fix(tracker): sample PostHog events at five percent

apps/desktop/layer/renderer/src/initialize/analytics.ts

@@ -1,5 +1,10 @@
 import { env } from "@follow/shared/env.desktop"
-import { setFirebaseTracker, setPostHogTracker, tracker } from "@follow/tracker"
+import {
+  createPostHogBeforeSend,
+  setFirebaseTracker,
+  setPostHogTracker,
+  tracker,
+} from "@follow/tracker"
 import { captureAttributionFromURL, getAttributionForAnalytics } from "@follow/utils"
 import type { AuthSessionResponse } from "@follow-app/client-sdk"
 import posthog from "posthog-js"
@@ -30,6 +35,7 @@ export const initAnalytics = async () => {
       api_host: env.VITE_POSTHOG_HOST,
       person_profiles: "identified_only",
       defaults: "2025-05-24",
+      before_send: createPostHogBeforeSend(),
       capture_exceptions: {
         capture_unhandled_errors: true,
         capture_unhandled_rejections: true,

apps/mobile/src/initialize/analytics.ts

@@ -1,5 +1,10 @@
 import { whoami } from "@follow/store/user/getters"
-import { setFirebaseTracker, setPostHogTracker, tracker } from "@follow/tracker"
+import {
+  createPostHogBeforeSend,
+  setFirebaseTracker,
+  setPostHogTracker,
+  tracker,
+} from "@follow/tracker"
 import type { AuthUser } from "@follow-app/client-sdk"
 import { getAnalytics } from "@react-native-firebase/analytics"
 import { nativeApplicationVersion, nativeBuildVersion } from "expo-application"
@@ -14,6 +19,7 @@ export const initAnalytics = async () => {
     setPostHogTracker(
       new PostHog(proxyEnv.POSTHOG_KEY, {
         host: proxyEnv.POSTHOG_HOST,
+        before_send: createPostHogBeforeSend(),
         errorTracking: {
           autocapture: {
             uncaughtExceptions: true,

packages/internal/tracker/src/adapters/index.ts

@@ -1,4 +1,9 @@
 export type { CaptureExceptionPayload, IdentifyPayload, TrackerAdapter, TrackPayload } from "./base"
 export { FirebaseAdapter, type FirebaseAdapterConfig } from "./firebase"
-export { PostHogAdapter, type PostHogAdapterConfig } from "./posthog"
+export {
+  createPostHogBeforeSend,
+  POSTHOG_SAMPLE_RATE,
+  PostHogAdapter,
+  type PostHogAdapterConfig,
+} from "./posthog"
 export { ProxyAdapter } from "./proxy"

packages/internal/tracker/src/adapters/posthog.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from "vitest"
+
+import { createPostHogBeforeSend, POSTHOG_SAMPLE_RATE } from "./posthog"
+
+describe("PostHogAdapter", () => {
+  it("uses a 5% PostHog sample rate", () => {
+    expect(POSTHOG_SAMPLE_RATE).toBe(0.05)
+  })
+
+  it("drops PostHog events when outside the configured sample rate", () => {
+    const beforeSend = createPostHogBeforeSend(0.05, () => 0.5)
+
+    expect(beforeSend({ event: "app init" })).toBeNull()
+  })
+
+  it("keeps PostHog events when inside the configured sample rate", () => {
+    const beforeSend = createPostHogBeforeSend(0.05, () => 0.04)
+    const event = { event: "app init" }
+
+    expect(beforeSend(event)).toBe(event)
+  })
+})

packages/internal/tracker/src/adapters/posthog.ts

@@ -3,6 +3,25 @@ import type PostHogReactNative from "posthog-react-native"
 
 import type { CaptureExceptionPayload, IdentifyPayload, TrackerAdapter, TrackPayload } from "./base"
 
+export const POSTHOG_SAMPLE_RATE = 0.05
+
+const normalizeSampleRate = (sampleRate: number): number => {
+  if (!Number.isFinite(sampleRate)) return 1
+
+  return Math.min(Math.max(sampleRate, 0), 1)
+}
+
+export const createPostHogBeforeSend = (
+  sampleRate = POSTHOG_SAMPLE_RATE,
+  sampleRandom: () => number = Math.random,
+) => {
+  const normalizedSampleRate = normalizeSampleRate(sampleRate)
+  const isSampled =
+    normalizedSampleRate >= 1 || (normalizedSampleRate > 0 && sampleRandom() < normalizedSampleRate)
+
+  return <T>(event: T): T | null => (isSampled ? event : null)
+}
+
 export interface PostHogAdapterConfig {
   instance: PostHog | PostHogReactNative
   enabled?: boolean

packages/internal/tracker/src/index.ts

@@ -9,9 +9,11 @@ export const tracker = new TrackerPoints()
 
 export {
   type CaptureExceptionPayload,
+  createPostHogBeforeSend,
   FirebaseAdapter,
   type FirebaseAdapterConfig,
   type IdentifyPayload,
+  POSTHOG_SAMPLE_RATE,
   PostHogAdapter,
   type PostHogAdapterConfig,
   ProxyAdapter,