feat: Add async feature flag evaluation

[marandaneto]

💡 Motivation and Context

Continues the async SDK stack for #103 by adding non-blocking feature flag and remote config APIs on top of AsyncPosthog.

This PR is stacked on #719. It replaces blocking /flags, flag definition polling, and remote config calls in the async client with awaitable HTTP helpers, while keeping FeatureFlagEvaluations.is_enabled() / .get_flag() synchronous for ergonomic branching. $feature_flag_called events are scheduled into the async capture queue and drained by await flush() / await shutdown().

Review feedback was addressed by making feature-flag-called scheduling gracefully no-op with a warning when accessed without a running event loop, asserting the async poller task is started, and adding parameterized remote config coverage.

💚 How did you test it?

• uv run ruff check posthog/async_client.py posthog/test/test_async_client.py posthog/test/test_async_feature_flags.py
• uv run --extra test pytest posthog/test/test_async_request.py posthog/test/test_async_client.py posthog/test/test_async_feature_flags.py --verbose --timeout=30
• uv run --extra test pytest posthog/test/test_evaluate_flags.py posthog/test/test_feature_flags.py --verbose --timeout=30
• uv run python -W error -c "import posthog; from posthog import AsyncPosthog"
• uv run --extra dev python .github/scripts/check_public_api.py

📝 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 pi. The main design choice was to keep flag snapshots synchronous to read after await evaluate_flags(...), and to schedule their $feature_flag_called side effects into the async client instead of making every flag accessor awaitable.

[github-actions]

posthog-python Compliance Report

Date: 2026-07-02 14:14:53 UTC
Duration: 177175ms

✅ All Tests Passed!

46/46 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	516ms
Format Validation.Event Has Uuid	✅	1508ms
Format Validation.Event Has Lib Properties	✅	1507ms
Format Validation.Distinct Id Is String	✅	1506ms
Format Validation.Token Is Present	✅	1506ms
Format Validation.Custom Properties Preserved	✅	1507ms
Format Validation.Event Has Timestamp	✅	1507ms
Retry Behavior.Retries On 503	✅	9515ms
Retry Behavior.Does Not Retry On 400	✅	3507ms
Retry Behavior.Does Not Retry On 401	✅	3507ms
Retry Behavior.Respects Retry After Header	✅	9514ms
Retry Behavior.Implements Backoff	✅	23516ms
Retry Behavior.Retries On 500	✅	7511ms
Retry Behavior.Retries On 502	✅	7512ms
Retry Behavior.Retries On 504	✅	7515ms
Retry Behavior.Max Retries Respected	✅	23528ms
Deduplication.Generates Unique Uuids	✅	1497ms
Deduplication.Preserves Uuid On Retry	✅	7515ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	14511ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7512ms
Deduplication.No Duplicate Events In Batch	✅	1507ms
Deduplication.Different Events Have Different Uuids	✅	1507ms
Compression.Sends Gzip When Enabled	✅	1506ms
Batch Format.Uses Proper Batch Structure	✅	1505ms
Batch Format.Flush With No Events Sends Nothing	✅	1005ms
Batch Format.Multiple Events Batched Together	✅	1505ms
Error Handling.Does Not Retry On 403	✅	3509ms
Error Handling.Does Not Retry On 413	✅	3508ms
Error Handling.Retries On 408	✅	7508ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	1007ms
Request Payload.Flags Request Uses V2 Query Param	✅	1006ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	1007ms
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	✅	1005ms
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	✅	1011ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	1001ms
Retry Behavior.Retries Flags On 502	✅	1004ms
Retry Behavior.Retries Flags On 504	✅	1007ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1509ms

[greptile-apps]

_{Reviews (1): Last reviewed commit: "Add async feature flag evaluation" | Re-trigger Greptile}