flutter_webmcp

Expose Flutter Web application actions as typed WebMCP tools without writing JavaScript interop code.

WebMCP is experimental. This package keeps the changing browser bindings behind a small Dart API and adds Flutter lifecycle management on top.

Try the live Flutter Web demo.

Compatibility

Runtime	Behavior
Flutter Web (JavaScript)	Supported when the browser exposes WebMCP
Flutter Web (WebAssembly)	Supported when the browser exposes WebMCP
Android, iOS, desktop, and Dart VM	Safe no-op detection; registration is unsupported

The package is tested against the WebMCP Draft Community Group Report dated 26 August 2026. It implements the current document.modelContext.registerTool shape, execution cancellation through AbortSignal, signal-based unregistration, tool annotations, and exposedTo origins. Because WebMCP is not yet a web standard, minor package releases may add compatibility shims for browser changes.

Some experimental Chromium builds return undefined from registerTool instead of the draft's Promise<void>. Both forms are supported.

Install

dependencies:
  flutter_webmcp: ^0.2.1

For a local checkout:

dependencies:
  flutter_webmcp:
    path: ../flutter_webmcp

Flutter usage

Create tools once and let WebMcpToolScope register them while the feature is mounted:

class TasksPageState extends State<TasksPage> {
  late final WebMcpTool addTaskTool;

  @override
  void initState() {
    super.initState();
    addTaskTool = WebMcpTypedTool<AddTaskInput>(
      name: 'add_task',
      title: 'Add task',
      description: 'Adds a task to the list currently open in the app.',
      inputSchema: const {
        'type': 'object',
        'properties': {
          'title': {'type': 'string'},
        },
        'required': ['title'],
      },
      decodeInput: AddTaskInput.fromJson,
      execute: (input, context) async {
        final task = await taskRepository.add(input.title);
        return WebMcpResult.structured(
          {'id': task.id, 'title': task.title},
          text: 'Task "${task.title}" was added.',
        );
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return WebMcpToolScope(
      tools: [addTaskTool],
      child: const TasksView(),
    );
  }
}

Keep tool instances stable, for example in initState, a ViewModel, or a dependency-injection container. Replacing a tool instance tells the scope to unregister the old configuration and register the new one.

Typed input

final class AddTaskInput {
  const AddTaskInput(this.title);

  factory AddTaskInput.fromJson(Map<String, Object?> json) {
    final title = json['title'];
    if (title is! String || title.trim().isEmpty) {
      throw const WebMcpToolException(
        code: 'invalid_title',
        message: 'Task title must be a non-empty string.',
      );
    }
    return AddTaskInput(title.trim());
  }

  final String title;
}

The original map-based WebMcpTool remains available for simple integrations.

Results and errors

Use helpers instead of building protocol maps manually:

return WebMcpResult.text('Done');

return WebMcpResult.structured({'taskId': task.id});

throw const WebMcpToolException(
  code: 'task_not_found',
  message: 'The selected task no longer exists.',
);

WebMcpToolException is converted into a structured isError result that an agent can understand. Unexpected Dart errors are logged and returned as a safe internal_error without exposing local details.

Feature detection

final support = WebMcp.support;
if (!support.isSupported) {
  debugPrint(support.message);
}

The result distinguishes unsupported platforms, insecure pages, and unavailable browser APIs. Permission-policy errors are reported when registration is attempted because current browsers do not expose a reliable read-only policy signal for WebMCP.

Logging

WebMcp.logger = (event) {
  debugPrint(
    '${event.toolName}: ${event.status} in ${event.duration.inMilliseconds}ms',
  );
};

Logs include the tool name, decoded JSON input, duration, result, and local error information. Logger failures never break tool execution.

Inputs and results can contain sensitive data. Redact them before forwarding events to production telemetry.

Security

WebMCP tools run with the same authority as your application code. Validate input in decodeInput, enforce authorization inside the handler, and require normal user confirmation for destructive or sensitive operations. Annotations are agent hints, not security boundaries.

Only use exposedTo with origins you trust. See SECURITY.md for reporting and deployment guidance.

Core Dart API

Code that does not need Flutter widgets can import the core library directly:

import 'package:flutter_webmcp/webmcp.dart';

Then call WebMcp.registerTool() and keep the returned WebMcpRegistration for manual cleanup.

Browser setup

WebMCP requires document.modelContext. It is available in ChatGPT's in-app browser. For local Chrome testing, enable chrome://flags/#enable-webmcp-testing and restart Chrome.

• Flutter Web compiled to JavaScript or WebAssembly is supported when the browser implements WebMCP.
• Android, iOS, macOS, Windows, Linux, and Dart VM report unsupported.

See the complete application in example/lib/main.dart.

Contributing

Issues and pull requests are welcome. Read CONTRIBUTING.md and follow the CODE_OF_CONDUCT.md. Report security issues privately as described in SECURITY.md.

License

flutter_webmcp is available under the MIT License.