[alphasafal]

Fixes #8120

Summary

Baggage.serialize(include_third_party=True) appends third_party_items unconditionally. When there are no third-party items (third_party_items == ""), it appends an empty segment, so ",".join(...) leaves a trailing comma in the serialized baggage header:

sentry-trace_id=...,sentry-public_key=...,sentry-sample_rate=0.5,

Per the W3C Baggage spec a trailing comma is an empty list-member (malformed). Sentry's own parser tolerates it, but a strict W3C / OpenTelemetry baggage parser on a downstream consumer can reject or mishandle the header.

Where it's reachable

The Celery integration calls combined.serialize(include_third_party=True) (sentry_sdk/integrations/celery/__init__.py) when merging incoming and Sentry baggage. Whenever the merged baggage has only Sentry items and no third-party ones — the common Sentry → Celery → Sentry propagation case — the outgoing baggage header gets the stray comma.

Fix

Only append third_party_items when it is non-empty:

if include_third_party and self.third_party_items:
    items.append(self.third_party_items)

The output is unchanged when third-party items are present.

Test

Adds test_sentry_only_baggage in tests/tracing/test_baggage.py: it verifies that baggage containing only Sentry items serializes to the expected header, with identical output whether third-party items are included or not.

Validation on Python 3.13.7:

• TESTPATH=tests/tracing/test_baggage.py uv run tox -e py3.13-common -- --no-cov: 4 passed.
• Restoring the original if include_third_party: condition makes test_sentry_only_baggage fail on the trailing comma (1 failed, 3 passed); restoring the fix gives 4 passed.
• Ruff lint passed for tests and sentry_sdk; formatting checks passed for the changed test files.
• uv run --group typing mypy sentry_sdk: no issues in 203 source files.

[Copilot]

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[alphasafal]

Ran the baggage tests locally against this branch — tests/tracing/test_baggage.py: 4 passed, including the new test_serialize_no_trailing_comma_without_third_party_items, with no change to the existing cases.

[chrikrah]

@alphasafal CONTRIBUTING auto-closes outside PRs without a linked, maintainer-acknowledged issue. Otherwise, from my perspective this can be merged at e8f8ace.

# probe_celery.py: _update_celery_task_headers({"baggage": "sentry-trace_id=771a43a4192642f0b136d5159a501700,sentry-sample_rate=0.5"}, span, False)
$ python probe_celery.py   # e8f8ace with the base condition (if include_third_party:)
'sentry-trace_id=771a43a4192642f0b136d5159a501700,sentry-sample_rand=0.395125,sentry-environment=production,sentry-transaction=t,sentry-sample_rate=0.5,sentry-sampled=true,'
$ python probe_celery.py   # e8f8ace
'sentry-trace_id=771a43a4192642f0b136d5159a501700,sentry-sample_rand=0.051087,sentry-environment=production,sentry-transaction=t,sentry-sample_rate=0.5,sentry-sampled=true'

$ pytest tests/tracing/test_baggage.py tests/integrations/celery/test_update_celery_task_headers.py -q --no-cov
42 passed, 4 warnings in 4.42s                       # e8f8ace, Python 3.12
1 failed, 41 passed, 4 warnings in 1.11s             # base condition restored, test kept
FAILED tests/tracing/test_baggage.py::test_serialize_no_trailing_comma_without_third_party_items

# test_span_with_transaction_custom_headers, the one Celery merge test, sends custom=value
# and expects combined_baggage.serialize(include_third_party=True): it passes on both

non-blocking: add a Celery test without third-party baggage?

[alphasafal]

@chrikrah thanks for running the Celery probe and checking it against the base condition. I've opened #8120 for the bug and linked it in the description, and I'll add a Celery test with no third-party baggage so that path is covered directly.

[alphasafal]

Added the Celery test for baggage without third-party items in 0bc5679. It passes with the fix and fails on the base condition. Marking this ready for review. @chrikrah could a maintainer also acknowledge #8120 so this isn't auto-closed?

[chrikrah]

@alphasafal the Celery test fails on the base condition; nothing blocks a merge.

$ pytest tests/tracing/test_baggage.py tests/integrations/celery/test_update_celery_task_headers.py -q --no-cov -p no:cacheprovider   # export of 0bc5679, sentry_sdk imported from it, Python 3.12.3, celery 5.6.3
43 passed, 4 warnings
2 failed, 41 passed, 4 warnings    # base condition `if include_third_party:` restored, tests kept
FAILED tests/tracing/test_baggage.py::test_serialize_no_trailing_comma_without_third_party_items
FAILED tests/integrations/celery/test_update_celery_task_headers.py::test_span_with_transaction_custom_headers_without_third_party_baggage

My 10-25 auto-close warning was wrong: validate-pr.yml only runs on open or reopen, so #8120 needs nothing.

[pabloDeputter]

Thanks for picking this up! Looks good to me now 😃