httpx Use the existing runloop for sync calls in async

The Fix

Upgrade to version 0.13.0 or later.

Based on closed encode/httpx issue #179 · 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

@@ -105,3 +105,38 @@ trio.run(main)
 !!! important
     `trio` must be installed to import and use the `TrioBackend`.
+
+## FAQ
+

repro.py

import httpx
...
class SomeClass(object):
    def get_some_url(self):
        r = httpx.get('some url')

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 0.13.0 or later.\nWhen NOT to use: Avoid using sync methods in an async environment to prevent blocking the event loop.\n\n

Why This Fix Works in Production

Trigger: self.client = httpx.Client(backend=kwargs.get('runloop'))
Mechanism: The sync methods in httpx use async calls, causing conflicts with existing event loops
Why the fix works: Document when to use AsyncClient to avoid blocking-style network calls in async environments. (first fixed release: 0.13.0).

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 3.6 in real deployments (not just unit tests).
The sync methods in httpx use async calls, causing conflicts with existing event loops
Surfaces as: self.client = httpx.Client(backend=kwargs.get('runloop'))

Proof / Evidence

GitHub issue: #179
Fix PR: https://github.com/encode/httpx/pull/534
First fixed release: 0.13.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.67

Discussion

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

Jump to Sources Open on GitHub

“> FWIW; I tried the following in a vein hope that it would do what I wanted: > > > > But alas, no success..…”

@cansarigol · 2019-07-31 · source

“Yeah, I figured that after I got the exception. I figured it would handle a standard runloop.”

@bennovakovic · 2019-07-31 · source

“There is no API to provide the event loop yourself, however there is a loop property in the AsyncioConcurrencyBackend that checks an existing private _loop…”

@yeraydiazdiaz · 2019-07-31 · source

“FWIW; I tried the following in a vein hope that it would do what I wanted: But alas, no success.. this the end of the…”

@bennovakovic · 2019-07-31 · source

Failure Signature (Search String)

self.client = httpx.Client(backend=kwargs.get('runloop'))

Error Message

Stack trace

error.txt

Error Message
-------------
self.client = httpx.Client(backend=kwargs.get('runloop'))
  File "/usr/local/lib/python3.6/site-packages/httpx/client.py", line 85, in __init__
    backend=backend,
  File "/usr/local/lib/python3.6/site-packages/httpx/dispatch/connection_pool.py", line 97, in __init__
    self.max_connections = self.backend.get_semaphore(pool_limits)
AttributeError: 'Loop' object has no attribute 'get_semaphore'

Minimal Reproduction

repro.py

import httpx
...
class SomeClass(object):
    def get_some_url(self):
        r = httpx.get('some url')

Environment

Python: 3.6

What Broke

Using sync methods in an async context leads to exceptions about running multiple event loops.

Why It Broke

The sync methods in httpx use async calls, causing conflicts with existing event loops

Fix Options (Details)

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

Upgrade to version 0.13.0 or later.

When NOT to use: Avoid using sync methods in an async environment to prevent blocking the event loop.

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

Fix reference: https://github.com/encode/httpx/pull/534

First fixed release: 0.13.0

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

When NOT to Use This Fix

Avoid using sync methods in an async environment to prevent blocking the event loop.

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

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.
Add a stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.

Version Compatibility Table

Version	Status
0.13.0	Fixed

Related Issues

No related fixes found.

Cluster: httpx:configuration-error All hubs All clusters

Related clusters: Data consistency Other Timeout

Sources

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

httpx Use the existing runloop for sync calls in async handler (Fix)