httpx Bug with merging base_url with relative path if colon

The Fix

Upgrade to version 0.18.0 or later.

Based on closed encode/httpx issue #1498 · 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

@@ -325,10 +325,19 @@ def _merge_url(self, url: URLTypes) -> URL:
         merge_url = URL(url)
         if merge_url.is_relative_url:
-            # We always ensure the base_url paths include the trailing '/',
-            # and always strip any leading '/' from the merge URL.
-            merge_url = merge_url.copy_with(raw_path=merge_url.raw_path.lstrip(b"/"))

repro.py

>> import httpx
>> httpx.Request("GET", "https://example.org/sub:path").url.raw
(b'https', b'example.org', None, b'/sub:path')  # ✅
>> httpx.Client(base_url="https://example.org").build_request("GET", "/sub:path").url.raw
(b'sub', b'', None, b'path')  # ❌
>> httpx.Client(base_url="https://example.org").build_request("GET", "sub:path").url.raw
(b'sub', b'', None, b'path')  # ✅ (Surprising, but in line with RFC 3986: `http:path` is treated as an authority-less HTTP URL)
>> httpx.Client(base_url="https://example.org").build_request("GET", "/sub%3Apath").url.raw
(b'https', b'example.org', None, b'/sub%3Apath')  # ✅
>> httpx.Client(base_url="https://example.org").build_request("GET", "/ok").url.raw
(b'https', b'example.org', None, b'/ok')  # ✅

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 0.18.0 or later.\nWhen NOT to use: This fix should not be used if the base_url is not properly formatted.\n\n

Why This Fix Works in Production

Trigger: E AssertionError: assert URL('sub:path') == 'https://www.example.com/sub%3Apath'
Mechanism: The merging of base_url with relative paths containing a colon was incorrectly handled
Why the fix works: Fixes an issue with merging base_url with relative paths containing a colon. (first fixed release: 0.18.0).

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 3.9 in real deployments (not just unit tests).
The merging of base_url with relative paths containing a colon was incorrectly handled
Surfaces as: E AssertionError: assert URL('sub:path') == 'https://www.example.com/sub%3Apath'

Proof / Evidence

GitHub issue: #1498
Fix PR: https://github.com/encode/httpx/pull/1532
First fixed release: 0.18.0
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.56

Discussion

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

Jump to Sources Open on GitHub

“cc @tomchristie — in case you've got any thoughts on this.”

@florimondmanca · 2021-03-04 · source

“I guess the first step here is a failing test case. I think the following change would work based on a bit of testing in…”

@lovelydinosaur · 2021-03-23 · source

“@gtors Thanks for reporting :-) Indeed, we've got an issue with interpreting colons when merging URLs: In 0.16.1 we get: So indeed, it looks like…”

@florimondmanca · 2021-03-04 · source

Failure Signature (Search String)

E AssertionError: assert URL('sub:path') == 'https://www.example.com/sub%3Apath'

Error Message

Stack trace

error.txt

Error Message
-------------
E       AssertionError: assert URL('sub:path') == 'https://www.example.com/sub%3Apath'
E        +  where URL('sub:path') = <Request('GET', 'sub:path')>.url

Minimal Reproduction

repro.py

>> import httpx
>> httpx.Request("GET", "https://example.org/sub:path").url.raw
(b'https', b'example.org', None, b'/sub:path')  # ✅
>> httpx.Client(base_url="https://example.org").build_request("GET", "/sub:path").url.raw
(b'sub', b'', None, b'path')  # ❌
>> httpx.Client(base_url="https://example.org").build_request("GET", "sub:path").url.raw
(b'sub', b'', None, b'path')  # ✅ (Surprising, but in line with RFC 3986: `http:path` is treated as an authority-less HTTP URL)
>> httpx.Client(base_url="https://example.org").build_request("GET", "/sub%3Apath").url.raw
(b'https', b'example.org', None, b'/sub%3Apath')  # ✅
>> httpx.Client(base_url="https://example.org").build_request("GET", "/ok").url.raw
(b'https', b'example.org', None, b'/ok')  # ✅

Environment

Python: 3.9
httpx: 0.17.0

What Broke

Requests with relative paths containing a colon resulted in unsupported protocol errors.

Why It Broke

The merging of base_url with relative paths containing a colon was incorrectly handled

Fix Options (Details)

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

Upgrade to version 0.18.0 or later.

When NOT to use: This fix should not be used if the base_url is not properly formatted.

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/1532

First fixed release: 0.18.0

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

This fix should not be used if the base_url is not properly formatted.
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
0.18.0	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 Bug with merging base_url with relative path if colon present in path (Fix)