httpx HTTP/2 download speeds / flow control settings (Fix)

The Fix

Upgrade to version 0.13.0 or later.

Based on closed encode/httpx issue #262 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -96,6 +96,7 @@ async def send_connection_init(self, timeout: Timeout) -> None:
 
         self.state.initiate_connection()
+        self.state.increment_flow_control_window(2 ** 24)
         data_to_send = self.state.data_to_send()
         await self.socket.write(data_to_send, timeout)

repro.py

import httpx
import time
import requests

url = "https://quic.aiortc.org/50000000"

start = time.time()
r = requests.get(url)
print("requests %.1f s" % (time.time() - start))

for http_version in ["HTTP/1.1", "HTTP/2"]:
    client = httpx.Client(http_versions=http_version)

    start = time.time()
    r = client.get(url)
    print("httpx %s %.1f s" % (http_version, time.time() - start))

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: This fix should not be applied if flow control settings are already optimized for your use case.\n\n

Why This Fix Works in Production

Trigger: url = "https://quic.aiortc.org/50000000"
Mechanism: Flow control window sizes were set too low, negatively impacting HTTP/2 download speeds
Why the fix works: Bump up flow control defaults to improve HTTP/2 download speeds. (first fixed release: 0.13.0).

Production impact:

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

Why This Breaks in Prod

Flow control window sizes were set too low, negatively impacting HTTP/2 download speeds
Production symptom (often without a traceback): url = "https://quic.aiortc.org/50000000"

Proof / Evidence

GitHub issue: #262
Fix PR: https://github.com/encode/httpx/pull/629
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: 0.55

Discussion

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

Jump to Sources Open on GitHub

“Actually the issue I reported was *download* speeds, and it does seem to be linked to HTTP/2. Here is a script to reproduce the issue:…”

@jlaine · 2019-08-21 · source

“Okay, at this point it's a bit more complicated, since we've also got different performance on the server side too”

@lovelydinosaur · 2019-08-21 · source

“Okay, that's really helpful, thanks”

@lovelydinosaur · 2019-08-21 · source

“The server mentioned (quic.aiortc.org) above is serving HTTP/1.1 and HTTP/2 using nginx talking (over HTTP/1.1) to uvicorn”

@jlaine · 2019-08-21 · source

Failure Signature (Search String)

url = "https://quic.aiortc.org/50000000"
url = "https://httpbin.org/stream-bytes/50000000"

Copy-friendly signature

signature.txt

Failure Signature
-----------------
url = "https://quic.aiortc.org/50000000"
url = "https://httpbin.org/stream-bytes/50000000"

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
url = "https://quic.aiortc.org/50000000"
url = "https://httpbin.org/stream-bytes/50000000"

Minimal Reproduction

repro.py

import httpx
import time
import requests

url = "https://quic.aiortc.org/50000000"

start = time.time()
r = requests.get(url)
print("requests %.1f s" % (time.time() - start))

for http_version in ["HTTP/1.1", "HTTP/2"]:
    client = httpx.Client(http_versions=http_version)

    start = time.time()
    r = client.get(url)
    print("httpx %s %.1f s" % (http_version, time.time() - start))

Why It Broke

Flow control window sizes were set too low, negatively impacting HTTP/2 download speeds

Fix Options (Details)

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

Upgrade to version 0.13.0 or later.

When NOT to use: This fix should not be applied if flow control settings are already optimized for your use case.

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/629

First fixed release: 0.13.0

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

When NOT to Use This Fix

This fix should not be applied if flow control settings are already optimized for your use case.

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.

Version Compatibility Table

Version	Status
0.13.0	Fixed

Related Issues

No related fixes found.

Cluster: httpx:data-consistency All hubs All clusters

Related clusters: Configuration error Other Timeout

Sources

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