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