aiohttp Regression when closing session (Fix)

The Fix

Fixes a regression where ClientSession.close() would hang indefinitely when HTTPS requests were made through an HTTP proxy.

Based on closed aio-libs/aiohttp issue #11273 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -0,0 +1 @@
@@ -0,0 +1 @@
+Fixed :py:meth:`ClientSession.close() <aiohttp.ClientSession.close>` hanging indefinitely when using HTTPS requests through HTTP proxies -- by :user:`bdraco`.
diff --git a/aiohttp/connector.py b/aiohttp/connector.py
index 1c96ff42457..e1288424bff 100644

repro.py

import asyncio
from aiohttp import ClientSession

async def main():
    proxy = ""
    session = ClientSession(proxy=proxy)

    async with session.get(
        "https://api.ipify.org/",
    ) as response:
        ip = await response.text()
    print("Closing session...")
    await session.close()
    print("Done")
    return ip

asyncio.run(main())

verify

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

fix.md

Option A — Apply the official fix\nFixes a regression where ClientSession.close() would hang indefinitely when HTTPS requests were made through an HTTP proxy.\nWhen NOT to use: This fix should not be used if the application relies on the previous behavior of connection pooling.\n\n

Why This Fix Works in Production

Trigger: Regression when closing session
Mechanism: The regression was caused by CONNECT tunnel connections being pooled with proxy=None, leading to hanging futures

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.13.5 in real deployments (not just unit tests).
The regression was caused by CONNECT tunnel connections being pooled with proxy=None, leading to hanging futures
Production symptom (often without a traceback): Regression when closing session

Proof / Evidence

GitHub issue: #11273
Fix PR: https://github.com/aio-libs/aiohttp/pull/11289
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.80
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.62

Discussion

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

Jump to Sources Open on GitHub

“So this only happens when the remote is ssl HTTP through proxy works fine, HTTPS through proxy: Creates a proxy=None connection (the CONNECT tunnel), session…”

@bdraco · 2025-07-09 · confirmation · source

“> Successfully installed aiohttp-3.12.13 > > > > import asyncio > from aiohttp import ClientSession > > async def main(): > proxy = "" >…”

@Nazar1ky · 2025-07-06 · confirmation · source

“I just saw the email show up. I'm traveling today and in flight. I'll look when I am on the ground and time adjusted”

@bdraco · 2025-07-08 · source

“We are going to try to do a release this week. I'm working on a PR”

@bdraco · 2025-07-09 · source

Failure Signature (Search String)

Regression when closing session
[V3.12.4](https://github.com/aio-libs/aiohttp/releases/tag/v3.12.4) Introduced a regression, in which, session with some proxies can't be closed.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Regression when closing session
[V3.12.4](https://github.com/aio-libs/aiohttp/releases/tag/v3.12.4) Introduced a regression, in which, session with some proxies can't be closed.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Regression when closing session
[V3.12.4](https://github.com/aio-libs/aiohttp/releases/tag/v3.12.4) Introduced a regression, in which, session with some proxies can't be closed.

Minimal Reproduction

repro.py

import asyncio
from aiohttp import ClientSession

async def main():
    proxy = ""
    session = ClientSession(proxy=proxy)

    async with session.get(
        "https://api.ipify.org/",
    ) as response:
        ip = await response.text()
    print("Closing session...")
    await session.close()
    print("Done")
    return ip

asyncio.run(main())

Environment

Python: 3.13.5

What Broke

Closing the session hangs indefinitely when using HTTPS requests through HTTP proxies.

Why It Broke

The regression was caused by CONNECT tunnel connections being pooled with proxy=None, leading to hanging futures

Fix Options (Details)

Option A — Apply the official fix

Fixes a regression where ClientSession.close() would hang indefinitely when HTTPS requests were made through an HTTP proxy.

When NOT to use: This fix should not be used if the application relies on the previous behavior of connection pooling.

Fix reference: https://github.com/aio-libs/aiohttp/pull/11289

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 behavior of connection pooling.

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?

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Prevention

Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.

Related Issues

No related fixes found.

Cluster: aiohttp:configuration-error All hubs All clusters

Related clusters: Data consistency Race condition Auth

Sources

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