feat(traces): before_span_send hook

[turnipdabeets]

💡 Motivation and Context

Adds the traces before_span_send option: a callable, or a list run in order, that receives each finished span as a plain dict (like the events before_send hook) and returns it edited, or None to drop it.

It is the documented place to scrub sensitive values, so a hook that raises drops the span rather than exporting it unscrubbed. trace_id, span_id and parent_span_id are read-only. Names, times, status and events are re-sanitized and the per-span limits re-applied to whatever the hook returns. The hook runs with no tracing lock held, and entries that are not callable are ignored with a warning.

Not reachable from the client yet.

Stack (PR 8 of 9, based on traces/07-span-limits):

1. feat(traces): W3C trace context ids and traceparent parsing #949 traces/01-ids-traceparent
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 ← this PR
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_pipeline.py and test_config.py cover edit, drop, raise, read-only ids, re-sanitizing and non-callable entries.

📝 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

[github-actions]

posthog-python Compliance Report

Date: 2026-09-16 18:11:46 UTC
Duration: 256066ms

✅ 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	✅	514ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	508ms
Required Headers.Has Authorization Bearer Header	✅	508ms
Required Headers.Has Content Type Json	✅	509ms
Required Headers.Has Posthog Sdk Info Format	✅	509ms
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	✅	509ms
Body Format.Body Has Created At And Batch	✅	508ms
Body Format.No Api Key In Body	✅	509ms
Body Format.No Sent At In Body	✅	509ms
Event Format.Event Has Required Root Fields	✅	508ms
Event Format.Event Uuid Is Valid	✅	509ms
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	✅	509ms
Event Format.Set Properties Preserved	✅	508ms
Event Format.Set Once Properties Preserved	✅	508ms
Event Format.Groups Properties Preserved	✅	509ms
Event Format.Sdk Generates Uuid If Not Provided	✅	511ms
Event Format.Event Has Required Root Fields Batch	✅	511ms
Event Format.Event Uuid Is Valid Batch	✅	511ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	511ms
Event Format.Distinct Id Is String Batch	✅	511ms
Event Format.Distinct Id At Root Not Properties Batch	✅	510ms
Event Format.Custom Properties Preserved Batch	✅	512ms
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	✅	511ms
Batch Behavior.Multiple Events In Single Batch	✅	514ms
Batch Behavior.Batch Envelope Smoke	✅	512ms
Batch Behavior.Flush With No Events Sends Nothing	✅	505ms
Batch Behavior.Flush At Triggers Batch	✅	1010ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	509ms
Deduplication.Generates Unique Uuids	✅	515ms
Deduplication.Different Events Same Content Different Uuids	✅	510ms
Deduplication.Preserves Uuid On Retry	✅	6517ms
Deduplication.Preserves Timestamp On Retry	✅	6517ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6517ms
Deduplication.No Duplicate Events In Batch	✅	514ms
Header Behavior On Retry.Attempt Header Starts At One	✅	509ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13524ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6518ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3017ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6513ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	510ms
Response Format Validation.Success Response Has Ok For Each Event	✅	511ms
Response Format Validation.Success No Retry After When All Ok	✅	510ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1514ms
Response Format Validation.Success No Retry After When Drop Only	✅	510ms
Response Format Validation.Response Echoes Request Id	✅	508ms
Retry Behavior.Retries On 408	✅	6517ms
Retry Behavior.Retries On 500	✅	6518ms
Retry Behavior.Retries On 503	✅	8516ms
Retry Behavior.Retries On 504	✅	6513ms
Retry Behavior.Retryable Errors Have Retry After	✅	3514ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11515ms
Retry Behavior.Does Not Retry On 400	✅	2512ms
Retry Behavior.Does Not Retry On 401	✅	2511ms
Retry Behavior.Does Not Retry On 402	✅	2512ms
Retry Behavior.Does Not Retry On 413	✅	2512ms
Retry Behavior.Does Not Retry On 415	✅	2511ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2511ms
Retry Behavior.Implements Backoff	✅	22532ms
Retry Behavior.Max Retries Respected	✅	22524ms
Partial Batch Handling.Handles 200 Full Success	✅	2511ms
Partial Batch Handling.Handles 200 With All Ok	✅	3513ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3514ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3511ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6521ms
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	✅	6515ms
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	✅	3512ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3517ms
Compression.Sends Gzip Content Encoding	✅	509ms
Compression.No Content Encoding When Disabled	✅	509ms
Compression.Compressed Body Is Decompressible	✅	508ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2511ms
Event Options.Cookieless Mode Override	✅	509ms
Event Options.Disable Skew Correction Override	✅	509ms
Event Options.Process Person Profile Override	✅	509ms
Event Options.Product Tour Id Override	✅	508ms
Event Options.Unset Options Omitted	✅	508ms
Event Options.Options Override In Batch	✅	512ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	510ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	509ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	508ms

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	✅	8ms
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	✅	311ms
Retry Behavior.Retries Flags On 504	✅	310ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	510ms

[greptile-apps]

Important Files Changed

Filename	Overview
posthog/tracing/_before_span_send.py	Runs hooks without tracing locks, restores span IDs, rebuilds safe records, and drops spans when a hook fails.
posthog/tracing/_pipeline.py	Runs before_span_send before queueing and safely handles close races and span setup errors.
posthog/tracing/_limits.py	Reapplies span and event limits after hooks. Callable values still reach the safe encoder path.
posthog/tracing/_config.py	Accepts one hook or a list and skips invalid entries with a warning.
posthog/test/tracing/test_pipeline.py	Covers hook edits, drops, errors, limits, lock use, close races, and span setup cleanup.

_{Reviews (2): Last reviewed commit: "fix(traces): keep a falsy status message..." | Re-trigger Greptile}