailable, nerEntities));
+
+    private static FakeOnnxNerModelRunner CreateNerRunner(
+        bool modelAvailable,
+        IReadOnlyList<Models.PiiEntity>? nerEntities) =>
+        new()
+        {
+            IsModelAvailable = modelAvailable,
+            EntitiesToReturn = nerEntities ?? []
+        };
+}
--- a/tests/PiiRedaction.Core.Tests/TestSupport/PromptScenario.cs
+++ b/tests/PiiRedaction.Core.Tests/TestSupport/PromptScenario.cs
@@ -0,0 +1,10 @@
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+public sealed record PromptScenario(
+    string Name,
+    string Prompt,
+    string ExpectedSanitized,
+    PiiEntityType[] ExpectedTypes,
+    string[] MustNotContainInSanitized);
--- a/tests/PiiRedaction.Core.Tests/TestSupport/PromptScenarioCatalog.cs
+++ b/tests/PiiRedaction.Core.Tests/TestSupport/PromptScenarioCatalog.cs
@@ -0,0 +1,68 @@
+using NUnit.Framework;
+using PiiRedaction.Core.Models;
+
+namespace PiiRedaction.Core.Tests.TestSupport;
+
+/// <summary>
+/// Focused end-to-end scenarios that exercise the full sanitizer pipeline beyond what unit tests cover in isolation.
+/// </summary>
+public static class PromptScenarioCatalog
+{
+    public static IEnumerable<TestCaseData> AllScenarios()
+    {
+        foreach (var scenario in BuildScenarios())
+        {
+            yield return new TestCaseData(scenario).SetName(scenario.Name);
+        }
+    }
+
+    private static IEnumerable<PromptScenario> BuildScenarios()
+    {
+        yield return CanonicalScenario();
+        yield return MultiRegexScenario();
+        yield return DuplicatePeopleScenario();
+        yield return OverlapScenario();
+        yield return NoPiiScenario();
+    }
+
+    private static PromptScenario CanonicalScenario()
+    {
+        const string prompt =
+            "Customer Ravi Kumar with email ravi.kumar@gmail.com and phone 9876543210 has LoanNumber LN-456789 and PAN ABCDE1234F. Please summarize this customer issue.";
+
+        return new PromptScenario(
+            "Canonical_DemoPrompt",
+            prompt,
+            "Customer <PERSON_1> with email <EMAIL_1> and phone <PHONE_1> has LoanNumber <LOAN_NUMBER_1> and PAN <PAN_1>. Please summarize this customer issue.",
+            [PiiEntityType.Person, PiiEntityType.Email, PiiEntityType.Phone, PiiEntityType.LoanNumber, PiiEntityType.Pan],
+            ["Ravi Kumar", "ravi.kumar@gmail.com", "9876543210", "LN-456789", "ABCDE1234F"]);
+    }
+
+    private static PromptScenario MultiRegexScenario() => new(
+        "Full_AllRegexTypes",
+        "Email a@b.co phone 9001234567 PAN ABCDE1234F aadhaar 1234 5678 9012 card 4111-1111-1111-1111.",
+        "Email <EMAIL_1> phone <PHONE_1> PAN <PAN_1> aadhaar <AADHAAR_1> card <CREDIT_CARD_1>.",
+        [PiiEntityType.Email, PiiEntityType.Phone, PiiEntityType.Pan, PiiEntityType.Aadhaar, PiiEntityType.CreditCard],
+        ["a@b.co", "9001234567", "ABCDE1234F", "1234 5678 9012", "4111-1111-1111-1111"]);
+
+    private static PromptScenario DuplicatePeopleScenario() => new(
+        "Duplicate_TwoPeople",
+        "Customer Ravi Kumar and Customer Priya Nair.",
+        "Customer <PERSON_2> and Customer <PERSON_1>.",
+        [PiiEntityType.Person, PiiEntityType.Person],
+        ["Ravi Kumar", "Priya Nair"]);
+
+    private static PromptScenario OverlapScenario() => new(
+        "Overlap_AadhaarAndPhone",
+        "Aadhaar 987654321012 and phone 9876543210.",
+        "Aadhaar <AADHAAR_1> and phone <PHONE_1>.",
+        [PiiEntityType.Aadhaar, PiiEntityType.Phone],
+        ["9876543210"]);
+
+    private static PromptScenario NoPiiScenario() => new(
+        "Negative_NoPii",
+        "Please summarize the general policy on refunds.",
+        "Please summarize the general policy on refunds.",
+        [],
+        []);
+}
--- a/tests/PiiRedaction.Infrastructure.Tests/Llm/MockLlmPromptServiceTests.cs
+++ b/tests/PiiRedaction.Infrastructure.Tests/Llm/MockLlmPromptServiceTests.cs
@@ -0,0 +1,62 @@
+using System.Runtime.CompilerServices;
+using FluentAssertions;
+using Microsoft.Extensions.AI;
+using PiiRedaction.Infrastructure.Llm;
+
+namespace PiiRedaction.Infrastructure.Tests.Llm;
+
+[TestFixture]
+public sealed class MockLlmPromptServiceTests
+{
+    [Test]
+    public async Task SendPromptAsync_ReturnsAssistantText()
+    {
+        var service = new MockLlmPromptService(new MockChatClient());
+
+        const string prompt = "Sanitized <EMAIL_1> content.";
+        var response = await service.SendPromptAsync(prompt);
+
+        response.Should().Contain("Mock LLM Response");
+        response.Should().Contain($"{prompt.Length} chars");
+    }
+
+    [Test]
+    public async Task SendPromptAsync_UsesUserRoleForOutboundMessage()
+    {
+        var capturingClient = new CapturingChatClient();
+        var service = new MockLlmPromptService(capturingClient);
+
+        await service.SendPromptAsync("Only placeholders <PHONE_1>.");
+
+        capturingClient.LastUserMessage.Should().Be("Only placeholders <PHONE_1>.");
+    }
+
+    private sealed class CapturingChatClient : IChatClient
+    {
+        public string? LastUserMessage { get; private set; }
+
+        public Task<ChatResponse> GetResponseAsync(
+            IEnumerable<ChatMessage> messages,
+            ChatOptions? options = null,
+            CancellationToken cancellationToken = default)
+        {
+            LastUserMessage = messages.LastOrDefault(message => message.Role == ChatRole.User)?.Text;
+            return Task.FromResult(new ChatResponse(new ChatMessage(ChatRole.Assistant, "ok")));
+        }
+
+        public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
+            IEnumerable<ChatMessage> messages,
+            ChatOptions? options = null,
+            [EnumeratorCancellation] CancellationToken cancellationToken = default)
+        {
+            await GetResponseAsync(messages, options, cancellationToken);
+            yield break;
+        }
+
+        public object? GetService(Type serviceType, object? serviceKey = null) => null;
+
+        public void Dispose()
+        {
+        }
+    }
+}
--- a/tests/PiiRedaction.Infrastructure.Tests/Onnx/OnnxNerModelRunnerTests.cs
+++ b/tests/PiiRedaction.Infrastructure.Tests/Onnx/OnnxNerModelRunnerTests.cs
@@ -0,0 +1,53 @@
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using PiiRedaction.Core.Configuration;
+using PiiRedaction.Core.Models;
+using PiiRedaction.Infrastructure.Onnx;
+
+namespace PiiRedaction.Infrastructure.Tests.Onnx;
+
+[TestFixture]
+public sealed class OnnxNerModelRunnerTests
+{
+    [Test]
+    public void Constructor_MissingModel_IsModelUnavailable()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"missing-ner-{Guid.NewGuid():N}.onnx");
+        using var runner = CreateRunner(path);
+
+        runner.IsModelAvailable.Should().BeFalse();
+    }
+
+    [Test]
+    public void PredictEntities_WhenModelUnavailable_ReturnsEmpty()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"missing-ner-{Guid.NewGuid():N}.onnx");
+        using var runner = CreateRunner(path);
+
+        runner.PredictEntities("Customer Ravi Kumar").Should().BeEmpty();
+    }
+
+    [Test]
+    public void Constructor_InvalidModelFile_IsModelUnavailable()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"invalid-ner-{Guid.NewGuid():N}.onnx");
+        File.WriteAllText(path, "not-a-valid-onnx-model");
+
+        try
+        {
+            using var runner = CreateRunner(path);
+            runner.IsModelAvailable.Should().BeFalse();
+        }
+        finally
+        {
+            File.Delete(path);
+        }
+    }
+
+    private static OnnxNerModelRunner CreateRunner(string modelPath)
+    {
+        var options = Options.Create(new PiiRedactionOptions { OnnxModelPath = modelPath });
+        return new OnnxNerModelRunner(options, NullLogger<OnnxNerModelRunner>.Instance);
+    }
+}
--- a/tests/PiiRedaction.Infrastructure.Tests/Onnx/RealNerModelRunnerTests.cs
+++ b/tests/PiiRedaction.Infrastructure.Tests/Onnx/RealNerModelRunnerTests.cs
@@ -0,0 +1,51 @@
+using FluentAssertions;
+
+using PiiRedaction.Core.Models;
+
+using PiiRedaction.Tests.Shared;
+
+
+
+namespace PiiRedaction.Infrastructure.Tests.Onnx;
+
+
+
+[TestFixture]
+
+[Category("RealModel")]
+
+public sealed class RealNerModelRunnerTests : RealNerModelFixture
+
+{
+
+    [TestCase("Customer Ravi Kumar called about billing.", "Ravi", "Ravi Kumar")]
+
+    [TestCase("Mr. John Smith called about a duplicate debit.", "John", "John Smith")]
+
+    public void PredictEntities_DetectsPersonWithCorrectSpan(string prompt, string expectedNamePart, string expectedValue)
+
+    {
+
+        var entities = Runner.PredictEntities(prompt);
+
+
+
+        entities.Should().Contain(entity =>
+
+            entity.Type == PiiEntityType.Person &&
+
+            entity.Source == PiiDetectionSource.Ner &&
+
+            entity.Value.Contains(expectedNamePart, StringComparison.Ordinal) &&
+
+            prompt.AsSpan(entity.StartIndex, entity.Length).ToString() == entity.Value);
+
+
+
+        entities.Should().Contain(entity => entity.Value == expectedValue);
+
+    }
+
+}
+
+
--- a/tests/PiiRedaction.Infrastructure.Tests/PiiRedaction.Infrastructure.Tests.csproj
+++ b/tests/PiiRedaction.Infrastructure.Tests/PiiRedaction.Infrastructure.Tests.csproj
@@ -0,0 +1,34 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="FluentAssertions" Version="8.10.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.0" />
+    <PackageReference Include="NUnit" Version="4.3.2" />
+    <PackageReference Include="NUnit.Analyzers" Version="4.7.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="5.0.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="NUnit.Framework" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="..\TestSupport.Shared\RealNerModelPaths.cs" Link="TestSupport.Shared\RealNerModelPaths.cs" />
+    <Compile Include="..\TestSupport.Shared\RealNerModelFixture.cs" Link="TestSupport.Shared\RealNerModelFixture.cs" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\PiiRedaction.Infrastructure\PiiRedaction.Infrastructure.csproj" />
+    <ProjectReference Include="..\..\src\PiiRedaction.Core\PiiRedaction.Core.csproj" />
+  </ItemGroup>
+
+</Project>
--- a/tests/TestSupport.Shared/RealNerModelFixture.cs
+++ b/tests/TestSupport.Shared/RealNerModelFixture.cs
@@ -0,0 +1,42 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using PiiRedaction.Core.Configuration;
+using PiiRedaction.Infrastructure.Onnx;
+
+namespace PiiRedaction.Tests.Shared;
+
+/// <summary>
+/// Reuses a single <see cref="OnnxNerModelRunner"/> per fixture for performance.
+/// Skips all tests in the class when the ONNX model is missing or cannot be loaded.
+/// </summary>
+public abstract class RealNerModelFixture
+{
+    protected OnnxNerModelRunner Runner { get; private set; } = null!;
+
+    protected string ModelPath { get; private set; } = null!;
+
+    [OneTimeSetUp]
+    public void OneTimeSetUpRealModel()
+    {
+        ModelPath = RealNerModelPaths.ResolveRepoModelPath();
+        if (!File.Exists(ModelPath))
+        {
+            Assert.Ignore(RealNerModelPaths.ModelMissingMessage);
+        }
+
+        var options = Options.Create(new PiiRedactionOptions { OnnxModelPath = ModelPath });
+        Runner = new OnnxNerModelRunner(options, NullLogger<OnnxNerModelRunner>.Instance);
+
+        if (!Runner.IsModelAvailable)
+        {
+            Runner.Dispose();
+            Assert.Ignore(RealNerModelPaths.ModelMissingMessage);
+        }
+    }
+
+    [OneTimeTearDown]
+    public void OneTimeTearDownRealModel()
+    {
+        Runner?.Dispose();
+    }
+}
--- a/tests/TestSupport.Shared/RealNerModelPaths.cs
+++ b/tests/TestSupport.Shared/RealNerModelPaths.cs
@@ -0,0 +1,24 @@
+namespace PiiRedaction.Tests.Shared;
+
+public static class RealNerModelPaths
+{
+    public const string ModelMissingMessage =
+        "ONNX model not found. Run scripts/download-ner-model.ps1 from the repository root.";
+
+    public static string ResolveRepoModelPath()
+    {
+        var directory = new DirectoryInfo(AppContext.BaseDirectory);
+        while (directory is not null)
+        {
+            var candidate = Path.Combine(directory.FullName, "models", "ner-model.onnx");
+            if (File.Exists(candidate))
+            {
+                return candidate;
+            }
+
+            directory = directory.Parent;
+        }
+
+        return Path.Combine(Environment.CurrentDirectory, "models", "ner-model.onnx");
+    }
+}