ln-772-error-handler-setup

SKILL.md

ln-772-error-handler-setup

Type: L3 Worker
Category: 7XX Project Bootstrap

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:

Middleware pipeline positioning

Exception type mapping to HTTP status codes

ProblemDetails (RFC 7807) format

Development vs Production error details

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:

Use MCP ref to get current ASP.NET Core exception handling patterns

Generate GlobalExceptionMiddleware with:

Exception type to HTTP status mapping

Logging of exceptions

ProblemDetails response format

Environment-aware detail level

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:

Use MCP ref to get current FastAPI exception handling patterns

Generate exception handlers with:

HTTPException handling

Custom AppException handling

Validation error handling

Request validation error handling

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:

Syntax check:

.NET: dotnet build --no-restore

Python: python -m py_compile exceptions/handlers.py

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