feat: add before send callback

[marandaneto]

💡 Motivation and Context

Customers need a configuration hook to inspect, modify, or drop events before upload, matching the before-send behavior available in other PostHog SDKs.

This adds PostHogOptions.BeforeSend, invoked after event enrichment (SDK properties, super properties, $is_server, groups, and feature flags) and before batch serialization. Returning null drops the event.

💚 How did you test it?

• dotnet test tests/UnitTests/UnitTests.csproj --filter 'FullyQualifiedName~PostHogClientTests.TheCaptureMethod'
• dotnet build src/PostHog/PostHog.csproj

📝 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: Human-driven (agent-assisted)

Implemented with the pi coding agent. The hook runs after the event is fully enriched by moving before-send processing into the batch resolution path, so feature flag enrichment is visible to the callback before serialization. A dropped event is filtered out of the outgoing batch.

[github-actions]

posthog-dotnet Compliance Report

Date: 2026-07-06 07:06:37 UTC
Duration: 4432ms

✅ All Tests Passed!

17/17 tests passed

---

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	246ms
Request Payload.Flags Request Uses V2 Query Param	✅	130ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	120ms
Request Payload.Flags Request Omits Authorization Header	✅	110ms
Request Payload.Token In Flags Body Matches Init	✅	111ms
Request Payload.Groups Round Trip	✅	112ms
Request Payload.Groups Default To Empty Object	✅	113ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	111ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	110ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	111ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	115ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	214ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	114ms
Retry Behavior.Retries Flags On 502	✅	1118ms
Retry Behavior.Retries Flags On 504	✅	1114ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	212ms

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat: add before send callback" | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "chore: fix before send changeset package..." | Re-trigger Greptile}

[ioannisj]

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