FastAPI patterns for agentic services

Dependency injection for shared clients

from fastapi import Depends

def get_llm_client():
    return LLMClient(api_key=settings.api_key)

@app.post("/generate")
async def generate(prompt: str, client: LLMClient = Depends(get_llm_client)):
    return await client.complete(prompt)

Streaming responses

from fastapi.responses import StreamingResponse

@app.post("/stream")
async def stream(prompt: str):
    async def event_generator():
        async for chunk in llm.stream(prompt):
            yield chunk
    return StreamingResponse(event_generator(), media_type="text/event-stream")

Background tasks for fire-and-forget work

from fastapi import BackgroundTasks

@app.post("/log-usage")
async def log_usage(payload: UsagePayload, background_tasks: BackgroundTasks):
    background_tasks.add_task(write_usage_log, payload)
    return {"status": "accepted"}