Agent Skills: ln-772-error-handler-setup

ln-772-error-handler-setup

Type: L3 Worker Category: 7XX Project Bootstrap Parent: ln-770-crosscutting-setup

Configures global error handling for .NET and Python backend applications.

Overview

| 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) |

Phase 1: Receive Context

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" }

Phase 2: Research Error Handling Patterns

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

Phase 3: Decision Points

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 |

Phase 4: Generate Configuration

.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
3. Generate custom exception classes

Registration Code:

app.UseMiddleware<GlobalExceptionMiddleware>();

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
3. Generate custom exception classes

Registration Code:

app.add_exception_handler(AppException, app_exception_handler)
app.add_exception_handler(RequestValidationError, validation_exception_handler)

Phase 5: Validate

Validation Steps:

1. Syntax check:

   • .NET: dotnet build --no-restore
   • Python: python -m py_compile exceptions/handlers.py

2. Test error handling:

   • Create test endpoint that throws exception
   • Verify error response format
   • Check that stack trace hidden in Production

Expected Error Response (ProblemDetails):

{
  "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"
}

Return to Coordinator

{
  "status": "success",
  "files_created": [
    "Middleware/GlobalExceptionMiddleware.cs",
    "Exceptions/AppException.cs",
    "Models/ErrorResponse.cs"
  ],
  "packages_added": [],
  "registration_code": "app.UseMiddleware<GlobalExceptionMiddleware>();",
  "message": "Configured global exception handling"
}

Reference Links

• ASP.NET Core Error Handling
• FastAPI Handling Errors
• RFC 7807 Problem Details

Critical Rules

• Use ProblemDetails (RFC 7807) by default — standardized error response format
• Hide stack traces in Production — environment-aware detail level is mandatory
• Use MCP ref for current patterns — do not hardcode middleware from memory
• Idempotent — if GlobalExceptionMiddleware or exception_handlers.py exists, return status: "skipped"
• Map all custom exceptions to HTTP status codes — no unhandled exception types reaching the client

Definition of Done

• Context Store received (stack, framework, environment)
• Error handling patterns researched via MCP tools
• GlobalExceptionMiddleware generated (.NET) or exception handlers generated (Python)
• Custom exception classes created (AppException, ValidationException, NotFoundException)
• Error response model created (ProblemDetails format)
• Syntax validated (dotnet build or py_compile)
• Structured JSON response returned to ln-770 coordinator

Version: 2.0.0 Last Updated: 2026-01-10