uvicorn Feature Request: reload after error (Fix)

The Fix

Upgrade to version 0.17.0 or later.

Based on closed Kludex/uvicorn issue #425 · 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%.

Jump to Verify Open PR/Commit

@@ -122,21 +122,6 @@ def test_cli_incomplete_app_parameter() -> None:
 
 
-def test_cli_reloader_incomplete_app_parameter(
-    capfd: pytest.CaptureFixture[str],
-) -> None:

repro.py

from uvicorn.supervisors.statreload import HANDLED_SIGNALS, StatReload

def statreload_run(self, target, *args, **kwargs):
    pid = os.getpid()
    logger = self.config.logger_instance

    logger.info("Started reloader process [{}]".format(pid))

    for sig in HANDLED_SIGNALS:
        signal.signal(sig, self.handle_exit)

    def get_subprocess():
        spawn = multiprocessing.get_context("spawn")
        try:
            fileno = sys.stdin.fileno()
        except OSError:
            fileno = None

        return spawn.Process(
            target=self.handle_fds, args=(target, fileno), kwargs=kwargs
        )

    process = get_subprocess()
    process.start()

    # Changes are in this loop. No check if the process is alive in the loop
    while not self.should_exit:
        time.sleep(0.3)
        if self.should_restart():
            self.clear()
            if process.is_alive():
                os.kill(process.pid, signal.SIGTERM)
                process.join()

            process = get_subprocess()
            process.start()
            self.reload_count += 1

    logger.info("Stopping reloader process [{}]".format(pid))

StatReload.run = statreload_run

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 0.17.0 or later.\nWhen NOT to use: This fix is not suitable for production environments where stability is critical.\n\n

Why This Fix Works in Production

Trigger: Feature Request: reload after error
Mechanism: The reload process exits when a subprocess crashes due to syntax errors
Why the fix works: Fixes the reload process behavior when an exception is raised, ensuring that the server reloads after a syntax error instead of exiting. (first fixed release: 0.17.0).

Production impact:

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

Shows up under Python 3.8.5 in real deployments (not just unit tests).
The reload process exits when a subprocess crashes due to syntax errors
Production symptom (often without a traceback): Feature Request: reload after error

Proof / Evidence

GitHub issue: #425
Fix PR: https://github.com/encode/uvicorn/pull/1313
First fixed release: 0.17.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.46

Discussion

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

Jump to Sources Open on GitHub

“It reliably crashes from syntax errors in my code”

@valgarf · 2019-10-08 · source

“> Still an issue for me in 0.16.0”

@Kludex · 2022-01-10 · source

“Slightly depends on what you mean”

@lovelydinosaur · 2019-10-07 · source

“Ah right, I was misunderstanding slightly - I though you were talking about runtime errors in a deployed version, rather than syntax errors during development…”

@lovelydinosaur · 2019-10-09 · source

Failure Signature (Search String)

Feature Request: reload after error
Even after the subprocess crashes, uvicorn should still reload, i.e. restart the process after the next file change. Maybe with an additional command line flag

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Feature Request: reload after error
Even after the subprocess crashes, uvicorn should still reload, i.e. restart the process after the next file change. Maybe with an additional command line flag (--reload-after-error).

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Feature Request: reload after error
Even after the subprocess crashes, uvicorn should still reload, i.e. restart the process after the next file change. Maybe with an additional command line flag (--reload-after-error).

Minimal Reproduction

repro.py

from uvicorn.supervisors.statreload import HANDLED_SIGNALS, StatReload

def statreload_run(self, target, *args, **kwargs):
    pid = os.getpid()
    logger = self.config.logger_instance

    logger.info("Started reloader process [{}]".format(pid))

    for sig in HANDLED_SIGNALS:
        signal.signal(sig, self.handle_exit)

    def get_subprocess():
        spawn = multiprocessing.get_context("spawn")
        try:
            fileno = sys.stdin.fileno()
        except OSError:
            fileno = None

        return spawn.Process(
            target=self.handle_fds, args=(target, fileno), kwargs=kwargs
        )

    process = get_subprocess()
    process.start()

    # Changes are in this loop. No check if the process is alive in the loop
    while not self.should_exit:
        time.sleep(0.3)
        if self.should_restart():
            self.clear()
            if process.is_alive():
                os.kill(process.pid, signal.SIGTERM)
                process.join()

            process = get_subprocess()
            process.start()
            self.reload_count += 1

    logger.info("Stopping reloader process [{}]".format(pid))

StatReload.run = statreload_run

Environment

Python: 3.8.5

What Broke

Developers experience interruptions as the server does not reload after syntax errors.

Why It Broke

The reload process exits when a subprocess crashes due to syntax errors

Fix Options (Details)

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

Upgrade to version 0.17.0 or later.

When NOT to use: This fix is not suitable for production environments where stability is critical.

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

Fix reference: https://github.com/encode/uvicorn/pull/1313

First fixed release: 0.17.0

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

When NOT to Use This Fix

This fix is not suitable for production environments where stability is critical.

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 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.
Track RSS + object counts after deployments; alert on monotonic growth and GC pressure.
Add a long-running test that repeats the failing call path and asserts stable memory.

Version Compatibility Table

Version	Status
0.17.0	Fixed

Related Issues

No related fixes found.

Cluster: uvicorn:configuration-error All hubs All clusters

Related clusters: Data consistency Race condition SSL handshake

Sources

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