> ## Documentation Index
> Fetch the complete documentation index at: https://mcp-server-langgraph.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# 61. FastAPI as Web Framework

> Architecture Decision Record: 61. FastAPI as Web Framework

# 61. FastAPI as Web Framework

Date: 2025-11-29

## Status

Accepted

## Category

Core Architecture

## Context

The MCP Server requires a web framework to expose HTTP APIs, handle WebSocket connections for real-time communication, and integrate with the Model Context Protocol. Key requirements include:

* **Async-first architecture**: LLM calls, database operations, and external API calls are I/O-bound
* **High performance**: Low latency for real-time agent interactions
* **Type safety**: Runtime validation of request/response payloads
* **OpenAPI generation**: Automatic API documentation for clients
* **WebSocket support**: Real-time streaming for agent responses
* **Dependency injection**: Clean separation of concerns for testing

### Frameworks Considered

1. **Flask**: Synchronous by default, WSGI-based, mature ecosystem
2. **Django**: Full-featured, ORM-included, admin interface
3. **FastAPI**: Async-native, Pydantic integration, automatic OpenAPI
4. **Starlette**: Lightweight async framework (FastAPI is built on it)
5. **Quart**: Flask-compatible async framework

## Decision

Use **FastAPI** as the web framework for all HTTP and WebSocket endpoints.

### Key Reasons

1. **Native Async Support**: Built on Starlette with first-class async/await support
2. **Pydantic Integration**: Request/response validation using the same models as the rest of the codebase (ADR-0014)
3. **Automatic OpenAPI**: Self-documenting APIs with Swagger UI and ReDoc
4. **Dependency Injection**: Clean DI system for authentication, database connections, etc.
5. **WebSocket Support**: Native WebSocket handling for streaming responses
6. **Performance**: One of the fastest Python frameworks (comparable to Node.js/Go)
7. **Type Hints**: Full IDE support with autocomplete and type checking

### Implementation Pattern

```python theme={null}
from fastapi import FastAPI, Depends, HTTPException
from pydantic import BaseModel

app = FastAPI(title="MCP Server LangGraph")

class ChatRequest(BaseModel):
    message: str
    session_id: str

class ChatResponse(BaseModel):
    response: str
    session_id: str

@app.post("/api/v1/chat", response_model=ChatResponse)
async def chat(
    request: ChatRequest,
    current_user: dict = Depends(get_current_user),
    agent: Agent = Depends(get_agent),
) -> ChatResponse:
    result = await agent.process(request.message)
    return ChatResponse(response=result, session_id=request.session_id)
```

## Consequences

### Positive

* **Developer Productivity**: Type hints + autocomplete reduce errors
* **Performance**: Async I/O handles concurrent requests efficiently
* **Documentation**: OpenAPI spec generated automatically
* **Testing**: Dependency injection makes mocking straightforward
* **Ecosystem**: Large community, extensive middleware available

### Negative

* **Learning Curve**: Async patterns require understanding of asyncio
* **Debugging**: Async stack traces can be harder to follow
* **Dependency**: Tied to Starlette's release cycle

## Alternatives Considered

### Flask

* **Rejected**: Synchronous by default; async support via extensions is bolted-on
* Would require Flask-SocketIO for WebSocket, Flask-Pydantic for validation

### Django

* **Rejected**: Heavy for an API-only service; ORM not needed (using raw asyncpg)
* Django REST Framework adds complexity without benefit for our use case

### Starlette

* **Considered**: FastAPI is built on Starlette, provides same performance
* **Rejected**: FastAPI adds valuable abstractions (Pydantic, OpenAPI, DI)

## References

* Implementation: `src/mcp_server_langgraph/api/` (all API endpoints)
* Configuration: `src/mcp_server_langgraph/core/server.py`
* Related ADRs: [ADR-0014](https://github.com/vishnu2kmohan/mcp-server-langgraph/blob/main/adr/adr-0014-pydantic-type-safety.md), [ADR-0019](https://github.com/vishnu2kmohan/mcp-server-langgraph/blob/main/adr/adr-0019-async-first-architecture.md)
* External: [FastAPI Documentation](https://fastapi.tiangolo.com/)