urllib3 request(..., preload_content=False).read(-1) raises

The Fix

pip install urllib3==2.2.2

Based on closed urllib3/urllib3 issue #3122 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -0,0 +1,2 @@
@@ -0,0 +1,2 @@
+Allowed passing negative integers as ``amt`` to read methods of
+:class:`http.client.HTTPResponse` as an alternative to ``None``.
diff --git a/src/urllib3/contrib/emscripten/response.py b/src/urllib3/contrib/emscripten/response.py

repro.py

import platform
import ssl
import urllib3

print("OS", platform.platform())
print("Python", platform.python_version())
print(ssl.OPENSSL_VERSION)
print("urllib3", urllib3.__version__)

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\npip install urllib3==2.2.2\nWhen NOT to use: Avoid using this fix if the application relies on the previous behavior of raising an exception for negative values.\n\n

Why This Fix Works in Production

Trigger: return self._decoded_buffer.get(amt)
Mechanism: The read method raises an exception when passing a negative integer instead of None
Why the fix works: Allows passing negative integers as `amt` to read methods, treating them the same as `None`, thus fixing the issue where `read(-1)` raises an unexpected exception. (first fixed release: 2.2.2).

Why This Breaks in Prod

Shows up under Python 3.11.4 in real deployments (not just unit tests).
The read method raises an exception when passing a negative integer instead of None
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #3122
Fix PR: https://github.com/urllib3/urllib3/pull/3356
First fixed release: 2.2.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.47

Discussion

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

Jump to Sources Open on GitHub

“@smason and I discussed the issue in https://github.com/urllib3/urllib3/pull/3186 too”

@illia-v · 2024-01-09 · source

“Our HTTPResponse object actually behaves a lot more like an io.BufferedIOBase instead of an io.RawIOBase object due to how .read() is implemented to always return…”

@sethmlarson · 2024-01-09 · source

Failure Signature (Search String)

return self._decoded_buffer.get(amt)

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/kosta/Library/Caches/pypoetry/virtualenvs/urllib3-read-IiCHxn1z-py3.11/lib/python3.11/site-packages/urllib3/response.py", line 877, in read
    return self._decoded_buffer.get(amt)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kosta/Library/Caches/pypoetry/virtualenvs/urllib3-read-IiCHxn1z-py3.11/lib/python3.11/site-packages/urllib3/response.py", line 255, in get
    raise RuntimeError("buffer is empty")

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/kosta/Library/Caches/pypoetry/virtualenvs/uai-snaky-path-TevYsdGm-py3.9/lib/python3.9/site-packages/urllib3/response.py", line 567, in read
    data = self._fp_read(amt) if not fp_closed else b""
  File "/Users/kosta/Library/Caches/pypoetry/virtualenvs/uai-snaky-path-TevYsdGm-py3.9/lib/python3.9/site-packages/urllib3/response.py", line 533, in _fp_read
    return self._fp.read(amt) if amt is not None else self._fp.read()
  File "/opt/homebrew/Cellar/[email protected]/3.9.17_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/http/client.py", line 462, in read
    b = bytearray(amt)
ValueError: negative count

Minimal Reproduction

repro.py

import platform
import ssl
import urllib3

print("OS", platform.platform())
print("Python", platform.python_version())
print(ssl.OPENSSL_VERSION)
print("urllib3", urllib3.__version__)

Environment

Python: 3.11.4
urllib3: 2.0.4

What Broke

Calling read(-1) results in a RuntimeError, disrupting data retrieval.

Why It Broke

The read method raises an exception when passing a negative integer instead of None

Fix Options (Details)

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

pip install urllib3==2.2.2

When NOT to use: Avoid using this fix if the application relies on the previous behavior of raising an exception for negative values.

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

Fix reference: https://github.com/urllib3/urllib3/pull/3356

First fixed release: 2.2.2

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

When NOT to Use This Fix

Avoid using this fix if the application relies on the previous behavior of raising an exception for negative values.

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
2.2.2	Fixed

Related Issues

No related fixes found.

Cluster: urllib3:ssl-handshake urllib3 hub urllib3 best practices 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.

urllib3 request(..., preload_content=False).read(-1) raises an unexpected Exception (Fix)