FastAPI poor quality traceback / useful stack frames not

The Fix

pip install fastapi==0.115.6

Based on closed fastapi/fastapi issue #13067 · PR/commit linked

Production note: This tends to surface only under concurrency. Reproduce with load tests and watch for lock contention/cancellation paths.

Jump to Verify Open PR/Commit

@@ -1,7 +1,7 @@
 from typing import AsyncGenerator, ContextManager, TypeVar
 
-import anyio
+import anyio.to_thread
 from anyio import CapacityLimiter

repro.py

File "...3.11/lib/python3.11/contextlib.py", line 650, in enter_async_context
    result = await _enter(cm)
             ^^^^^^^^^^^^^^^^
  File "...3.11/lib/python3.11/contextlib.py", line 210, in __aenter__
    return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File ".../full-stack-fastapi-template/backend/.venv/lib/python3.11/site-packages/fastapi/concurrency.py", line 35, in contextmanager_in_threadpool
    raise e
Exception: error

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 fastapi==0.115.6\nWhen NOT to use: This fix should not be applied if the application does not use synchronous dependencies with yield.\n\n

Why This Fix Works in Production

Trigger: poor quality traceback / useful stack frames not present when exceptions raised in sync dependencies
Mechanism: The traceback was missing when exceptions were raised in synchronous dependencies using yield
Why the fix works: Preserves the traceback when an exception is raised in a synchronous dependency with yield, addressing the issue of missing traceback information. (first fixed release: 0.115.6).

Production impact:

If left unfixed, failures can be intermittent under concurrency (hard to reproduce; shows up as sporadic 5xx/timeouts).

Why This Breaks in Prod

Shows up under Python 3.11 in real deployments (not just unit tests).
The traceback was missing when exceptions were raised in synchronous dependencies using yield
Surfaces as: File "...3.11/lib/python3.11/contextlib.py", line 650, in enter_async_context\n result = await _enter(cm)\n ^^^^^^^^^^^^^^^^\n File "...3.11/lib/python3.11/contextlib.py", line…

Proof / Evidence

GitHub issue: #13067
Fix PR: https://github.com/fastapi/fastapi/pull/5823
First fixed release: 0.115.6
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.56

Discussion

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

Jump to Sources Open on GitHub

“Close enough: Dropped a breakpoint, does indeed appear that __traceback__ is None, but why?”

@cjw296 · 2024-12-13 · source

“@Kludex - about the issue/discussions thing..”

@cjw296 · 2024-12-13 · source

“I don't really have the power or motivation to do that. **I don't maintain FastAPI.** If you want, you can create a GitHub issue about…”

@Kludex · 2024-12-13 · source

“For reference, this issue was solved in the last release: https://github.com/fastapi/fastapi/releases/tag/0.115.6 - Specifically on https://github.com/fastapi/fastapi/pull/5823”

@Kludex · 2024-12-13 · source

Failure Signature (Search String)

poor quality traceback / useful stack frames not present when exceptions raised in sync dependencies

Error Message

Stack trace

error.txt

Error Message
-------------
File "...3.11/lib/python3.11/contextlib.py", line 650, in enter_async_context\n    result = await _enter(cm)\n             ^^^^^^^^^^^^^^^^\n  File "...3.11/lib/python3.11/contextlib.py", line 210, in __aenter__\n    return await anext(self.gen)\n           ^^^^^^^^^^^^^^^^^^^^^\n  File ".../full-stack-fastapi-template/backend/.venv/lib/python3.11/site-packages/fastapi/concurrency.py", line 35, in contextmanager_in_threadpool\n    raise e

Minimal Reproduction

repro.py

File "...3.11/lib/python3.11/contextlib.py", line 650, in enter_async_context
    result = await _enter(cm)
             ^^^^^^^^^^^^^^^^
  File "...3.11/lib/python3.11/contextlib.py", line 210, in __aenter__
    return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File ".../full-stack-fastapi-template/backend/.venv/lib/python3.11/site-packages/fastapi/concurrency.py", line 35, in contextmanager_in_threadpool
    raise e
Exception: error

Environment

Python: 3.11

What Broke

Developers received unhelpful tracebacks, making debugging difficult during development.

Why It Broke

The traceback was missing when exceptions were raised in synchronous dependencies using yield

Fix Options (Details)

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

pip install fastapi==0.115.6

When NOT to use: This fix should not be applied if the application does not use synchronous dependencies with yield.

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

Fix reference: https://github.com/fastapi/fastapi/pull/5823

First fixed release: 0.115.6

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

This fix should not be applied if the application does not use synchronous dependencies with yield.

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 stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.

Version Compatibility Table

Version	Status
0.115.6	Fixed

Related Issues

No related fixes found.

Cluster: fastapi:race-condition FastAPI hub FastAPI best practices All hubs All clusters

Related clusters: Configuration error Data consistency Rate limit

Sources

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

FastAPI poor quality traceback / useful stack frames not present when exceptions raised in sync (Fix)