fix(ai): preserve Anthropic messages stream API

[marandaneto]

💡 Motivation and Context

PostHog's Anthropic wrapper returned tracked generators from messages.stream() instead of Anthropic's native stream managers. That broke drop-in compatibility for sync and async callers relying on the provider's with/async with contract, text_stream, response helpers, and close behavior. The async wrapper also incorrectly required awaiting stream().

This change preserves Anthropic's native manager API while instrumenting the raw stream underneath it. It uses base Anthropic resources to avoid re-entering the tracked create(stream=True) override on older supported Anthropic versions, preventing double capture and preserving privacy options.

💚 How did you test it?

• python -m pytest -q posthog/test/ai/anthropic/test_anthropic.py (37 passed, 1 skipped)
• ruff check posthog/ai/stream.py posthog/ai/anthropic/anthropic.py posthog/ai/anthropic/anthropic_async.py posthog/test/ai/anthropic/test_anthropic.py
• ruff format --check posthog/ai/stream.py posthog/ai/anthropic/anthropic.py posthog/ai/anthropic/anthropic_async.py posthog/test/ai/anthropic/test_anthropic.py
• Focused mypy validation of the three runtime modules
• git diff --check

📝 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)

A human directed a Pi worker to implement and prepare this compatibility fix. Pi autoreview identified an older-version double-tracking/privacy risk; the implementation was adjusted to construct base Anthropic resources, and the final autoreview rerun had no actionable findings. Human review is still required before merge.

[github-actions]

posthog-python Compliance Report

Date: 2026-08-01 06:46:10 UTC
Duration: 338704ms

✅ 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	✅	1009ms
Required Headers.Has Authorization Bearer Header	✅	1009ms
Required Headers.Has Content Type Json	✅	1009ms
Required Headers.Has Posthog Sdk Info Format	✅	1008ms
Required Headers.Has Posthog Attempt Header	✅	1009ms
Required Headers.Has Posthog Request Id	✅	1008ms
Required Headers.Has Posthog Request Timestamp	✅	1008ms
Required Headers.Has User Agent	✅	1009ms
Body Format.Body Has Created At And Batch	✅	1008ms
Body Format.No Api Key In Body	✅	1009ms
Body Format.No Sent At In Body	✅	1008ms
Event Format.Event Has Required Root Fields	✅	1008ms
Event Format.Event Uuid Is Valid	✅	1008ms
Event Format.Event Timestamp Is Rfc3339	✅	1008ms
Event Format.Distinct Id Is String	✅	1009ms
Event Format.Distinct Id At Root Not Properties	✅	1009ms
Event Format.Custom Properties Preserved	✅	1009ms
Event Format.Set Properties Preserved	✅	1009ms
Event Format.Set Once Properties Preserved	✅	1007ms
Event Format.Groups Properties Preserved	✅	1008ms
Event Format.Sdk Generates Uuid If Not Provided	✅	1010ms
Event Format.Event Has Required Root Fields Batch	✅	1013ms
Event Format.Event Uuid Is Valid Batch	✅	1011ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	1012ms
Event Format.Distinct Id Is String Batch	✅	1012ms
Event Format.Distinct Id At Root Not Properties Batch	✅	1011ms
Event Format.Custom Properties Preserved Batch	✅	1012ms
Event Format.Set Properties Preserved Batch	✅	1013ms
Event Format.Set Once Properties Preserved Batch	✅	1012ms
Event Format.Groups Properties Preserved Batch	✅	1012ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	1012ms
Batch Behavior.Multiple Events In Single Batch	✅	1506ms
Batch Behavior.Batch Envelope Smoke	✅	1014ms
Batch Behavior.Flush With No Events Sends Nothing	✅	1005ms
Batch Behavior.Flush At Triggers Batch	✅	1508ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	1010ms
Deduplication.Generates Unique Uuids	✅	1506ms
Deduplication.Different Events Same Content Different Uuids	✅	1506ms
Deduplication.Preserves Uuid On Retry	✅	7514ms
Deduplication.Preserves Timestamp On Retry	✅	7507ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7512ms
Deduplication.No Duplicate Events In Batch	✅	1507ms
Header Behavior On Retry.Attempt Header Starts At One	✅	1008ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	14520ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	7508ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3516ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	7512ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	1006ms
Response Format Validation.Success Response Has Ok For Each Event	✅	1507ms
Response Format Validation.Success No Retry After When All Ok	✅	1506ms
Response Format Validation.Success Retry After Present When Retry Events	✅	2510ms
Response Format Validation.Success No Retry After When Drop Only	✅	1506ms
Response Format Validation.Response Echoes Request Id	✅	1009ms
Retry Behavior.Retries On 408	✅	7513ms
Retry Behavior.Retries On 500	✅	7513ms
Retry Behavior.Retries On 503	✅	9515ms
Retry Behavior.Retries On 504	✅	7511ms
Retry Behavior.Retryable Errors Have Retry After	✅	4508ms
Retry Behavior.Respects Retry After On Retryable Error	✅	12516ms
Retry Behavior.Does Not Retry On 400	✅	3503ms
Retry Behavior.Does Not Retry On 401	✅	3507ms
Retry Behavior.Does Not Retry On 402	✅	3507ms
Retry Behavior.Does Not Retry On 413	✅	3507ms
Retry Behavior.Does Not Retry On 415	✅	3507ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	3508ms
Retry Behavior.Implements Backoff	✅	23529ms
Retry Behavior.Max Retries Respected	✅	23520ms
Partial Batch Handling.Handles 200 Full Success	✅	3001ms
Partial Batch Handling.Handles 200 With All Ok	✅	4507ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	4508ms
Partial Batch Handling.Does Not Retry Limited Events	✅	4507ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	7513ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	7510ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	7512ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	7506ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	7516ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	7511ms
Partial Batch Handling.Respects Retry After On Partial	✅	9510ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	4507ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	4508ms
Compression.Sends Gzip Content Encoding	✅	1006ms
Compression.No Content Encoding When Disabled	✅	1008ms
Compression.Compressed Body Is Decompressible	✅	1009ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	3507ms
Event Options.Cookieless Mode Override	✅	1008ms
Event Options.Disable Skew Correction Override	✅	1008ms
Event Options.Process Person Profile Override	✅	1008ms
Event Options.Product Tour Id Override	✅	1009ms
Event Options.Unset Options Omitted	✅	1009ms
Event Options.Options Override In Batch	✅	1011ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	1008ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	1009ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	1008ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	1006ms
Request Payload.Flags Request Uses V2 Query Param	✅	1006ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	1006ms
Request Payload.Flags Request Omits Authorization Header	✅	1006ms
Request Payload.Token In Flags Body Matches Init	✅	1006ms
Request Payload.Groups Round Trip	✅	1006ms
Request Payload.Groups Default To Empty Object	✅	1006ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	1006ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	1006ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	1006ms
Request Lifecycle.No Flags Request On Init Alone	✅	503ms
Request Lifecycle.No Flags Request On Normal Capture	✅	1507ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1010ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	1002ms
Retry Behavior.Retries Flags On 502	✅	1006ms
Retry Behavior.Retries Flags On 504	✅	1006ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1509ms

[marandaneto]

@PostHog/team-ai-observability not sure if intentional so pls take a look at it

[greptile-apps]

_{Reviews (1): Last reviewed commit: "chore(ai): keep sync stream wrapper inte..." | Re-trigger Greptile}

[ioannisj]

Looks reasonable