aiohttp Documentation not mentioning treatment of invalid

The Fix

Adds test coverage for invalid content type handling and documents the expected behavior, clarifying that invalid Content-Type headers default to 'text/plain'.

Based on closed aio-libs/aiohttp issue #9522 · 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

@@ -1474,7 +1474,12 @@ Response object
          Returns value is ``'application/octet-stream'`` if no
          Content-Type header present in HTTP headers according to
-         :rfc:`2616`. To make sure Content-Type header is not present in
+         :rfc:`9110`. If the *Content-Type* header is invalid (e.g., ``jpg``
+         instead of ``image/jpeg``), the value is ``text/plain`` by default

repro.py

from email.parser import HeaderParser
hp = HeaderParser()
hp.parsestr('Content-Type: jpg').get_content_type() # 'text/plain'
hp.parsestr('Content-Type: image/jpg').get_content_type() # 'image/jpg'
hp.parsestr('Content-Type: /jpg').get_content_type() # '/jpg'

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Apply the official fix\nAdds test coverage for invalid content type handling and documents the expected behavior, clarifying that invalid Content-Type headers default to 'text/plain'.\nWhen NOT to use: This fix does not apply if the Content-Type header is valid and correctly formatted.\n\n

Why This Fix Works in Production

Trigger: Documentation not mentioning treatment of invalid Content-Type headers in ClientResponse (mismatch of header content and property value)
Mechanism: Invalid Content-Type headers lead to mismatched values in ClientResponse properties

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 3.9.17 in real deployments (not just unit tests).
Invalid Content-Type headers lead to mismatched values in ClientResponse properties
Production symptom (often without a traceback): Documentation not mentioning treatment of invalid Content-Type headers in ClientResponse (mismatch of header content and property value)

Proof / Evidence

GitHub issue: #9522
Fix PR: https://github.com/aio-libs/aiohttp/pull/10880
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.70
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.64

Discussion

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

Jump to Sources Open on GitHub

“### Describe the bug Recently a client script of mine received responses (status=200) for image-fetches denoting 'text/plain' in the content_type property of the response, while the body was still an image and not an error message (as would”

Issue thread · issue description · source

Failure Signature (Search String)

Documentation not mentioning treatment of invalid Content-Type headers in ClientResponse (mismatch of header content and property value)
```python-traceback

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Documentation not mentioning treatment of invalid Content-Type headers in ClientResponse (mismatch of header content and property value)
```python-traceback

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Documentation not mentioning treatment of invalid Content-Type headers in ClientResponse (mismatch of header content and property value)
```python-traceback

Minimal Reproduction

repro.py

from email.parser import HeaderParser
hp = HeaderParser()
hp.parsestr('Content-Type: jpg').get_content_type() # 'text/plain'
hp.parsestr('Content-Type: image/jpg').get_content_type() # 'image/jpg'
hp.parsestr('Content-Type: /jpg').get_content_type() # '/jpg'

Environment

Python: 3.9.17

What Broke

Clients receive incorrect content type, causing confusion and potential processing errors.

Why It Broke

Invalid Content-Type headers lead to mismatched values in ClientResponse properties

Fix Options (Details)

Option A — Apply the official fix

Adds test coverage for invalid content type handling and documents the expected behavior, clarifying that invalid Content-Type headers default to 'text/plain'.

When NOT to use: This fix does not apply if the Content-Type header is valid and correctly formatted.

Fix reference: https://github.com/aio-libs/aiohttp/pull/10880

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

When NOT to Use This Fix

This fix does not apply if the Content-Type header is valid and correctly formatted.

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

Make timeouts explicit and test them (unit + integration) to avoid silent behavior changes.
Instrument retries (attempt count + reason) and alert on spikes to catch dependency slowdowns.

Related Issues

No related fixes found.

Cluster: aiohttp:data-consistency All hubs All clusters

Related clusters: Configuration error Race condition Auth

Sources

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

aiohttp Documentation not mentioning treatment of invalid Content-Type headers in ClientResponse (Fix)