Designing APIs for Long-Running Agent Tasks
Standard web APIs return in 80ms. An autonomous agent researching 10 websites can take 3 minutes. Holding synchronous HTTP connections open triggers 504 Gateway Timeouts. Master HTTP 202 Accepted, Server-Sent Events (SSE), signed webhooks, and distributed cancellation.
1. Core Pillars of Asynchronous Agent APIs
Select an API patternAsync API & Stream Inspector
# 1. FASTAPI ASYNC JOB TICKET PATTERN
from fastapi import FastAPI, BackgroundTasks, status
from fastapi.responses import JSONResponse
import uuid
app = FastAPI()
tasks_db = {}
@app.post("/api/v1/agent/research", status_code=status.HTTP_202_ACCEPTED)
async def submit_agent_task(prompt: str, bg_tasks: BackgroundTasks):
task_id = str(uuid.uuid4())
tasks_db[task_id] = {"status": "queued", "progress": 0}
# Offload execution to background worker
bg_tasks.add_task(run_agent_pipeline, task_id, prompt)
return {
"task_id": task_id,
"status": "queued",
"poll_url": f"/api/v1/agent/tasks/{task_id}"
}2. Interactive Async Job API Studio
HTTP 202 Polling vs SSE StreamerLong-Running Agent API Architecture Studio
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Common Engineering Traps
Using FastAPI's built-in BackgroundTasks in production. If your web container restarts or auto-scales down while an agent is mid-execution, all in-flight jobs die silently with zero recovery. Always push jobs to an external persistent queue like Celery or Redis Streams.
When a user closes their browser tab, the frontend disconnects. If your backend doesn't register an abort signal or check connection liveness, the agent worker keeps invoking expensive LLM tools for minutes on behalf of a user who is no longer there.
Key Architectural Takeaways
- 1.HTTP 202 Decoupling: Immediately return a ticket ID so clients can poll or stream status without hitting HTTP 504 timeouts.
- 2.SSE Streaming: Use Server-Sent Events to provide real-time token and tool transparency to end users.
- 3.Distributed Cancellation: Check Redis cancellation flags on every agent graph turn to abort abandoned tasks instantly.
Deploying Agents in Worker Node Architectures
Keep your web servers lightweight. Learn how to decouple HTTP intake from compute-heavy agent loops using Redis Streams, Celery, and auto-scaling worker pools.