[posthog]

Why

Two SDKs changed their before-send behavior this week, in the same direction, because the spec never said how many times the hook runs or which assembly steps must already be done when it does.

• posthog-android#885 — the hook chain ran twice for every event captured through the stateful client (build event → run hooks → delegate to the stateless path, which rebuilt the properties and ran the hooks again). Hooks with side effects ran twice, random or uuid-keyed sampling hooks kept about half their configured rate, and hook edits to distinctId, timestamp and uuid were discarded by the rebuild. The fix queues the event the single hook pass returned, and minimizes $feature_flag_called before the hook, as the stateless path and posthog-ios already did.
• posthog-rs#307 — groups were merged into $groups after the hook, so a hook could not see or change them, and minimized $feature_flag_called events were trimmed after the hook, so properties a hook added were silently dropped. Both steps now run before the hook, matching posthog-python.

What changed in the spec

before-send-hook gains one requirement: the hook chain runs exactly once per captured event; the event handed to it is the event that would be sent if the hook returned it unchanged (groups already merged, minimized flag-called events already trimmed); and the SDK enqueues what the hook returned instead of rebuilding it. The existing Flutter wrapper-layer divergence is carried over unchanged.

Proposed, applied and archived on this branch under openspec/changes/archive/2026-10-27-run-before-send-once-on-the-sent-event/, as AGENTS.md asks.

Uncertain / for review

• The requirement names only the two assembly steps the SDK changes actually moved (group merge, flag-called trim) rather than a full ordered assembly list. Consent gating, sanitizers and dynamic-context building are still ordered differently across SDKs and are left alone.
• acceptance/private/before-send-hook.feature is untouched: the harness has no steps for invocation counts, group merging, or minimal flag-called events. The four new scenarios live in spec.md only and have not been run against an adapter.
• openspec validate --specs --strict was not run — the OpenSpec CLI is not installed in this environment.

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