starlette `HTTPMiddleware` breaks `BackgroundTasks` in the

The Fix

Upgrade to version 0.29.0 or later.

Based on closed Kludex/starlette issue #2093 · PR/commit linked

Production note: Watch p95/p99 latency and retry volume; timeouts can turn into retry storms and duplicate side-effects.

Jump to Verify Open PR/Commit

@@ -170,6 +170,8 @@ async def body_stream() -> typing.AsyncGenerator[bytes, None]:
                         if body:
                             yield body
+                        if not message.get("more_body", False):
+                            break

repro.py

import logging
from time import sleep

from starlette.applications import Starlette
from starlette.background import BackgroundTasks
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
from starlette.requests import Request
from starlette.responses import JSONResponse, Response
from starlette.routing import Route

logging.basicConfig(
    level=logging.INFO,
    format="[%(asctime)s] [%(levelname)s] %(message)s",
    datefmt="%Y-%m-%d %H:%M:%S",
)


class CustomMiddleware(BaseHTTPMiddleware):
    async def dispatch(
        self, request: Request, call_next: RequestResponseEndpoint
    ) -> Response:
        return await call_next(request)


def create_task(request: Request) -> JSONResponse:
    logging.info("Entered POST method")
    tasks = BackgroundTasks()
    tasks.add_task(background_task)
    message = {"status": "Task created"}
    return JSONResponse(message, background=tasks, status_code=201)


def background_task() -> None:
    logging.info("Started background task")
    sleep(10)
    logging.info("Finished background task")


routes = [Route("/task", endpoint=create_task, methods=["POST"])]
middleware = [Middleware(CustomMiddleware)]
app = Starlette(routes=routes, middleware=middleware)

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 0.29.0 or later.\nWhen NOT to use: This fix should not be used if the application relies on the previous sequential execution behavior.\n\n

Why This Fix Works in Production

Trigger: `HTTPMiddleware` breaks `BackgroundTasks` in the same connection
Mechanism: The HTTPMiddleware caused BackgroundTasks to execute sequentially in the same connection due to improper handling of the body stream
Why the fix works: Stops the body stream in case more_body is False, resolving the issue where BackgroundTasks were executed sequentially in the same connection. (first fixed release: 0.29.0).

Production impact:

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

Why This Breaks in Prod

The HTTPMiddleware caused BackgroundTasks to execute sequentially in the same connection due to improper handling of the body stream
Production symptom (often without a traceback): `HTTPMiddleware` breaks `BackgroundTasks` in the same connection

Proof / Evidence

GitHub issue: #2093
Fix PR: https://github.com/kludex/starlette/pull/2194
First fixed release: 0.29.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.43

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: starlette
Fixed: 0.29.0
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

Discussion

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

Jump to Sources Open on GitHub

“@Kludex Is it ok to PR regarding this issue for a review?”

@jsg921019 · 2023-04-26 · source

“You mean that you want to create a PR? Yeah, go ahead.”

@Kludex · 2023-04-26 · source

“Is anyone working on this issue? It's quite important for us and I think whether it is necessary to fix it myself or someone is…”

@nikita-b · 2023-05-17 · source

“@nikita-b No one is working on this issue. Go ahead. 🙏”

@Kludex · 2023-05-24 · source

Failure Signature (Search String)

`HTTPMiddleware` breaks `BackgroundTasks` in the same connection
Which results in a sequential execution of background tasks (and read timeout from the client as a consequence):

Copy-friendly signature

signature.txt

Failure Signature
-----------------
`HTTPMiddleware` breaks `BackgroundTasks` in the same connection
Which results in a sequential execution of background tasks (and read timeout from the client as a consequence):

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
`HTTPMiddleware` breaks `BackgroundTasks` in the same connection
Which results in a sequential execution of background tasks (and read timeout from the client as a consequence):

Minimal Reproduction

repro.py

import logging
from time import sleep

from starlette.applications import Starlette
from starlette.background import BackgroundTasks
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
from starlette.requests import Request
from starlette.responses import JSONResponse, Response
from starlette.routing import Route

logging.basicConfig(
    level=logging.INFO,
    format="[%(asctime)s] [%(levelname)s] %(message)s",
    datefmt="%Y-%m-%d %H:%M:%S",
)


class CustomMiddleware(BaseHTTPMiddleware):
    async def dispatch(
        self, request: Request, call_next: RequestResponseEndpoint
    ) -> Response:
        return await call_next(request)


def create_task(request: Request) -> JSONResponse:
    logging.info("Entered POST method")
    tasks = BackgroundTasks()
    tasks.add_task(background_task)
    message = {"status": "Task created"}
    return JSONResponse(message, background=tasks, status_code=201)


def background_task() -> None:
    logging.info("Started background task")
    sleep(10)
    logging.info("Finished background task")


routes = [Route("/task", endpoint=create_task, methods=["POST"])]
middleware = [Middleware(CustomMiddleware)]
app = Starlette(routes=routes, middleware=middleware)

What Broke

Clients experienced read timeouts when executing multiple background tasks in the same connection.

Why It Broke

The HTTPMiddleware caused BackgroundTasks to execute sequentially in the same connection due to improper handling of the body stream

Fix Options (Details)

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

Upgrade to version 0.29.0 or later.

When NOT to use: This fix should not be used if the application relies on the previous sequential execution behavior.

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/2194

First fixed release: 0.29.0

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

When NOT to Use This Fix

This fix should not be used if the application relies on the previous sequential execution behavior.

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

Make timeouts explicit and test them (unit + integration) to avoid silent behavior changes.
Instrument retries (attempt count + reason) and alert on spikes to catch dependency slowdowns.

Version Compatibility Table

Version	Status
0.29.0	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 `HTTPMiddleware` breaks `BackgroundTasks` in the same connection (Fix)