# Implementation Plan: OAuth2 MCP Authorization Library

**Branch**: `001-oauth2-mcp-authorization-library` | **Date**: 2025-01-27 | **Spec**: `/specs/001-oauth2-mcp-authorization-library/spec.md`
**Input**: Feature specification from `/specs/001-oauth2-mcp-authorization-library/spec.md`

**Note**: This template is filled in by the `/speckit.plan` command. See `.specify/templates/commands/plan.md` for the execution workflow.

## Summary

A minimal, secure OAuth2 authorization library written in Python for MCP (Model Context Protocol) servers. The library provides simple middleware integration for FastAPI applications to authenticate requests using standard OAuth2 JWT tokens. The implementation focuses on essential OAuth 2.1 security requirements with minimal configuration and secure-by-default behavior.

## Technical Context

**Language/Version**: Python 3.13.5+  
**Primary Dependencies**: FastAPI, Pydantic v2, PyJWT, httpx, uv package manager  
**Storage**: In-memory caching for JWKS (no persistent storage)  
**Testing**: pytest with >95% coverage requirement  
**Target Platform**: Cross-platform (Windows, macOS, Linux), Docker containers  
**Project Type**: Python library package  
**Performance Goals**: <100ms token validation, <500ms JWKS fetch, <50ms middleware overhead  
**Constraints**: <10MB memory usage, <1MB storage, HTTPS only, fail-fast on JWKS errors  
**Scale/Scope**: 100+ concurrent requests, generic OAuth2 provider support, FastAPI integration

## Constitution Check

*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*

### Minimal API Surface ✅
- Single-line middleware integration: `app.add_middleware(OAuth2Middleware, config=config)`
- Transparent token validation with automatic user injection
- Minimal configuration with sensible defaults
- Auto-discovery of JWKS endpoints

### Enterprise-Grade Security ✅
- OAuth 2.1 compliance with audience binding and issuer validation
- HTTPS-only communications with proper certificate validation
- Secure JWT token validation with signature verification
- Fail-fast behavior on JWKS fetch failures

### Developer Experience First ✅
- Type-safe API with Pydantic v2 validation
- Detailed error messages for authentication failures
- >95% test coverage requirement
- Clear documentation and examples

### Performance and Scalability ✅
- Performance targets defined in spec.md Performance Specification section
- <100ms token validation, <500ms JWKS fetching, <50ms middleware overhead
- <10MB memory usage, 100+ concurrent requests supported

**GATE STATUS**: ✅ PASSED - All constitutional requirements met

## Project Structure

### Documentation (this feature)

```
specs/[###-feature]/
├── plan.md              # This file (/speckit.plan command output)
├── research.md          # Phase 0 output (/speckit.plan command)
├── data-model.md        # Phase 1 output (/speckit.plan command)
├── quickstart.md        # Phase 1 output (/speckit.plan command)
├── contracts/           # Phase 1 output (/speckit.plan command)
└── tasks.md             # Phase 2 output (/speckit.tasks command - NOT created by /speckit.plan)
```

### Source Code (repository root)

```
mcp_oauth2/
├── __init__.py                 # Main API exports
├── middleware.py               # OAuth2 middleware for FastAPI
├── token_validation.py         # JWT token validation
├── config.py                   # Configuration models
├── models.py                   # Data models
├── exceptions.py               # Custom exception classes
└── utils.py                    # Utility functions

tests/
├── unit/                       # Unit tests
├── integration/                # Integration tests
└── conftest.py                 # Test configuration

docs/
├── README.md                   # Main documentation
└── examples/                   # Usage examples

pyproject.toml                  # Python project configuration
Makefile                        # Development commands
```

**Structure Decision**: Single Python library package structure optimized for FastAPI integration. The library follows standard Python packaging conventions with clear separation between core modules (middleware, token validation, configuration, models) and supporting files (exceptions, utilities). Test structure supports both unit and integration testing with proper fixtures.

## Phase 1 Completion Status

✅ **Research Phase Complete**: All technology decisions documented in `research.md`
✅ **Data Model Complete**: Comprehensive data models defined in `data-model.md`
✅ **API Contracts Complete**: OpenAPI specification created in `contracts/oauth2-middleware-api.json`
✅ **Quickstart Guide Complete**: Step-by-step integration guide in `quickstart.md`
✅ **Agent Context Updated**: Cursor IDE context updated with project technology stack

### Generated Artifacts

- **research.md**: Technology stack decisions and rationale
- **data-model.md**: Pydantic v2 models with validation rules
- **contracts/oauth2-middleware-api.json**: OpenAPI 3.0 specification
- **quickstart.md**: Complete integration guide with examples
- **.cursor/rules/specify-rules.mdc**: Updated Cursor IDE context

## Constitution Check (Post-Design)

*Re-evaluation after Phase 1 design completion*

### Minimal API Surface ✅
- ✅ Single-line middleware integration implemented
- ✅ Transparent token validation with automatic user injection
- ✅ Minimal configuration with sensible defaults
- ✅ Auto-discovery of JWKS endpoints

### Enterprise-Grade Security ✅
- ✅ OAuth 2.1 compliance with audience binding and issuer validation
- ✅ HTTPS-only communications enforced in data models
- ✅ Secure JWT token validation with signature verification
- ✅ Fail-fast behavior on JWKS fetch failures

### Developer Experience First ✅
- ✅ Type-safe API with Pydantic v2 validation and comprehensive models
- ✅ Detailed error messages with specific error codes
- ✅ >95% test coverage requirement specified
- ✅ Clear documentation with complete quickstart guide

### Performance and Scalability ✅
- ✅ <100ms token validation performance target specified
- ✅ <500ms JWKS fetching with 1-hour caching strategy
- ✅ <10MB memory usage constraint defined
- ✅ Support for 100+ concurrent requests

**GATE STATUS**: ✅ PASSED - All constitutional requirements met and validated through design artifacts

## Complexity Tracking

*No constitutional violations detected - all requirements met within minimal scope*
