httpx Too much data for declared Content-Length when

The Fix

Upgrade to version 0.18.0 or later.

Based on closed encode/httpx issue #1482 · 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

@@ -40,11 +40,7 @@ def render_headers(self) -> bytes:
     def render_data(self) -> bytes:
         if not hasattr(self, "_data"):
-            self._data = (
-                self.value
-                if isinstance(self.value, bytes)

repro.py

import httpx

response = httpx.post(
    "https://httpbin.org/post",
    files={
        'upload-file': ('example.txt', '\u00E9', 'text/plain; charset=utf-8')
    }
)
response.raise_for_status()
print(response.read())

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 application relies on the original Content-Length behavior.\n\n

Why This Fix Works in Production

Trigger: response = httpx.post(
Mechanism: Content-Length is calculated using Unicode length instead of byte length for non-ascii characters
Why the fix works: Fixes the Content-Length calculation for unicode file contents in multipart requests, addressing the issue where an exception was raised for non-ascii characters. (first fixed release: 0.18.0).

Production impact:

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

Why This Breaks in Prod

Shows up under Python 3.9 in real deployments (not just unit tests).
Content-Length is calculated using Unicode length instead of byte length for non-ascii characters
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #1482
Fix PR: https://github.com/encode/httpx/pull/1537
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: 0.44

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: httpx
Fixed: 0.18.0
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

b'{\n  "args": {}, \n  "data": "", \n  "files": {\n    "upload-file": "\\u00e9"\n  }, \n  "form": {}, \n  "headers": {\n    "Accept": "*/*", \n    "Accept-Encoding": "gzip, deflate", \n    "Content-Length": "197", \n    "Content-Type": "multipart/form-data; boundary=8e29a4c1fcee5377fe1b2ecfbd64760a", \n    "Host": "httpbin.org", \n    "User-Agent": "python-httpx/0.18.0", \n    "X-Amzn-Trace-Id": "Root=1-698cb453-60705b9b741ee1964ad3415c"\n  }, \n  "json": null, \n  "origin": "34.38.131.152", \n  "url": "https://httpbin.org/post"\n}\n'

Discussion

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

Jump to Sources Open on GitHub

“Looks like we calculate Content-Length using Unicode length: https://github.com/encode/httpx/blob/0f280af8b170ed5cc48c12a894f71a8b5762f748/httpx/_multipart.py#L90-L91 but sends with bytes: https://github.com/encode/httpx/blob/0f280af8b170ed5cc48c12a894f71a8b57”

@j178 · 2021-02-24 · source

Failure Signature (Search String)

response = httpx.post(

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "/***redacted***/httpx_bug.py", line 5, in <module>
    response = httpx.post(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_api.py", line 296, in post
    return request(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_api.py", line 93, in request
    return client.request(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 733, in request
    return self.send(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 767, in send
    response = self._send_handling_auth(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 805, in _send_handling_auth
    response = self._send_handling_redirects(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 837, in _send_handling_redirects
    response = self._send_single_request(request, timeout)
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpx/_client.py", line 861, in _send_single_request
    (status_code, headers, stream, ext) = transport.request(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpcore/_sync/connection_pool.py", line 218, in request
    response = connection.request(
  File "/***redacted***/.venv/lib/python3.9/site-packages/httpcore/_sync/connection.py", line 106, in request
    return self.connection.r
... (truncated) ...

Minimal Reproduction

repro.py

import httpx

response = httpx.post(
    "https://httpbin.org/post",
    files={
        'upload-file': ('example.txt', '\u00E9', 'text/plain; charset=utf-8')
    }
)
response.raise_for_status()
print(response.read())

Environment

Python: 3.9

What Broke

An exception is raised when uploading files with non-ascii characters, causing failed requests.

Why It Broke

Content-Length is calculated using Unicode length instead of byte length for non-ascii characters

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 application relies on the original Content-Length behavior.

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/1537

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 application relies on the original Content-Length behavior.

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: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 Too much data for declared Content-Length when passing string with non-ascii characters via (Fix)