Static Mixpanel.init() passes 5 args to the 6-arg native initialize — fatal EXC_BAD_ACCESS at boot on RN New Architecture (iOS)

[tomas-reuser]

Summary

The static Mixpanel.init() helper passes 5 arguments to the native initialize method, which declares 6 arguments (it omits useGzipCompression). On React Native's New Architecture, where legacy modules run through the TurboModule interop layer, the missing argument shifts the promise resolve/reject blocks into the wrong NSInvocation slots — the module invocation then retains a garbage pointer and the app crashes with a fatal EXC_BAD_ACCESS (KERN_PROTECTION_FAILURE) in objc_retain the moment init is called (for most apps: at boot).

The instance API is unaffected (new Mixpanel(...) + mixpanel.init() passes all 6 arguments), which is likely why this hasn't been widely reported — only apps using the static helper crash.

Affected versions

• mixpanel-react-native 3.3.0 and 3.4.0 (both verified; the bug was presumably introduced when useGzipCompression was added to the native signature without updating the static JS helper)
• React Native 0.83.6 (Expo SDK 55), New Architecture (module runs via interop — the package has no codegen spec)
• iOS 26.5, reproduced on device (iPhone 16 Pro Max); crashes identically in Debug and Release
• Version 3.0.9 is fine (predates useGzipCompression)

The mismatch

index.js — static helper passes 5 args:

static async init(token, trackAutomaticEvents, optOutTrackingDefault = DEFAULT_OPT_OUT) {
  await MixpanelReactNative.initialize(
    token,
    trackAutomaticEvents,
    optOutTrackingDefault,
    Helper.getMetaData(),
    "https://api.mixpanel.com"
    // <-- useGzipCompression missing
  );
  ...
}

index.js — instance method correctly passes 6:

async init(optOutTrackingDefault = DEFAULT_OPT_OUT, superProperties = {}, serverURL = "...", useGzipCompression = false) {
  await this.mixpanelImpl.initialize(
    this.token, this.trackAutomaticEvents, optOutTrackingDefault,
    {...Helper.getMetaData(), ...superProperties}, serverURL, useGzipCompression
  );
}

ios/MixpanelReactNative.m — native export declares 6 args + resolver/rejecter:

RCT_EXTERN_METHOD(initialize:(NSString *)token trackAutomaticEvents:(BOOL)trackAutomaticEvents optOutTrackingByDefault:(BOOL)optOutTrackingByDefault properties:(NSDictionary *)properties serverURL:(NSString *)serverURL useGzipCompression:(BOOL)useGzipCompression resolver:(RCTPromiseResolveBlock)resolve rejecter:(RCTPromiseRejectBlock)reject)

The legacy bridge tolerated the short call; the New Architecture interop layer does not.

Crash stack (symbolicated, from Sentry)

EXC_BAD_ACCESS: Exception 1, Code 2 — KERN_PROTECTION_FAILURE
  at objc_retain
  at facebook::react::ObjCTurboModule::createPromise::lambda::operator()
  at MixpanelReactNative.initialize (MixpanelReactNative.swift:31)
  at __invoking___
  at -[NSInvocation invoke]
  at -[NSInvocation invokeWithTarget:]
  at facebook::react::ObjCTurboModule::performMethodInvocation

Reproduction

1. RN 0.83+ app with New Architecture (default), iOS device on iOS 26
2. const mp = await Mixpanel.init("<token>", true); → immediate fatal crash

Workaround

Use the instance API instead of the static helper:

const mixpanel = new Mixpanel("<token>", true);
await mixpanel.init(); // passes all 6 args — no crash

Suggested fix

Add the missing argument in the static helper (and consider forwarding superProperties/serverURL/useGzipCompression params so both APIs stay in sync):

await MixpanelReactNative.initialize(
  token, trackAutomaticEvents, optOutTrackingDefault,
  Helper.getMetaData(), "https://api.mixpanel.com", false
);

A codegen'd TurboModule spec would also catch this class of bug at build time.

[linear-code]

SDK-115