ln-772-error-handler-setup

| Aspect | Details |

|--------|---------|

| Input | Context Store from ln-770 |

| Output | Exception handling middleware and custom exceptions |

| Stacks | .NET (ASP.NET Core Middleware), Python (FastAPI exception handlers) |

---

Accept Context Store from coordinator.

Required Context:

• STACK: .NET or Python
• FRAMEWORK: ASP.NET Core or FastAPI
• PROJECT_ROOT: Project directory path
• ENVIRONMENT: Development or Production

Idempotency Check:

• .NET: Grep for GlobalExceptionMiddleware or UseExceptionHandler
• Python: Grep for @app.exception_handler or exception_handlers.py
• If found: Return { "status": "skipped" }

---

Use MCP tools to get up-to-date documentation.

For .NET:

```

MCP ref: "ASP.NET Core global exception handling middleware"

Context7: /dotnet/aspnetcore

```

For Python:

```

MCP ref: "FastAPI exception handlers custom exceptions"

Context7: /tiangolo/fastapi

```

Key Patterns to Research:

1. Middleware pipeline positioning
2. Exception type mapping to HTTP status codes
3. ProblemDetails (RFC 7807) format
4. Development vs Production error details
5. Logging integration

---

Q1: Error Response Format

| Option | Description |

|--------|-------------|

| ProblemDetails (RFC 7807) (Recommended) | Standardized format, widely adopted |

| Custom Format | Project-specific requirements |

Q2: Error Detail Level

| Environment | Stack Trace | Inner Exceptions | Request Details |

|-------------|-------------|------------------|-----------------|

| Development | ✓ Show | ✓ Show | ✓ Show |

| Production | ✗ Hide | ✗ Hide | ✗ Hide |

Q3: Error Taxonomy

Define standard error codes:

| Code | HTTP Status | Description |

|------|-------------|-------------|

| VALIDATION_ERROR | 400 | Invalid input data |

| UNAUTHORIZED | 401 | Authentication required |

| FORBIDDEN | 403 | Insufficient permissions |

| NOT_FOUND | 404 | Resource not found |

| CONFLICT | 409 | Resource state conflict |

| INTERNAL_ERROR | 500 | Unexpected server error |

---

.NET Output Files

| File | Purpose |

|------|---------|

| Middleware/GlobalExceptionMiddleware.cs | Exception handling middleware |

| Exceptions/AppException.cs | Base exception class |

| Exceptions/ValidationException.cs | Validation errors |

| Exceptions/NotFoundException.cs | Not found errors |

| Models/ErrorResponse.cs | Error response model |

Generation Process:

1. Use MCP ref to get current ASP.NET Core exception handling patterns
2. Generate GlobalExceptionMiddleware with:

- Exception type to HTTP status mapping

- Logging of exceptions

- ProblemDetails response format

- Environment-aware detail level

1. Generate custom exception classes

Registration Code:

```csharp

app.UseMiddleware();

```

Python Output Files

| File | Purpose |

|------|---------|

| exceptions/app_exceptions.py | Custom exception classes |

| exceptions/handlers.py | FastAPI exception handlers |

| models/error_response.py | Pydantic error models |

Generation Process:

1. Use MCP ref to get current FastAPI exception handling patterns
2. Generate exception handlers with:

- HTTPException handling

- Custom AppException handling

- Validation error handling

- Request validation error handling

1. Generate custom exception classes

Registration Code:

```python

app.add_exception_handler(AppException, app_exception_handler)

app.add_exception_handler(RequestValidationError, validation_exception_handler)

```

---

Validation Steps:

1. Syntax check:

- .NET: dotnet build --no-restore

- Python: python -m py_compile exceptions/handlers.py

1. Test error handling:

- Create test endpoint that throws exception

- Verify error response format

- Check that stack trace hidden in Production

Expected Error Response (ProblemDetails):

```json

{

"type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",

"title": "Validation Error",

"status": 400,

"detail": "Invalid input data",

"instance": "/api/users",

"errors": [

{ "field": "email", "message": "Invalid email format" }

],

"traceId": "abc-123-def-456"

}

```

---

```json

{

"status": "success",

"files_created": [

"Middleware/GlobalExceptionMiddleware.cs",

"Exceptions/AppException.cs",

"Models/ErrorResponse.cs"

],

"packages_added": [],

"registration_code": "app.UseMiddleware();",

"message": "Configured global exception handling"

}

```

---

• [ASP.NET Core Error Handling](https://learn.microsoft.com/aspnet/core/web-api/handle-errors)
• [FastAPI Handling Errors](https://fastapi.tiangolo.com/tutorial/handling-errors/)
• [RFC 7807 Problem Details](https://tools.ietf.org/html/rfc7807)

---

Version: 2.0.0

Last Updated: 2026-01-10