starlette Background tasks are cancelled if the client

The Fix

Upgrade to version 0.21.0 or later.

Based on closed Kludex/starlette issue #1438 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -4,12 +4,13 @@
 from starlette.requests import Request
 from starlette.responses import Response, StreamingResponse
-from starlette.types import ASGIApp, Receive, Scope, Send
+from starlette.types import ASGIApp, Message, Receive, Scope, Send

repro.py

import traceback

import anyio
from starlette.applications import Starlette
from starlette.background import BackgroundTasks
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.responses import Response
from starlette.routing import Route


async def passthrough(request, call_next):
    return await call_next(request)


async def _sleep(identifier, delay):
    print(identifier, "started")
    try:
        await anyio.sleep(delay)
        print(identifier, "completed")
    except BaseException:
        print(identifier, "error")
        traceback.print_exc()
        raise


async def response_with_sleeps(request):
    background_tasks = BackgroundTasks()
    background_tasks.add_task(_sleep, "background task 1", 2)
    background_tasks.add_task(_sleep, "background task 2", 2)
    return Response(background=background_tasks)


application = Starlette(
    middleware=[
        Middleware(BaseHTTPMiddleware, dispatch=passthrough),
    ],
    routes=[
        Route("/", response_with_sleeps),
    ],
)

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.21.0 or later.\nWhen NOT to use: This fix should not be applied if the application does not use BaseHTTPMiddleware.\n\n

Why This Fix Works in Production

Trigger: background task 1 started
Mechanism: Background tasks are cancelled due to improper handling of client disconnection in BaseHTTPMiddleware
Why the fix works: Replaces task cancellation in BaseHTTPMiddleware with a mechanism to handle http.disconnect, preventing background tasks from being cancelled when the client disconnects. (first fixed release: 0.21.0).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

Shows up under Python 3.9 in real deployments (not just unit tests).
Background tasks are cancelled due to improper handling of client disconnection in BaseHTTPMiddleware
Surfaces as: background task 1 started

Proof / Evidence

GitHub issue: #1438
Fix PR: https://github.com/kludex/starlette/pull/1715
First fixed release: 0.21.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.40

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“Thanks for the precise report @jhominal :) I've implemented a solution on #1441. Let's see how the discussion around that goes.”

@Kludex · 2022-01-28 · source

“I looked the code that BaseHTTPMiddleware and StreamingResponse use the same task_group, when one scope receive http.disconnect, task_group cancel all taskes. But I don't know…”

@xingdongzhe · 2022-04-03 · source

“The PR that solves the issue has all the details. EDIT: My PR had some regressions. I've closed it.”

@Kludex · 2022-04-03 · source

“The background tasks need to be shielded.”

@kigawas · 2022-06-20 · source

Failure Signature (Search String)

background task 1 started

Error Message

Stack trace

error.txt

Error Message
-------------
background task 1 started
background task 1 error
Traceback (most recent call last):
  File "/Users/jean/PycharmProjects/starlette-bg-cancelled/./repro.py", line 19, in _sleep
    await anyio.sleep(delay)
  File "/Users/jean/PycharmProjects/starlette-bg-cancelled/venv/lib/python3.9/site-packages/anyio/_core/_eventloop.py", line 69, in sleep
    return await get_asynclib().sleep(delay)
  File "/usr/local/Cellar/[email protected]/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/tasks.py", line 654, in sleep
    return await future
asyncio.exceptions.CancelledError

Minimal Reproduction

repro.py

import traceback

import anyio
from starlette.applications import Starlette
from starlette.background import BackgroundTasks
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.responses import Response
from starlette.routing import Route


async def passthrough(request, call_next):
    return await call_next(request)


async def _sleep(identifier, delay):
    print(identifier, "started")
    try:
        await anyio.sleep(delay)
        print(identifier, "completed")
    except BaseException:
        print(identifier, "error")
        traceback.print_exc()
        raise


async def response_with_sleeps(request):
    background_tasks = BackgroundTasks()
    background_tasks.add_task(_sleep, "background task 1", 2)
    background_tasks.add_task(_sleep, "background task 2", 2)
    return Response(background=background_tasks)


application = Starlette(
    middleware=[
        Middleware(BaseHTTPMiddleware, dispatch=passthrough),
    ],
    routes=[
        Route("/", response_with_sleeps),
    ],
)

Environment

Python: 3.9

What Broke

Background tasks fail to complete when the client disconnects, leading to incomplete processing.

Why It Broke

Background tasks are cancelled due to improper handling of client disconnection in BaseHTTPMiddleware

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Upgrade to version 0.21.0 or later.

When NOT to use: This fix should not be applied if the application does not use BaseHTTPMiddleware.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/kludex/starlette/pull/1715

First fixed release: 0.21.0

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be applied if the application does not use BaseHTTPMiddleware.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
0.21.0	Fixed

Related Issues

No related fixes found.

Cluster: starlette:data-consistency All hubs All clusters

Related clusters: Configuration error SSL handshake Other

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

starlette Background tasks are cancelled if the client closes connection (Fix)