fix(tracing): keep propagated tags intact across baggage headers

[lets-order-some-fries]

What does this PR do?

No linked issue. I found this while reading propagation.py.

propagate_attributes(tags=[...], as_baggage=True) puts the tag list itself into baggage (langfuse/_client/propagation.py:606 at 0bc5897), and OTel's W3CBaggagePropagator writes every value with str(value). The receiving service passes the decoded string straight through (propagation.py:495), so its spans get langfuse.trace.tags set to the string "['tag-a', 'tag-b']" instead of a list. Only tags are affected.

The wire format stays, because Langfuse's AI gateway already parses this form (parse_tags in ai-gateway/src/telemetry/context.rs, from langfuse/langfuse#17549). So the fix is read-side only: a langfuse_tags value starting with [ is parsed as a JSON array, then as a Python list literal, and kept only if it is a list of strings. Anything else is split on ,, as the JS SDK writes it. ast.literal_eval runs no code, OTel drops baggage entries over 4096 bytes, and malformed or deeply nested input raises and falls through to the split.

Tags with commas now survive a Python-to-Python hop. A JS service still splits Python-written tags on commas; that fix belongs in langfuse-js.

The existing baggage tests stay in one process, where the in-context list hides the baggage value. The new tests extract a real header into an empty context; on main the first fails with assert "['tag-a', 'comma,tag']" == ('tag-a', 'comma,tag').

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor
• Documentation update
• Tooling, CI, or repo maintenance

Verification

List the main commands you ran:

uv sync --locked
uv run --frozen ruff check .                        # All checks passed!
uv run --frozen ruff format --check .               # flags only tests/unit/test_media.py, same on main
uv run --frozen mypy langfuse --no-error-summary    # exit 0
uv run --frozen pytest tests/unit/test_propagate_attributes.py   # 136 passed on 3.11.8, 3.10.21, 3.14.6
uv run --frozen pytest -n auto --dist worksteal tests/unit       # 712 passed, 2 skipped

The gateway's tests/fixtures/python-baggage.json carrier now reads back as its ten tags, and the SDK writes a byte-identical langfuse_tags entry for them. I did not run e2e or live_provider, since unit tests cover the change without a server.

Checklist

• I self-reviewed the diff using code_review.md.
• I added or updated tests for behavior changes.
• I updated docs, examples, or .env.template if needed. (not needed)
• I did not hand-edit generated files; if generated files changed, I used the upstream regeneration path.
• I did not commit secrets or credentials.

🤖 Generated with Claude Code

Confidence Score: 4/5

The parsing change has a narrow tag-corruption edge case, and the repository’s import-placement requirement must be satisfied before merging.

Summary

The PR reads cross-service baggage tags as lists while leaving the Python baggage wire format unchanged.

• Parses JSON arrays and Python list representations, with a comma-delimited fallback.
• Adds header-extraction tests for Python, JS-style, and JSON values.

_{Reviews (1) · Last reviewed commit: "fix(tracing): keep propagated tags intac..."}

[claude]

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[greptile-apps]

List-shaped tags change meaning A JS-originated single tag whose literal value is ["red","blue"] is parsed as two tags here. The downstream span records red and blue instead of the original tag, making trace tags unreliable for this valid value.

Knowledge Base Used: OpenTelemetry propagation and export

Prompt To Fix With AI

This is a comment left during a code review.
Path: langfuse/_client/propagation.py
Line: 563-564

Comment:
**List-shaped tags change meaning** A JS-originated single tag whose literal value is `["red","blue"]` is parsed as two tags here. The downstream span records `red` and `blue` instead of the original tag, making trace tags unreliable for this valid value.

**Knowledge Base Used:** [OpenTelemetry propagation and export](https://app.greptile.com/personal-org-4986/-/custom-context/knowledge-base/langfuse/langfuse-python/-/docs/opentelemetry-propagation-and-export.md)

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

[lets-order-some-fries]

Good catch on the edge case. A single tag whose text is itself a JSON array is ambiguous on the wire: the AI gateway's parse_tags reads ["red","blue"] as two tags as well, and the JS SDK's comma split already breaks that tag apart today. I kept the reader consistent with the gateway, but I'm happy to narrow it if you'd prefer.

[greptile-apps]

That’s a fair clarification. Given that the wire format is already ambiguous and the gateway’s parse_tags applies the same interpretation, keeping the reader aligned with the gateway is the least surprising behavior. The JS SDK’s comma-delimited format also cannot preserve a tag containing commas, so narrowing only the Python reader would not restore round-trip fidelity. I’m withdrawing this edge-case finding; no change is needed here.

Tip: You can customize Greptile's behavior for this repo with .greptile/rules.md and .greptile/config.json.