The Fix
Adds a sentinel type for _FAILEDTELL, replacing the object type with a Final Enum. This change improves type safety in the set_file_position function.
Based on closed urllib3/urllib3 issue #2512 · 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%.
@@ -41,7 +41,7 @@
from .util.connection import is_connection_dropped
from .util.proxy import connection_requires_http_tunnel
-from .util.request import set_file_position
+from .util.request import _TYPE_BODY_POSITION, set_file_position
from .util.response import assert_header_parsing
Option A — Apply the official fix\nAdds a sentinel type for _FAILEDTELL, replacing the object type with a Final Enum. This change improves type safety in the set_file_position function.\nWhen NOT to use: This fix is not applicable if the sentinel type is not required for your implementation.\n\n
Why This Fix Works in Production
- Trigger: Change set_file_position() type to use proper sentinel with _FAILEDTELL
- Mechanism: The sentinel type for _FAILEDTELL was incorrectly using object instead of a Final Enum
- 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
- The sentinel type for _FAILEDTELL was incorrectly using object instead of a Final Enum
- Production symptom (often without a traceback): Change set_file_position() type to use proper sentinel with _FAILEDTELL
Proof / Evidence
- GitHub issue: #2512
- Fix PR: https://github.com/urllib3/urllib3/pull/2519
- Reproduced locally: No (not executed)
- Last verified: 2026-02-11
- Confidence: 0.70
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.72
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Currently _FAILEDTELL in urllib3.util.request uses object as a sentinel type, we should give this object the same treatment as _DEFAULT_TIMEOUT and switch to a Final[Enum]. This type should be used wherever body_pos is used. cc @hramezani”
Failure Signature (Search String)
- Change set_file_position() type to use proper sentinel with _FAILEDTELL
- Currently `_FAILEDTELL` in `urllib3.util.request` uses `object` as a sentinel type, we should give this object the same treatment as `_DEFAULT_TIMEOUT` and switch to a
Copy-friendly signature
Failure Signature
-----------------
Change set_file_position() type to use proper sentinel with _FAILEDTELL
Currently `_FAILEDTELL` in `urllib3.util.request` uses `object` as a sentinel type, we should give this object the same treatment as `_DEFAULT_TIMEOUT` and switch to a `Final[Enum]`. This type should be used wherever `body_pos` is used.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Change set_file_position() type to use proper sentinel with _FAILEDTELL
Currently `_FAILEDTELL` in `urllib3.util.request` uses `object` as a sentinel type, we should give this object the same treatment as `_DEFAULT_TIMEOUT` and switch to a `Final[Enum]`. This type should be used wherever `body_pos` is used.
What Broke
Potential type errors when using body_pos in set_file_position function.
Why It Broke
The sentinel type for _FAILEDTELL was incorrectly using object instead of a Final Enum
Fix Options (Details)
Option A — Apply the official fix
Adds a sentinel type for _FAILEDTELL, replacing the object type with a Final Enum. This change improves type safety in the set_file_position function.
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)
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)
Fix reference: https://github.com/urllib3/urllib3/pull/2519
Last verified: 2026-02-11. Validate in your environment.
When NOT to Use This Fix
- This fix is not applicable if the sentinel type is not required for your implementation.
- 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.
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.
- 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.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.