[firebase_auth][web] signInWithCustomToken crashes with "Int64 accessor not supported by dart2js"

[Mwatchorn26]

Is there an existing issue for this?

• I have searched the existing issues.

Which plugins are affected?

Auth

Which platforms are affected?

Web

Description

FirebaseAuth.instance.signInWithCustomToken(token) crashes on Flutter
web (dart2js) with "Unsupported operation: Int64 accessor not supported
by dart2js."

Root cause (found by reading the installed package source):
firebase_auth_platform_interface-9.0.6/lib/src/pigeon/messages.pigeon.dart,
_PigeonCodec.writeValue:

​dart class _PigeonCodec extends StandardMessageCodec { const _PigeonCodec(); @override void writeValue(WriteBuffer buffer, Object? value) { if (value is int) { buffer.putUint8(4); buffer.putInt64(value); // <-- unconditional, no kIsWeb guard } else if (value is ActionCodeInfoOperation) { ... ​

Any plain Dart int reaching this codec crashes on web, since
WriteBuffer.putInt64 isn't supported by dart2js.

Prior art: this is the same bug class as #17924
("cloud_functions_web: dartify() throws 'Int64 accessor not supported by
dart2js' when serializing callable function request"), closed 2026-03-04
after a kIsWeb-guarded fix (PR #18066) landed in cloud_functions. See
cloud_functions-6.3.6/lib/src/https_callable.dart:

​dart dynamic _updateRawDataToList(dynamic value) { if (value is Uint8List || value is Int32List || // Int64List is not supported by dart2js, skip the check on web. (!kIsWeb && value is Int64List) || ... ​

That fix was scoped to cloud_functions only.
firebase_auth_platform_interface has the identical unguarded pattern in
its own generated pigeon codec and was apparently never covered by an
equivalent fix. I also spot-checked cloud_firestore_platform_interface's
pigeon codec (8.0.6) and found the same unguarded if (value is int)
pattern there too, though I have not reproduced an actual crash through
cloud_firestore — flagging it as a likely-affected sibling package in
case a shared/generated-code fix makes sense across all three rather than
patching each plugin individually.

Expected: signInWithCustomToken (and any other firebase_auth
pigeon-serialized call carrying an int) should work on web without
crashing — either by adding the same kIsWeb-guarded conversion used in
cloud_functions, or by avoiding raw int in the pigeon payload for web
builds.

### Reproducing the issue

1. Mint a Firebase custom token server-side, e.g. via the Admin SDK:
   `admin.auth().createCustomToken(uid)`.
2. In a Flutter web app (release or profile build, i.e. dart2js — this
   is a compile-target-specific bug, not reproducible in debug/JIT mode),
   call:
   `await FirebaseAuth.instance.signInWithCustomToken(token);`
3. Observe the crash below instead of a successful sign-in.

### Firebase Core version

4.13.0

### Flutter Version

Flutter 3.41.9 • channel stable Dart 3.11.5

### Relevant Log Output

```shell
switchToChildSession failed: Unsupported operation: Int64 accessor not supported by dart2js.

    at Object.wrapException (main.dart.js:8448:47)
    at NativeTypedData.setInt64$3 (main.dart.js:116592:15)
    at WriteBuffer.putInt64$1 (main.dart.js:201711:32)
    at _PigeonCodec.writeValue$2 (main.dart.js:185592:16)
    at StandardMessageCodec_writeValue_closure.call$2 (main.dart.js:268561:10)
    at JsLinkedHashMap.forEach$1 (main.dart.js:115808:16)
    at _PigeonCodec.writeValue$2 (main.dart.js:268424:12)
    at _PigeonCodec.writeValue$2 (main.dart.js:185594:14)
    at _PigeonCodec.writeValue$2 (main.dart.js:268419:17)
    at _PigeonCodec.writeValue$2 (main.dart.js:185594:14)

Flutter dependencies

Expand Flutter dependencies snippet

- audioplayers 5.2.1 [audioplayers_android audioplayers_darwin audioplayers_linux audioplayers_platform_interface audioplayers_web audioplayers_windows file flutter http meta path_provider synchronized uuid]
- battery_plus 4.1.0 [flutter flutter_web_plugins battery_plus_platform_interface meta upower]
- cloud_firestore 6.8.0 [cloud_firestore_platform_interface cloud_firestore_web collection firebase_core firebase_core_platform_interface flutter meta]
- cloud_functions 6.3.6 [cloud_functions_platform_interface cloud_functions_web firebase_core firebase_core_platform_interface flutter]
- connectivity_plus 5.0.2 [flutter flutter_web_plugins connectivity_plus_platform_interface js meta nm]
- crypto 3.0.7 [typed_data]
- cupertino_icons 1.0.9
- excel 4.0.6 [archive xml collection equatable]
- file_picker 8.3.7 [flutter flutter_web_plugins flutter_plugin_android_lifecycle plugin_platform_interface ffi path win32 cross_file web]
- firebase_auth 6.5.7 [firebase_auth_platform_interface firebase_auth_web firebase_core firebase_core_platform_interface flutter meta]
- firebase_core 4.13.0 [firebase_core_platform_interface firebase_core_web flutter meta]
- firebase_storage 13.4.6 [firebase_core firebase_core_platform_interface firebase_storage_platform_interface firebase_storage_web flutter mime]
- flutter_svg 2.2.4 [flutter http vector_graphics vector_graphics_codec vector_graphics_compiler]
- google_sign_in 6.3.0 [flutter google_sign_in_android google_sign_in_ios google_sign_in_platform_interface google_sign_in_web]
- http 1.6.0 [async http_parser meta web]
- intl 0.20.2 [clock meta path]
- path_provider 2.1.5 [flutter path_provider_android path_provider_foundation path_provider_linux path_provider_platform_interface path_provider_windows]
- qr_flutter 4.1.0 [flutter qr]
- shared_preferences 2.5.5 [flutter shared_preferences_android shared_preferences_foundation shared_preferences_linux shared_preferences_platform_interface shared_preferences_web shared_preferences_windows]
- sign_in_with_apple 6.1.4 [flutter meta sign_in_with_apple_platform_interface sign_in_with_apple_web]
- three_js 0.0.1 [flutter three_js_math three_js_advanced_loaders three_js_animations three_js_controls three_js_core three_js_core_loaders three_js_curves three_js_simple_loaders three_js_text]
- timezone 0.9.4 [path]
- url_launcher 6.3.2 [flutter url_launcher_android url_launcher_ios url_launcher_linux url_launcher_macos url_launcher_platform_interface url_launcher_web url_launcher_windows]
- uuid 4.5.3 [crypto fixnum]

Additional context and comments

No response

[SelaseKay]

Hi @Mwatchorn26, thanks for the report. I'm looking into it.

[SelaseKay]

Hi @Mwatchorn26, thanks for the detailed report — the dart2js stack and the pointer at _PigeonCodec.writeValue were especially helpful.

I reproduced the Int64 crash on the generated Pigeon codec, and we also checked the signInWithCustomToken path on web. The two don't line up the way it first looks.

On web, FirebaseAuth.instance.signInWithCustomToken goes through firebase_auth_web (JS interop), not Pigeon / method channels. Even if that codec were used, the custom-token payload is only [AuthPigeonFirebaseApp, String] — no ints. We encoded that payload in Chrome and it succeeded.

What does crash on dart2js is encoding any Dart int through _PigeonCodec (WriteBuffer.putInt64). Your stack also shows JsLinkedHashMap.forEach, which means a Map containing an int was being encoded. That is not the custom-token request.

Could you share the body of switchToChildSession, or a minimal profile/release web app that only calls signInWithCustomToken?

Thanks — happy to keep digging once we can see the rest of that path.

[Mwatchorn26]

Thanks for digging into signInWithCustomToken — you were right, I've now confirmed with a from-scratch minimal repro (pinned to the exact same package versions as the reporting app: firebase_core: 4.13.0, firebase_auth: 6.5.7, cloud_functions: 6.3.6, cloud_firestore: 6.8.0) that signInWithCustomToken on web is clean in isolation. Sorry for the misattribution in the original report.

Extending that same repro to walk the real production chain end-to-end (guardian sign-in → call a cloud_functions callable → signInWithCustomToken with the returned token → Firestore read) isolated the actual crash site: it's FirebaseFunctions.instance.httpsCallable(...).call(...) on web, not firebase_auth at all. To rule out the call arguments as the cause, I also tried calling with zero arguments (callable.call()), and it crashes identically:

Unsupported operation: Int64 accessor not supported by dart2js.
    at Object.wrapException
    at NativeTypedData.setInt64$3
    at WriteBuffer.putInt64$1
    at _PigeonCodec.writeValue$2
    at StandardMessageCodec_writeValue_closure.call$2
    at JsLinkedHashMap.forEach$1
    at _PigeonCodec.writeValue$2
    ...

So the crash is argument-independent — it happens on any callable invocation, regardless of what (if anything) is passed to it.

This turned out not to be a bug in the normal web call path at all. I initially assumed httpsCallable(...).call(...) on web routes through cloud_functions_web's JS-interop delegate (HttpsCallableWeb, in cloud_functions_web/lib/https_callable_web.dart) ,

and reading that file confirms it does, using dart:js_interop (.jsify()/.toJS), never touching Pigeon or method channels. That path should be clean, the same way firebase_auth_web is.

So I added a diagnostic print of FirebaseFunctionsPlatform.instance.runtimeType right before the callable call, expecting to see FirebaseFunctionsWeb. Instead it prints:

FirebaseFunctionsPlatform.instance.runtimeType = MethodChannelFirebaseFunctions

• confirmed on two separate clean runs, immediately after Firebase.initializeApp and after FirebaseFunctions.instance.useFunctionsEmulator(...) had already been called. cloud_functions_web's FirebaseFunctionsWeb.registerWith(registrar) is present and correctly invoked in the generated web_plugin_registrant.dart for my repro app (verified by reading .dart_tool/dartpad/web_plugin_registrant.dart directly), so registration isn't simply missing from the build — yet FirebaseFunctionsPlatform.instance (the static default in cloud_functions_platform_interface, which falls back to MethodChannelFirebaseFunctions.instance per platform_interface_firebase_functions.dart:47 if no other implementation has been set) is still resolving to the method-channel implementation, not the web one, at the point httpsCallable(...).call(...) runs.

That fallback is what's actually crashing: MethodChannelHttpsCallable.call (cloud_functions_platform_interface/lib/src/method_channel/method_channel_https_callable.dart) sends a request map that unconditionally includes at least one plain int ('timeout': options.timeout.inMilliseconds). Any int in that map hits the generated _PigeonCodec.writeValue:

class _PigeonCodec extends StandardMessageCodec {
  const _PigeonCodec();
  @override
  void writeValue(WriteBuffer buffer, Object? value) {
    if (value is int) {
      buffer.putUint8(4);
      buffer.putInt64(value);   // no kIsWeb guard
    } else {
      super.writeValue(buffer, value);
    }
  }
  ...
}

which throws on dart2js. This is the same class of bug already fixed elsewhere in this plugin — cloud_functions-6.3.6's own lib/src/https_callable.dart:83 has a !kIsWeb guard, credited to PR #5369 in the CHANGELOG — but the same guard doesn't appear in this generated codec in cloud_functions_platform_interface.

What I haven't nailed down, and would value your read on since you know this plugin's registration flow: why FirebaseFunctionsPlatform.instance is landing on the method-channel default instead of the web delegate in this app, given the web plugin's registerWith is present and wired in the generated registrant. I don't yet know if that's a timing/ordering issue specific to how I'm calling things in my repro's initState, a real registration bug in cloud_functions_web/cloud_functions_platform_interface for this version combination, or something about running against the Firebase emulator suite specifically. I'd rather flag that gap than guess at it.

Versions: cloud_functions_platform_interface resolves to 6.0.6 and cloud_functions_web to 5.1.12 for cloud_functions: 6.3.6 confirmed via flutter pub deps in the repro app, not just inferred from my local pub cache.

I packaged and attached the repro app itself. It fully reproduces both the registration-fallback symptom and the resulting crash.

signin_repro.zip

[SelaseKay]

Hi @Mwatchorn26, thanks for the additional feedback. I'll run the repro and investigate further.

[SelaseKay]

Hi @Mwatchorn26, I've run the attached repro but I'm still unable to reproduce this issue. It successfully went through the interop path in my case. See screenshot attached below:

Could you try running this to see if issue still persists?

flutter clean
flutter pub get
flutter run -d chrome --profile