fix(integrations): align tracing-header capture across frameworks

[dustinbyrne]

💡 Motivation and Context

Restore browser-to-backend attribution without requiring a second opt-in after installing a request integration, and align Python's configuration with server SDK peers.

• Add capture_tracing_headers for ASGI/Flask and POSTHOG_MW_CAPTURE_TRACING_HEADERS for Django, enabled by default once installed.
• Keep the trust-named options as deprecated aliases. Explicit legacy opt-outs still work; the capture-named option wins when both are configured. Preserve ASGI's existing positional arguments and mutable compatibility attributes.
• Share sanitization and attribution logic across all three integrations. Repeated ASGI tracing headers use the first usable value instead of letting an empty value suppress later valid attribution.
• Keep explicit event IDs and $session_id properties authoritative. Preserve Django's existing authenticated-user fallback and request-context lifecycle/privacy behavior.

Compared current Node, Ruby/Rails, PHP, Go, Java, .NET, Elixir and Rust implementations against the tracing-header specification. The spec permits default-on server extraction after middleware installation and does not prescribe one server option name. Peer authentication policies differ, so this change preserves Python's existing identity precedence rather than changing it incidentally. Tracing IDs remain analytics context, never authentication or authorization evidence.

Compatibility/release: The effective default becomes enabled for all three integrations; applications that omit the options will now consume attribution headers. Explicit False remains an opt-out. Includes a minor Sampo changeset, configuration docs and public API snapshot updates. The generated posthoganalytics mirror receives the same behavior.

💚 How did you test it?

• Full suite: 4,692 passed, 16 skipped, 56 subtests passed.
• Request integration suite: 236 passed, including all four public framework paths (Django sync/async, ASGI and Flask).
• Regression: blank/control-only first ASGI duplicate failed before the fix and passed afterward; valid first values still win.
• Cross-framework tests cover defaults/opt-outs, alias precedence/deprecation, sanitization, explicit capture/application identity precedence, session-only personless capture, no-identity flag evaluation without network calls, and context restoration.
• Ruff lint/format, mypy baseline, public API snapshot, warnings-as-errors import and whitespace checks pass.
• Isolated generated-mirror smoke tests passed all four framework paths alongside an application-owned posthog module.
• Inspected the current backend normalization/storage consumers for the existing distinct_id, $session_id and personless properties; no wire-format changes.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• Default behavior and compatibility changes are documented in the changeset.

If releasing new changes

• Ran sampo add to generate a changeset file.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented using Pi with read-only scout/reviewer subagents, uv, pytest, Ruff, mypy and GitHub CLI. Peer comparison and simplification preceded the independent pre-PR review. The integration layer handles sanitized request attribution; the existing capture pipeline remains responsible for explicit-value precedence. Human review is required before merging.

[stamphog]

Not approved — this change needs a human reviewer.

Re-adding the stamphog label gives the same result unless the changed files change.

stamphog won't auto-review this pull request because two gates refused it. The deny-list gate flagged it for touching public_api, which here is the references/public_api_snapshot.txt update and the new capture_tracing_headers and POSTHOG_MW_CAPTURE_TRACING_HEADERS options. The tier gate classified it as T2-never because it is a cross-cutting fix (623 lines across 12 files, touching the ASGI, Django and Flask integrations together). The size gate passed, so the refusal is about the public API surface and the cross-cutting scope, not the line count.

Please ask a human reviewer to look at it, since the PR description already notes that human review is required. If you want it to be easier to review, you could split the shared _tracing_headers.py helper and the per-framework changes into separate pull requests, and keep the public API and default-behavior change on its own.

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✗	matches: public_api
size	✓	262L, 4F substantive, 623L/12F incl. docs/generated/snapshots — within ceiling
tier	✗	classified as T2-never: T2-never (623L, 12F, cross-cutting, fix)
stamphog 2.5.1		.stamphog/policy.yml @ unknown · reviewed head 1432df6

[github-actions]

posthog-python Compliance Report

Date: 2026-10-14T18:27:44.065459+00:00
Duration: 259971ms

✅ All Tests Passed!

121/121 tests passed

---

Capture_V1 Tests

✅ 95/95 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	518ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	512ms
Required Headers.Has Authorization Bearer Header	✅	511ms
Required Headers.Has Content Type Json	✅	511ms
Required Headers.Has Posthog Sdk Info Format	✅	511ms
Required Headers.Has Posthog Attempt Header	✅	511ms
Required Headers.Has Posthog Request Id	✅	510ms
Required Headers.Has Posthog Request Timestamp	✅	512ms
Required Headers.Has User Agent	✅	511ms
Body Format.Body Has Created At And Batch	✅	510ms
Body Format.No Api Key In Body	✅	511ms
Body Format.No Sent At In Body	✅	511ms
Event Format.Event Has Required Root Fields	✅	510ms
Event Format.Event Uuid Is Valid	✅	511ms
Event Format.Event Timestamp Is Rfc3339	✅	511ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	519ms
Event Format.Distinct Id Is String	✅	518ms
Event Format.Distinct Id At Root Not Properties	✅	511ms
Event Format.Custom Properties Preserved	✅	510ms
Event Format.Set Properties Preserved	✅	512ms
Event Format.Set Once Properties Preserved	✅	511ms
Event Format.Groups Properties Preserved	✅	510ms
Event Format.Sdk Generates Uuid If Not Provided	✅	512ms
Event Format.Event Has Required Root Fields Batch	✅	514ms
Event Format.Event Uuid Is Valid Batch	✅	514ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	514ms
Event Format.Distinct Id Is String Batch	✅	514ms
Event Format.Distinct Id At Root Not Properties Batch	✅	514ms
Event Format.Custom Properties Preserved Batch	✅	514ms
Event Format.Set Properties Preserved Batch	✅	515ms
Event Format.Set Once Properties Preserved Batch	✅	514ms
Event Format.Groups Properties Preserved Batch	✅	514ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	515ms
Batch Behavior.Multiple Events In Single Batch	✅	519ms
Batch Behavior.Batch Envelope Smoke	✅	515ms
Batch Behavior.Flush With No Events Sends Nothing	✅	507ms
Batch Behavior.Flush At Triggers Batch	✅	1010ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	511ms
Deduplication.Generates Unique Uuids	✅	519ms
Deduplication.Different Events Same Content Different Uuids	✅	512ms
Deduplication.Preserves Uuid On Retry	✅	6521ms
Deduplication.Preserves Timestamp On Retry	✅	6520ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6524ms
Deduplication.No Duplicate Events In Batch	✅	519ms
Header Behavior On Retry.Attempt Header Starts At One	✅	512ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13529ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6516ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3021ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6520ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	511ms
Response Format Validation.Success Response Has Ok For Each Event	✅	515ms
Response Format Validation.Success No Retry After When All Ok	✅	513ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1516ms
Response Format Validation.Success No Retry After When Drop Only	✅	513ms
Response Format Validation.Response Echoes Request Id	✅	511ms
Retry Behavior.Retries On 408	✅	6520ms
Retry Behavior.Retries On 500	✅	6520ms
Retry Behavior.Retries On 503	✅	8521ms
Retry Behavior.Retries On 504	✅	6520ms
Retry Behavior.Retryable Errors Have Retry After	✅	3516ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11524ms
Retry Behavior.Does Not Retry On 400	✅	2514ms
Retry Behavior.Does Not Retry On 401	✅	2515ms
Retry Behavior.Does Not Retry On 402	✅	2513ms
Retry Behavior.Does Not Retry On 413	✅	2514ms
Retry Behavior.Does Not Retry On 415	✅	2515ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2514ms
Retry Behavior.Implements Backoff	✅	22532ms
Retry Behavior.Max Retries Respected	✅	22534ms
Partial Batch Handling.Handles 200 Full Success	✅	2513ms
Partial Batch Handling.Handles 200 With All Ok	✅	3517ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3517ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3519ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6522ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6522ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6524ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6518ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6521ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6519ms
Partial Batch Handling.Respects Retry After On Partial	✅	8518ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3518ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3518ms
Compression.Sends Gzip Content Encoding	✅	512ms
Compression.No Content Encoding When Disabled	✅	510ms
Compression.Compressed Body Is Decompressible	✅	511ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2513ms
Event Options.Cookieless Mode Override	✅	512ms
Event Options.Disable Skew Correction Override	✅	511ms
Event Options.Process Person Profile Override	✅	510ms
Event Options.Product Tour Id Override	✅	511ms
Event Options.Unset Options Omitted	✅	510ms
Event Options.Options Override In Batch	✅	515ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	510ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	511ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	510ms

Capture_Ai Tests

✅ 5/5 tests passed

View Details

Test	Status	Duration
Routing.Capture Ai Posts To Ai Endpoint	✅	510ms
Routing.Capture Does Not Reroute Ai Named Events	✅	511ms
Identity.Capture Ai Event Has Uuid	✅	510ms
Identity.Capture Ai Keeps Supplied Uuid	✅	511ms
Timestamp Format.Non Utc Event Timestamp Is Converted To Utc	✅	511ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	12ms
Request Payload.Flags Request Uses V2 Query Param	✅	11ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	11ms
Request Payload.Flags Request Omits Authorization Header	✅	10ms
Request Payload.Token In Flags Body Matches Init	✅	9ms
Request Payload.Groups Round Trip	✅	10ms
Request Payload.Groups Default To Empty Object	✅	11ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	11ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	10ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	9ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	510ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	15ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	10ms
Retry Behavior.Retries Flags On 502	✅	314ms
Retry Behavior.Retries Flags On 504	✅	314ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	513ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	74ms
Versioned Boolean Matching.Matching Version 1	✅	70ms
Versioned Boolean Matching.Matching Version 2	✅	70ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	40ms