fix: chain beforeSend hooks and fall back on exception instead of dropping

[posthog]

💡 Motivation and Context

Brings the Before Send Hook contract closer to compliance with the before-send-hook spec tracked in compliance/posthog-android.md note n17 (and the identical posthog-java.md note n2, since both posthog-android and posthog-server share this code path via PostHogStateless).

The spec requires that when multiple beforeSend hooks are registered, each one receives the previous hook's output, and that a hook throwing an exception must not silently drop the event.

PostHogStateless.buildEvent() had two bugs:

1. Chaining bug — the loop called beforeSend.run(postHogEvent) (the original, pre-chain event) on every iteration instead of beforeSend.run(eventChecked) (the running value). With 2+ hooks, hook Bugfix: fix screen event name back to screen and change key to screen… #2 never saw hook Change screen key to $screen_name to match ios #1's mutation.
2. Exception handling — a throwing hook returned null from buildEvent(), dropping the event entirely, rather than falling back to the last good (pre-exception) event.

Why this is backward-compatible

• Fix (1) only changes behavior for callers with 2+ registered hooks where an earlier hook mutates the event — a case that was silently broken before, so there's no previously-correct behavior to regress.
• Fix (2) is strictly less destructive than today's behavior: an event that used to be dropped on a hook exception is now delivered instead. No signature changes on any public API.

Added as a patch changeset for both posthog and posthog-server (they share the fixed code).

💚 How did you test it?

• Added two new tests to PostHogBeforeSendTest.kt: one asserting a second hook sees the first hook's mutation, one asserting the event is still delivered (with the last-good properties) when a hook throws.
• Updated the existing PostHogStatelessTest.kt > beforeSend error handling does not crash test, which had asserted the old (buggy) drop-on-exception behavior — it now asserts the event is delivered.
• Ran ./gradlew :posthog:test :posthog-server:test, spotlessCheck, and :posthog:apiCheck :posthog-server:apiCheck locally — all green, no public API surface changes.

📝 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

🤖 Agent context

Autonomy: Fully autonomous

This PR was opened by an unattended daily SDK-compliance-implementer loop (Claude Code) that reads the compliance matrices in PostHog/sdk-specs and implements the single best-scoped, backward-compatible gap each run. This contract/gap was selected over ~90 other open gaps across 12 SDKs for being backward-compatible, high-impact (core capture pipeline correctness), narrowly scoped (a two-bug fix confined to one function), and not already covered by an open PR. No human directed this specific change — please assign an owner on review.

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Created with PostHog Desktop

[github-actions]

posthog-android Compliance Report

Date: 2026-08-11 16:25:40 UTC
Duration: 118725ms

✅ All Tests Passed!

46/46 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	435ms
Format Validation.Event Has Uuid	✅	36ms
Format Validation.Event Has Lib Properties	✅	36ms
Format Validation.Distinct Id Is String	✅	30ms
Format Validation.Token Is Present	✅	36ms
Format Validation.Custom Properties Preserved	✅	37ms
Format Validation.Event Has Timestamp	✅	33ms
Retry Behavior.Retries On 503	✅	7034ms
Retry Behavior.Does Not Retry On 400	✅	4028ms
Retry Behavior.Does Not Retry On 401	✅	4032ms
Retry Behavior.Respects Retry After Header	✅	7032ms
Retry Behavior.Implements Backoff	✅	17030ms
Retry Behavior.Retries On 500	✅	7021ms
Retry Behavior.Retries On 502	✅	7022ms
Retry Behavior.Retries On 504	✅	7023ms
Retry Behavior.Max Retries Respected	✅	17040ms
Deduplication.Generates Unique Uuids	✅	48ms
Deduplication.Preserves Uuid On Retry	✅	7019ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	12034ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7016ms
Deduplication.No Duplicate Events In Batch	✅	58ms
Deduplication.Different Events Have Different Uuids	✅	33ms
Compression.Sends Gzip When Enabled	✅	26ms
Batch Format.Uses Proper Batch Structure	✅	23ms
Batch Format.Flush With No Events Sends Nothing	✅	16ms
Batch Format.Multiple Events Batched Together	✅	45ms
Error Handling.Does Not Retry On 403	✅	4025ms
Error Handling.Does Not Retry On 413	✅	4024ms
Error Handling.Retries On 408	✅	5029ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	39ms
Request Payload.Flags Request Uses V2 Query Param	✅	32ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	25ms
Request Payload.Flags Request Omits Authorization Header	✅	30ms
Request Payload.Token In Flags Body Matches Init	✅	27ms
Request Payload.Groups Round Trip	✅	28ms
Request Payload.Groups Default To Empty Object	✅	27ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	28ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	25ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	30ms
Request Lifecycle.No Flags Request On Init Alone	✅	14ms
Request Lifecycle.No Flags Request On Normal Capture	✅	28ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	51ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	26ms
Retry Behavior.Retries Flags On 502	✅	329ms
Retry Behavior.Retries Flags On 504	✅	329ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	28ms