fix(observability): start every lfx serve request task from a clean context

[ogabrielluiz]

lfx serve never opts into uvicorn's per-request context reset, so the defect fixed for langflow in #14769 is still live on the runtime that actually serves traffic. Reported by Rafael Gil on 27 August while verifying that fix.

The gap

#14769 set reset_contextvars on both langflow launch paths. lfx serve runs the same HTTP instrumentation (instrument_fastapi_app is documented as serving both runtimes) but has three launch paths of its own, and none of them set it:

• commands.py single worker, the default
• commands.py multi-worker on Windows
• LFXUvicornWorker, which declared no CONFIG_KWARGS and so inherited only the base worker's loop and http

Mechanism

A request that arrives while another is still in flight on the same connection is queued as a pipelined request, and uvicorn starts it from inside the finishing request's task (httptools_impl.on_response_complete → _start_asgi_task). create_task copies the context, so the new request begins with the previous request's already-ended server span still current. OpenTelemetry's ASGI middleware reads that as nesting and emits the request as an INTERNAL child of an unrelated finished request rather than as a SERVER root.

Unrelated traces merge into one whose duration includes idle time, and most HTTP traffic disappears from RED metrics and service maps.

Measured, not assumed

A real lfx serve with an OTLP receiver attached, sending 12 GET /health in a single socket write so uvicorn pipelines requests 2..12. Same build, same flow, the only difference being the flag:

	request spans	SERVER	demoted	traces
before	13	2	11 (84.6%)	2
after	13	13	0 (0%)	13

Sequential requests never reproduce it, which is why this survived normal testing. Rafael measured 92% demoted on his probe; same shape.

Tests

Two, covering all three paths:

• the gunicorn worker, asserted through a real uvicorn.Config so a renamed or rejected option fails rather than a dict key check passing. It also asserts the inherited keys survive: CONFIG_KWARGS is spread over the base class's, and replacing it outright would silently drop uvicorn's loop and http selection.
• the two uvicorn.run calls, read from the source with ast rather than by patching uvicorn.run. This is the guard that would have caught the original gap, and it fails for a fourth launch path added later that does not opt in.

Mutation checked: dropping the flag from either uvicorn.run, dropping it from the worker, and replacing CONFIG_KWARGS instead of spreading it each fail, and each fails in the test that should catch it.

Known gaps

test_serve_env_isolation_integration.py has two multi-worker tests that fail intermittently under full-suite load with RemoteDisconnected. I ran the full CLI suite three times: on the unmodified base one of them fails, with this change the other one does, and both pass in isolation. Different subset each run, so it looks like flakiness in that file rather than anything here, but I have not chased it down.

Rafael also reported that lfx serve --workers 2 exports no spans at all (0 against 48 for single worker). That is separate from this and not addressed here. He offered a fork/exporter-thread mechanism as a candidate rather than a claim, and I have not measured it.

Summary by CodeRabbit

• Bug Fixes

  • Improved request context isolation for all server launch modes.
  • Prevented telemetry and request context from carrying over between pipelined requests.
  • Ensured consistent behavior for single-worker, multi-worker Windows, and Gunicorn-based deployments.

• Tests

  • Added coverage verifying clean request contexts across supported server configurations.

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Walkthrough

The lfx serve command now resets context variables for single-worker, Windows multi-worker, and Gunicorn launches. New tests verify the reset flag and preservation of inherited Uvicorn worker options.

Changes

Request context isolation

Layer / File(s)	Summary
Serve launch configuration src/lfx/src/lfx/cli/commands.py, src/lfx/src/lfx/cli/serve_gunicorn.py	All Uvicorn launch paths set reset_contextvars=True. LFXUvicornWorker preserves base configuration options while adding the reset setting.
Serve launch validation src/lfx/tests/unit/cli/test_serve_request_context_isolation.py	Tests verify the Gunicorn configuration and inspect every uvicorn.run(...) call for a literal reset_contextvars=True argument.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🟡 Moderate · up to 73067

This change fixes request-trace isolation across all LFX serve modes, but Uvicorn 0.34.3 rejects the new configuration option and can prevent the service from starting at all. The PR is not merge-ready until it requires a compatible Uvicorn version or handles older versions explicitly.

Suggested reviewers: jordanrfrazier

🚥 Pre-merge checks | ✅ 7 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Test Quality And Coverage	⚠️ Warning	The new tests validate the Unix worker configuration through uvicorn.Config and inspect both uvicorn.run call sites. However, line 25 applies pytest.importorskip("gunicorn") at module scope. `gu…	Move the Gunicorn availability skip into test_the_gunicorn_worker_resets_the_context_and_keeps_its_inherited_options (or mark only that test as Unix-only). Keep the Uvicorn AST test independent of Gunicorn so it runs on Windows. Also add …

✅ Passed checks (7 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Test Coverage For New Implementations	✅ Passed	The PR adds a new regression test file at src/lfx/tests/unit/cli/test_serve_request_context_isolation.py, which follows the project's test_*.py convention. The worker test passes `LFXUvicornWorker…
Test File Naming And Structure	✅ Passed	The added backend test is correctly named test_serve_request_context_isolation.py and is located under src/lfx/tests/unit/cli. LFX pytest configuration discovers test_*.py files and test_* fun…
Excessive Mock Usage Warning	✅ Passed	PASS: The added test file uses no mock objects or mock frameworks. It validates the worker with a real uvicorn.Config and inspects uvicorn.run calls with Python AST parsing. Existing mocks in othe…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: enabling clean request context for every lfx serve launch path.

Full details: Test Coverage For New Implementations

Explanation

The PR adds a new regression test file at src/lfx/tests/unit/cli/test_serve_request_context_isolation.py, which follows the project's test_*.py convention. The worker test passes LFXUvicornWorker.CONFIG_KWARGS to a real uvicorn.Config and checks reset_contextvars=True plus inherited options. The AST-based test checks both uvicorn.run launch sites in commands.py. These tests directly cover the three changed launch paths, so the explicit coverage requirements are met.

Full details: Test Quality And Coverage

Explanation

The new tests validate the Unix worker configuration through uvicorn.Config and inspect both uvicorn.run call sites. However, line 25 applies pytest.importorskip("gunicorn") at module scope. gunicorn is conditional on sys_platform != 'win32' in src/lfx/pyproject.toml, so Windows skips both tests, including the only new assertion for the Windows multi-worker launch. The existing Windows test checks the fallback and factory=True, but it does not check reset_contextvars. The added coverage therefore does not cover all three launch paths on a supported platform.

Resolution

Move the Gunicorn availability skip into test_the_gunicorn_worker_resets_the_context_and_keeps_its_inherited_options (or mark only that test as Unix-only). Keep the Uvicorn AST test independent of Gunicorn so it runs on Windows. Also add assert called["kwargs"]["reset_contextvars"] is True to the existing Windows fallback test, and assert the equivalent keyword in the single-worker launch test, so the runtime launch paths are checked in addition to the source-level guard.

Full details: Test File Naming And Structure

Explanation

The added backend test is correctly named test_serve_request_context_isolation.py and is located under src/lfx/tests/unit/cli. LFX pytest configuration discovers test_*.py files and test_* functions. Both test functions have descriptive names and use standard pytest assertions. The tests cover the worker configuration and both uvicorn.run launch paths. No integration test is required because these are unit-level source and configuration checks, and no setup or teardown is needed because the tests do not allocate external resources.

Full details: Excessive Mock Usage Warning

Explanation

PASS: The added test file uses no mock objects or mock frameworks. It validates the worker with a real uvicorn.Config and inspects uvicorn.run calls with Python AST parsing. Existing mocks in other CLI tests are unchanged and do not indicate excessive mock usage introduced by this pull request.

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• Commit unit tests in branch fix-lfx-serve-contextvars

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✅ Test Coverage Advisor

No source changes detected without accompanying tests. Thanks for keeping coverage up! 🎉

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Actionable comments posted: 2

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Inline comments:
In `@src/lfx/src/lfx/cli/commands.py`:
- Line 574: Require a Uvicorn version that supports reset_contextvars in
uvicorn.run and Config, or remove that keyword for compatibility with older
releases. Update both uvicorn.run sites in src/lfx/src/lfx/cli/commands.py
(lines 574-574 and 732-732) and the UvicornWorker CONFIG_KWARGS site in
src/lfx/src/lfx/cli/serve_gunicorn.py (line 43); ensure the dependency
constraint covers every affected path.

In `@src/lfx/tests/unit/cli/test_serve_request_context_isolation.py`:
- Line 25: Remove the module-level Gunicorn import skip and place the skip
inside
test_the_gunicorn_worker_resets_the_context_and_keeps_its_inherited_options, so
test_every_uvicorn_launch_in_the_serve_command_resets_the_context still runs
when Gunicorn is unavailable.

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📒 Files selected for processing (3)

• src/lfx/src/lfx/cli/commands.py
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Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release-1.13.0@8a56d1b). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             release-1.13.0   #14908   +/-   ##
=================================================
  Coverage                  ?   66.49%           
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  Lines                     ?   261603           
  Branches                  ?    36772           
=================================================
  Hits                      ?   173956           
  Misses                    ?    85483           
  Partials                  ?     2164           

Flag	Coverage Δ
backend	74.14% <ø> (?)
lfx	64.86% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/lfx/src/lfx/cli/commands.py	76.61% <100.00%> (ø)
src/lfx/src/lfx/cli/serve_gunicorn.py	73.46% <100.00%> (ø)

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Frontend Unit Test Coverage Report

Coverage Summary

Lines	Statements	Branches	Functions
	55.65% (84754/152292)	72.47% (12552/17320)	50.95% (2000/3925)

Unit Test Results

Tests	Skipped	Failures	Errors	Time
6641	0 💤	0 ❌	0 🔥	27m 9s ⏱️

[erichare]

@ogabrielluiz I pushed d1f16a4 to address both review findings: standalone LFX now requires Uvicorn >=0.45.0, and the Gunicorn skip is scoped to the worker-specific test so the independent launch-path check still runs without Gunicorn. Local verification: 77 focused serve tests passed, Ruff/format and uv lock --check passed, and Uvicorn 0.45.0 accepts reset_contextvars. Approved; replacement CI is running.

[ogabrielluiz]

Thanks Eric, both of those were right and I had not caught the uvicorn one before you pushed.

I checked the floor independently: reset_contextvars is absent from 0.34.3 through 0.44.0 and first appears in 0.45.0, so >=0.45.0 is exactly the boundary, and it matches what langflow already requires. Without it lfx serve would have raised a TypeError at startup on a resolver that picked an older uvicorn.

Re-ran the two tests on your head, both pass, and the flag is still on all three paths after the 1.13 merge.

One thing worth saying out loud since the base moved to release-1.13.0: this does not ship in 1.12, so 1.12 still demotes most lfx serve request spans. Fine by me, just want it written down somewhere.