flutter_mcp_harness 0.1.0

Programmatic E2E harness for Flutter apps: build and launch a debug binary, attach to its Dart VM service, drive widgets through the MCP toolkit service extensions, and assert with composable scenarios.

flutter_mcp_harness

Programmatic E2E harness for Flutter apps: own the whole lifecycle — build the app, launch the process, attach to its Dart VM service, and only then drive/assert. The driving layer speaks the same ext.mcp.toolkit.* service extensions the MCP server exposes — as a plain Dart API, no MCP transport required.

This is the client-side counterpart of mcp_toolkit, extracted from a production E2E loop (see ADR-0015). A guided tour lives in the E2E scenarios guide.

Quick start

import 'package:flutter_mcp_harness/flutter_mcp_harness.dart';

LaunchedApp? app;
var passed = false;
try {
  final scenario = Scenario('smoke', steps: [
    ('launch', (context) async {
      app = await MacosAppTarget(
        projectDir: 'my_app',
        binaryPath: 'my_app/build/macos/Build/Products/Debug/My App.app/Contents/MacOS/My App',
      ).launch(); // full `flutter build macos --debug`, then run the binary
      await app!.stdout.waitFor('app ready'); // wait on logs, never sleep
      context.report.pass('attached at ${app!.vmUri}');
    }),
    ('drive + assert', (context) async {
      final driver = WidgetDriver(await app!.vm());
      final ref = await driver.findRef('login');
      if (ref == null) return context.report.fail('no login button');
      await driver.enterText(await driver.findRef('email'), '[e-mail]');
      await driver.tap(ref);
      final banner = await driver.findValue((v) => v.contains('Welcome'));
      banner != null
          ? context.report.pass('login confirmed')
          : context.report.fail('no welcome banner');
    }),
  ]);
  passed = await scenario.run();
  scenario.report.printSummary();
} finally {
  await app?.stop(); // cleanup always runs
}
exit(passed ? 0 : 1);

Full two-instance composition root (host + controller pairing): example/desktop_pair.dart.

Why this shape

An earlier harness attempt was a declarative YAML runner on top of the MCP server — and never owned the hard parts. The valuable 80% is the boring 20%:

1. Own the build. Run flutter build <os> --debug yourself, then launch the binary directly. No flutter run, whose incremental-kernel and tool-respawn behavior is flaky under automation.
2. Own the VM. Scrape the VM service URI from the child's own stdout and connect over the vm_service package ([VmClient]). From there: list extensions, call toolkit extensions directly (semantic snapshot, tap, enter text), evaluate, hot-reload — no subprocess, no JSON parsing, no sticky state files.
3. Own the process lifecycle. [LaunchedApp] keeps the Process, a broadcast [LogTap] on its stdout, and the [VmClient]. Scenarios stop what they started, in finally.

Only on top of those is a [Scenario] worth anything: named steps over a HarnessContext (a report + a bag for passing values between steps), first failure aborts, cleanup still runs, exit code reflects the result.

API map

You want to…	Reach for
Launch an owning interactive flutter run (hot reload, showcase)	FlutterRunTarget
Fresh full build → direct binary launch (no compile channel after)	BinaryAppTarget, MacosAppTarget, WindowsAppTarget
Attach to an app someone else owns (dev runner session, ADR-0014)	construct LaunchedApp around the owning process
Wait for log lines / assert on output	LogTap.waitFor / firstMatch / count / tail
Drive the UI	WidgetDriver: snapshot, findRef, tap, tapUntil, enterText, scroll, findValue
Structure steps, assertions, cleanup	Scenario, Check, ScenarioReport, HarnessContext, retry
Evaluate Dart / hot-reload / discover extensions in the app	VmClient.evaluate / hotReload / extensionNames
Reference the toolkit verb names	ToolkitExtensions (mirrors mcp_toolkit's interaction toolkit)

Layout

• lib/src/log_tap.dart — line buffer + waitFor(pattern) (no sleeps).
• lib/src/vm_client.dart — WebSocket VM service connection, extension calls, evaluate, hot reload.
• lib/src/flutter_app.dart — AppTarget/LaunchedApp; [BinaryAppTarget] with MacosAppTarget/WindowsAppTarget presets. Android/iOS device bring-up belongs to the owning dev session (see oka_harness, which follows the runner-session contract).
• lib/src/flutter_run.dart — [FlutterRunTarget], the owning interactive flutter run session (hot reload via stdin; the showcase launch path).
• lib/src/widget_driver.dart — typed snapshot/tap/enterText/findValue over the toolkit service extensions.
• lib/src/toolkit_extensions.dart — the extension verb names the driver calls (source of truth: mcp_toolkit's interaction toolkit).
• lib/src/scenario.dart, lib/src/check.dart — steps, checks, report.
• example/desktop_pair.dart — a full two-instance composition root.
• tool/showcase.dart — this repo's showcase launcher (macOS / --web / --stop), the Dart rewrite of the former scripts/*.sh showcase.
• tool/intentcall_session.dart — IntentCall doors against a running showcase (discover / bridge ping / MCP serve), a checked-in composition root — nothing under .showcase/ is generated at runtime.

Running

cd packages/harness
dart test
dart run example/desktop_pair.dart --skip-build

Scenario entrypoints live in the consuming project (composition roots are project knowledge, not package code).

Showcase

The repo showcase is itself a composition root over this package (tool/showcase.dart, the Dart rewrite of the former scripts/run_showcase.sh family):

make showcase        # macOS showcase, interactive foreground (r/R/q relayed)
make web-showcase    # Chrome with WebMCP flags (--web --detach for CI-shaped runs)
make showcase-stop   # idempotent teardown of stray sessions and the VM port

Logs and pid files land under .showcase/; the detached web variant prints WS_URI=… and exits once the VM service is reachable.

IntentCall doors against the running showcase (second terminal):

dart run packages/harness/tool/intentcall_session.dart demo         # discover + bridge ping
dart run packages/harness/tool/intentcall_session.dart serve-debug  # MCP door pinned to this VM

The session tool resolves the VM service URI from the freshest announcement in the showcase log (or --vm-service-uri) and the IntentCall CLI from INTENTCALL_ROOT, sibling checkouts, or PATH.

Web targets

Chrome/Chromium scenarios run through [ChromeAppTarget] (lib/src/chrome_app_target.dart): same ownership philosophy — own the process, publish the endpoint, attach the client — but the protocol client is universal_browser_cdp (CDP), not the VM service, so it produces a [LaunchedChrome] rather than a [LaunchedApp]. The compile step is deliberately absent: serve the web build yourself and point startUrl at it. A port already answering CDP is adopted as a borrowed session and never killed. Real-Chrome smoke: XS_TEST_CHROME=1 dart test.

Relationship to the MCP server

The server (mcp_server_dart) exposes the same toolkit primitives as MCP tools (fmt_*) for interactive agent sessions. This package is the programmatic path: repeatable E2E scenarios as checked-in Dart, no MCP client needed. It adds no toolkit verbs and never owns app compilation; when an external dev runner owns the session, connect to its published vm_service_uri directly (ADR-0014).