fix(queue): 413 no config mutation

[turnipdabeets]

The 413 adaptive batch sizing currently mutates the user-provided PostHogConfig, halving config.maxBatchSize and config.flushAt directly. That makes the user's configured values silently change at runtime — surprising for callers that read those fields back, and it leaks internal queue state across SDK boundaries.

It also halves from the configured cap rather than the actual batch size that triggered the 413. If a 413 fires on a batch of 10 with maxBatchSize=50, we halve from 50 → 25 → 12 → ... wasting halvings on sizes the server already proved (or is about to prove) too large.

Move the cap and flushAt into a private BatchLimits class owned by PostHogQueue, initialized from config once at construction. Halve from min(cap, actualBatchSize) so a partial batch's 413 doesn't waste cycles on larger caps the server hasn't accepted, and clamp flushAt to cap on each halve so we never buffer more events than a single batch can drain (e.g. cap=1 + flushAt=10 would otherwise pile 10 events to send 1 at a time). config.maxBatchSize and config.flushAt are now read-only as far as the queue is concerned.

Matches the equivalent iOS fix in posthog-ios PR #585

💡 Motivation and Context

💚 How did you test it?

📝 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 pnpm changeset to generate a changeset file

[github-actions]

⚠️ Changeset declares `posthog-android` but no source files in that package changed

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Changesets in this PR:

• posthog — patch
• posthog-android — patch

[github-actions]

posthog-android Compliance Report

Date: 2026-05-05 18:29:48 UTC
Duration: 2865ms

⚠️ Some Tests Failed

0/16 tests passed, 16 failed

---

Feature_Flags Tests

⚠️ 0/16 tests passed, 16 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	❌	280ms
Request Payload.Flags Request Uses V2 Query Param	❌	35ms
Request Payload.Flags Request Hits Flags Path Not Decide	❌	43ms
Request Payload.Flags Request Omits Authorization Header	❌	26ms
Request Payload.Token In Flags Body Matches Init	❌	28ms
Request Payload.Groups Round Trip	❌	24ms
Request Payload.Groups Default To Empty Object	❌	23ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	❌	21ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	❌	18ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	20ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	❌	16ms
Request Lifecycle.No Flags Request On Init Alone	❌	13ms
Request Lifecycle.No Flags Request On Normal Capture	❌	2051ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	❌	15ms
Request Lifecycle.Mock Response Value Is Returned To Caller	❌	17ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	❌	16ms

Failures

request_payload.request_with_person_properties_device_id

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.flags_request_uses_v2_query_param

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.flags_request_hits_flags_path_not_decide

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.flags_request_omits_authorization_header

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.token_in_flags_body_matches_init

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.groups_round_trip

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.groups_default_to_empty_object

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.person_properties_distinct_id_auto_populated_when_caller_omits_it

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.disable_geoip_false_propagates_as_geoip_disable_false

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.disable_geoip_omitted_defaults_to_false

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.flag_keys_to_evaluate_contains_only_requested_key

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_lifecycle.no_flags_request_on_init_alone

Expected 0 /flags requests, got 1

request_lifecycle.no_flags_request_on_normal_capture

Expected 0 /flags requests, got 1

request_lifecycle.two_flag_calls_produce_two_remote_requests

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_lifecycle.mock_response_value_is_returned_to_caller

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

side_effect_events.get_feature_flag_captures_feature_flag_called_event

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'