httpx Query params should support list-of-strings values.

The Fix

Upgrade to version 0.7.5 or later.

Based on closed encode/httpx issue #366 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -32,6 +32,7 @@
 from .status_codes import StatusCode
 from .utils import (
+    flatten_queryparams,
     guess_json_utf,
     is_known_encoding,

repro.py

from httpx.models import QueryParams
from requests.models import RequestEncodingMixin

dict_params = {
    "documentIds": ["a55b2f894b90dd1213201471_25", "b54b2f894b90dd1213201471_9"]
}

httpx_params = str(QueryParams(dict_params))
print("httpx", httpx_params)

requests_params = RequestEncodingMixin._encode_params(dict_params)
print("requests", requests_params)

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 0.7.5 or later.\nWhen NOT to use: This fix is not applicable if the query parameters are not intended to be lists.\n\n

Why This Fix Works in Production

Trigger: API calls failed to return expected results due to incorrect query parameter encoding.
Mechanism: Multivalued query parameters were incorrectly processed as a Python list instead of being properly encoded
Why the fix works: Allows lists in query parameters to be properly encoded, fixing the issue where they were incorrectly processed as a Python list. (first fixed release: 0.7.5).

Production impact:

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

Why This Breaks in Prod

Multivalued query parameters were incorrectly processed as a Python list instead of being properly encoded
Production symptom (often without a traceback): API calls failed to return expected results due to incorrect query parameter encoding.

Proof / Evidence

GitHub issue: #366
Fix PR: https://github.com/encode/httpx/pull/386
First fixed release: 0.7.5
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

“We should look at adding some requests compatibility tests for this if they exist. :)”

@sethmlarson · 2019-09-23 · source

“Here's a bit more "live" example: Results:”

@senpos · 2019-09-21 · source

“Hi @Senpos! Thank you for reporting this”

@florimondmanca · 2019-09-21 · source

“Thanks for opening the issue! Retitled to make this more clearly specific.”

@lovelydinosaur · 2019-09-25 · source

Failure Signature (Search String)

API calls failed to return expected results due to incorrect query parameter encoding.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
API calls failed to return expected results due to incorrect query parameter encoding.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
API calls failed to return expected results due to incorrect query parameter encoding.

Minimal Reproduction

repro.py

from httpx.models import QueryParams
from requests.models import RequestEncodingMixin

dict_params = {
    "documentIds": ["a55b2f894b90dd1213201471_25", "b54b2f894b90dd1213201471_9"]
}

httpx_params = str(QueryParams(dict_params))
print("httpx", httpx_params)

requests_params = RequestEncodingMixin._encode_params(dict_params)
print("requests", requests_params)

What Broke

API calls failed to return expected results due to incorrect query parameter encoding.

Why It Broke

Multivalued query parameters were incorrectly processed as a Python list instead of being properly encoded

Fix Options (Details)

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

Upgrade to version 0.7.5 or later.

When NOT to use: This fix is not applicable if the query parameters are not intended to be lists.

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/386

First fixed release: 0.7.5

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

When NOT to Use This Fix

This fix is not applicable if the query parameters are not intended to be lists.

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
0.7.5	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.

httpx Query params should support list-of-strings values. (Fix)