[luantaraschi]

💡 Motivation and Context

When the distinct id is a uuid.UUID, get_feature_flag, feature_enabled, get_all_flags and evaluate_flags return missing flags and no request reaches /flags. ID_TYPES includes UUID, and capture() handles it, because events go through clean(), which turns UUIDs into strings. The flags request body skips clean() and goes straight to json.dumps(..., cls=DatetimeSerializer), which raises TypeError: Object of type UUID is not JSON serializable. The SDK logs it and treats every flag as missing. A UUID group key or a UUID inside person_properties or group_properties fails the same way, in both Client and AsyncClient.

DatetimeSerializer.default now returns str(uuid) for UUIDs, which is what clean() already does for captured events. I put the fix in the serializer instead of stringifying the distinct id at the call site so that group keys and flag properties are covered too, and the sync and async request paths share it.

💚 How did you test it?

• Three new tests: test_uuid_serialization for the serializer, test_flags_sends_uuid_inputs_as_strings for sync flags() with a UUID distinct id, group key and person property, and test_async_flags_sends_uuid_inputs_as_strings for async_flags. All three fail on main with the TypeError and pass with the fix.
• End to end against a local HTTP server that returns one enabled flag: on main, client.get_feature_flag("beta", some_uuid) returns None and the server sees no /flags request. With the fix it returns True after one request. Passing str(some_uuid) gives True before and after.
• ruff format --check, ruff check (0.12.2, the version in uv.lock), the mypy baseline and python -W error -c "import posthog" are clean.
• Full suite in a python:3.12-slim container: 5850 passed on main, 5853 passed with this change, no failures in either.

📝 Checklist

• All commits are signed (see Commit signing)
• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Claude Code with Claude Opus 5.5 found this while checking posthog-python against the server scenarios in PostHog/sdk-specs, reproduced it against a local mock server, and wrote the fix and the tests. Luan directed the work.

[marandaneto]

The code looks good; no qualifying findings. Verdict: correct.

[marandaneto]

thanks @luantaraschi