httpx Lazy instantiation of ssl_context in a Client or

The Fix

Upgrade to version 0.17.0 or later.

Based on closed encode/httpx issue #1374 · 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

@@ -1040,12 +1040,13 @@ class HelloWorldTransport(httpcore.SyncHTTPTransport):
     """
 
-    def request(self, method, url, headers=None, stream=None, timeout=None):
+    def request(self, method, url, headers=None, stream=None, ext=None):
         message = {"text": "Hello, world!"}

repro.py

import pypki2config

class Client(httpx.Client):
    def _transport_for_url(self, url):
        if url.host in AUDITED_DOMAINS:
            ctx = pypki2config.ssl_context()
            return self._init_transport(verify=ctx)
        else:
            ctx = ssl.create_default_context()
            return self._init_transport(verify=ctx)

### and the same override code for AsyncClient

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 0.17.0 or later.\nWhen NOT to use: This fix should not be used if immediate SSL context initialization is required.\n\n

Why This Fix Works in Production

Trigger: Lazy instantiation of ssl_context in a Client or Transport
Mechanism: The SSL context was being instantiated eagerly, causing unnecessary password prompts
Why the fix works: Add support for Mount API to allow users to control which transport an outgoing request should be routed via. (first fixed release: 0.17.0).

Why This Breaks in Prod

The SSL context was being instantiated eagerly, causing unnecessary password prompts
Production symptom (often without a traceback): Lazy instantiation of ssl_context in a Client or Transport

Proof / Evidence

GitHub issue: #1374
Fix PR: https://github.com/encode/httpx/pull/1362
First fixed release: 0.17.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.61

Discussion

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

Jump to Sources Open on GitHub

“Thanks @tomchristie ! I totally missed _transport_for_url or misunderstood it”

@kafonek · 2020-11-12 · source

“So, some options here..”

@lovelydinosaur · 2020-11-12 · source

Failure Signature (Search String)

Lazy instantiation of ssl_context in a Client or Transport
* Change the `httpcore.Transport` `ssl_context` init variable to accept a callback, and that callback wouldn't be executed until needed (such as when `._open_socket` is invoked)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Lazy instantiation of ssl_context in a Client or Transport
* Change the `httpcore.Transport` `ssl_context` init variable to accept a callback, and that callback wouldn't be executed until needed (such as when `._open_socket` is invoked)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Lazy instantiation of ssl_context in a Client or Transport
* Change the `httpcore.Transport` `ssl_context` init variable to accept a callback, and that callback wouldn't be executed until needed (such as when `._open_socket` is invoked)

Minimal Reproduction

repro.py

import pypki2config

class Client(httpx.Client):
    def _transport_for_url(self, url):
        if url.host in AUDITED_DOMAINS:
            ctx = pypki2config.ssl_context()
            return self._init_transport(verify=ctx)
        else:
            ctx = ssl.create_default_context()
            return self._init_transport(verify=ctx)

### and the same override code for AsyncClient

Environment

httpx: 0.16

What Broke

Users experienced password prompts during client initialization, leading to confusion and delays.

Why It Broke

The SSL context was being instantiated eagerly, causing unnecessary password prompts

Fix Options (Details)

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

Upgrade to version 0.17.0 or later.

When NOT to use: This fix should not be used if immediate SSL context initialization is required.

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

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/encode/httpx/pull/1362

First fixed release: 0.17.0

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

When NOT to Use This Fix

This fix should not be used if immediate SSL context initialization is required.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

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 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.

Version Compatibility Table

Version	Status
0.17.0	Fixed

Related Issues

No related fixes found.

Cluster: httpx:ssl-handshake All hubs All clusters

Related clusters: Configuration error Data consistency Other

Sources

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

httpx Lazy instantiation of ssl_context in a Client or Transport (Fix)