feat(traces): W3C trace context ids and traceparent parsing

[turnipdabeets]

💡 Motivation and Context

First slice of native tracing for the Python SDK. Starts the posthog.tracing package with W3C Trace Context: random 16-byte trace ids and 8-byte span ids (never all zeros), plus traceparent / tracestate parsing per the spec. Invalid headers (uppercase hex, version ff, version 00 with trailing fields) are rejected, a higher version is echoed whole, and only the sampled flag is kept. A tracestate with more than 32 members or non-printable characters is discarded; one over 512 characters is trimmed by whole members.

Not reachable from the client yet.

Stack (PR 1 of 9, based on main):

1. feat(traces): W3C trace context ids and traceparent parsing #949 traces/01-ids-traceparent ← this PR
2. feat(traces): OTLP span encoding and client-side validity #950 traces/02-otlp-encoding
3. feat(traces): span handles #951 traces/03-span-handles
4. feat(traces): span batch transport #952 traces/04-transport
5. feat(traces): span pipeline #953 traces/05-pipeline
6. feat(traces): span export queue with retries #954 traces/06-export
7. feat(traces): per-span limits and exception stacktraces #955 traces/07-span-limits
8. feat(traces): before_span_send hook #956 traces/08-before-span-send
9. feat(traces): wire tracing into the client #957 traces/09-client-wiring

💚 How did you test it?

Unit tests in posthog/test/tracing/test_ids.py and test_traceparent.py cover id generation and every header validity rule above.

📝 Checklist

• 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)

Implemented with Claude Code (Claude Opus 5) against the traces spec, one commit per slice so each PR reviews on its own. Rebased onto main and opened as a stacked draft in a later Claude Code session (Claude Fable 5.1).

🤖 Generated with Claude Code

https://claude.ai/code/session_012o7CtHLfcypjmXL7g9ZGRC

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
posthog/tracing/_traceparent.py:117-121
**Malformed tracestate members accepted**

`sanitize_tracestate` only checks whether each nonempty member contains `=`, so it accepts empty members, invalid keys, empty values, multiple separators, and duplicate keys. When this value is propagated, downstream W3C implementations may discard the entire header and lose vendor state. Validate each member against the W3C key/value grammar, reject duplicates and empty members, and update the test at `posthog/test/tracing/test_traceparent.py:147-148` that currently requires empty-member preservation.

### Issue 2
posthog/tracing/_traceparent.py:13-15
**Invalid future extensions accepted**

The future-version pattern accepts any printable suffix, including invalid values such as `-extra`, spaces, or a dangling `-`. W3C future extensions are restricted to nonempty lowercase hexadecimal/hyphen fields, so malformed inbound headers can be continued and echoed instead of causing a new trace. Restrict the suffix to the future-version extension grammar and replace the tests at `posthog/test/tracing/test_traceparent.py:35-37` and `posthog/test/tracing/test_traceparent.py:98-102` with valid extension values.

---

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

_{Reviews (1): Last reviewed commit: "feat(traces): W3C trace context ids and ..." | Re-trigger Greptile}

[github-actions]

posthog-python Compliance Report

Date: 2026-09-17 03:42:54 UTC
Duration: 256033ms

✅ All Tests Passed!

111/111 tests passed

---

Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	513ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	509ms
Required Headers.Has Authorization Bearer Header	✅	508ms
Required Headers.Has Content Type Json	✅	508ms
Required Headers.Has Posthog Sdk Info Format	✅	508ms
Required Headers.Has Posthog Attempt Header	✅	508ms
Required Headers.Has Posthog Request Id	✅	509ms
Required Headers.Has Posthog Request Timestamp	✅	508ms
Required Headers.Has User Agent	✅	508ms
Body Format.Body Has Created At And Batch	✅	508ms
Body Format.No Api Key In Body	✅	509ms
Body Format.No Sent At In Body	✅	508ms
Event Format.Event Has Required Root Fields	✅	509ms
Event Format.Event Uuid Is Valid	✅	508ms
Event Format.Event Timestamp Is Rfc3339	✅	508ms
Event Format.Distinct Id Is String	✅	508ms
Event Format.Distinct Id At Root Not Properties	✅	508ms
Event Format.Custom Properties Preserved	✅	508ms
Event Format.Set Properties Preserved	✅	508ms
Event Format.Set Once Properties Preserved	✅	509ms
Event Format.Groups Properties Preserved	✅	508ms
Event Format.Sdk Generates Uuid If Not Provided	✅	508ms
Event Format.Event Has Required Root Fields Batch	✅	511ms
Event Format.Event Uuid Is Valid Batch	✅	511ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	510ms
Event Format.Distinct Id Is String Batch	✅	511ms
Event Format.Distinct Id At Root Not Properties Batch	✅	511ms
Event Format.Custom Properties Preserved Batch	✅	511ms
Event Format.Set Properties Preserved Batch	✅	511ms
Event Format.Set Once Properties Preserved Batch	✅	511ms
Event Format.Groups Properties Preserved Batch	✅	511ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	510ms
Batch Behavior.Multiple Events In Single Batch	✅	514ms
Batch Behavior.Batch Envelope Smoke	✅	513ms
Batch Behavior.Flush With No Events Sends Nothing	✅	505ms
Batch Behavior.Flush At Triggers Batch	✅	1009ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	509ms
Deduplication.Generates Unique Uuids	✅	513ms
Deduplication.Different Events Same Content Different Uuids	✅	510ms
Deduplication.Preserves Uuid On Retry	✅	6512ms
Deduplication.Preserves Timestamp On Retry	✅	6517ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6515ms
Deduplication.No Duplicate Events In Batch	✅	514ms
Header Behavior On Retry.Attempt Header Starts At One	✅	508ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13517ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6517ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3017ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6518ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	508ms
Response Format Validation.Success Response Has Ok For Each Event	✅	512ms
Response Format Validation.Success No Retry After When All Ok	✅	510ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1513ms
Response Format Validation.Success No Retry After When Drop Only	✅	510ms
Response Format Validation.Response Echoes Request Id	✅	508ms
Retry Behavior.Retries On 408	✅	6516ms
Retry Behavior.Retries On 500	✅	6518ms
Retry Behavior.Retries On 503	✅	8520ms
Retry Behavior.Retries On 504	✅	6513ms
Retry Behavior.Retryable Errors Have Retry After	✅	3514ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11514ms
Retry Behavior.Does Not Retry On 400	✅	2512ms
Retry Behavior.Does Not Retry On 401	✅	2511ms
Retry Behavior.Does Not Retry On 402	✅	2511ms
Retry Behavior.Does Not Retry On 413	✅	2512ms
Retry Behavior.Does Not Retry On 415	✅	2512ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2510ms
Retry Behavior.Implements Backoff	✅	22532ms
Retry Behavior.Max Retries Respected	✅	22520ms
Partial Batch Handling.Handles 200 Full Success	✅	2511ms
Partial Batch Handling.Handles 200 With All Ok	✅	3515ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3511ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3513ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6519ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6519ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6519ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6518ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6519ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6518ms
Partial Batch Handling.Respects Retry After On Partial	✅	8517ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3514ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3516ms
Compression.Sends Gzip Content Encoding	✅	509ms
Compression.No Content Encoding When Disabled	✅	508ms
Compression.Compressed Body Is Decompressible	✅	509ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2510ms
Event Options.Cookieless Mode Override	✅	508ms
Event Options.Disable Skew Correction Override	✅	508ms
Event Options.Process Person Profile Override	✅	508ms
Event Options.Product Tour Id Override	✅	508ms
Event Options.Unset Options Omitted	✅	508ms
Event Options.Options Override In Batch	✅	511ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	508ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	508ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	509ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	9ms
Request Payload.Flags Request Uses V2 Query Param	✅	7ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	7ms
Request Payload.Flags Request Omits Authorization Header	✅	7ms
Request Payload.Token In Flags Body Matches Init	✅	7ms
Request Payload.Groups Round Trip	✅	7ms
Request Payload.Groups Default To Empty Object	✅	7ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	7ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	7ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	7ms
Request Lifecycle.No Flags Request On Init Alone	✅	2ms
Request Lifecycle.No Flags Request On Normal Capture	✅	508ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	11ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	7ms
Retry Behavior.Retries Flags On 502	✅	310ms
Retry Behavior.Retries Flags On 504	✅	310ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	510ms

[marandaneto]

Automated advisory code review.

[jzhu13]

Reviewed against main. Tracing tests pass at the head, ruff and mypy are clean, and the traceparent parsing checks out against the W3C spec (version ff, trailing fields on version 00, all-zero ids, short higher-version headers, unknown flag bits). No blocking issues. Approving with a few non-blocking notes.

Non-blocking

1. posthog/tracing/_traceparent.py:90 traceparent_header: an unusable explicit parent ends up handled three different ways downstream. A two-element header list becomes a child of the active span (PR 5), a garbage string starts a new root, and inert_span returns NOOP_SPAN even when a pass-through span is active (PR 3). Worth picking one fallback here, documenting it, and aligning #951 and #953.
2. posthog/tracing/_traceparent.py:106 sanitize_tracestate is half-validation: it rejects members without = but forwards empty keys, uppercase keys, vendor=a=b, and duplicate keys. The spec allows pass-through, so either validate the full key/value grammar (OTel Python does) or treat the header as opaque printable ASCII and say so. The current = check protects nothing a strict downstream would not drop anyway.
3. posthog/tracing/_ids.py:1 docstring says every id is validated before it ships. Nothing in the stack validates at export; ids are valid by construction (generated here or accepted from a validated header). Reword, and drop the is_valid_* re-check inside the regex-matched parser except for the all-zero compare.
4. TRACE_FLAGS_SAMPLED is "01" here and 0x01 in #950's _otlp.py. Same name, two types, same package. Rename one.
5. Nits: the length-cap comment at _traceparent.py:17 talks about tracestate; _TRACESTATE_FORBIDDEN_RE also validates traceparent trailing fields; a bytes header value (raw ASGI scope) silently becomes "no parent".

Reviewed with Claude Code (Claude Fable 5.1). Behaviors above were checked by probe against this branch head.

[turnipdabeets]

Thanks for the review. Addressed in 205db65:

• 3: parser keeps only the all-zero compare, unused validators removed, docstring reworded.
• 4: the int constant is now SAMPLED_FLAG_BIT (in feat(traces): OTLP span encoding and client-side validity #950).
• 5: comment and regex renamed, a bytes header value is decoded.
• 1: a blank string now means the default parent (feat(traces): span pipeline #953). Unifying the other malformed cases is still open.
• 2: declining for now, our sanitizer matches Node's exactly. Aligning both SDKs on the full grammar is a follow-up.

[turnipdabeets]

Follow-up: item 1 is now done in #953 (78b6318). A non-blank unusable parent (garbage string, two-value list, foreign span object) starts a new trace, in both the recording and inert paths. Ready for another look.

[turnipdabeets]

One more on item 5, d331639: an end-to-end run showed a bytes traceparent still started a new trace, because the pipeline checks for str before the parser sees it. The header helper now decodes it (a pipeline test is in #953).