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Provide output logs when the process launch fails. #293

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d8a6c43
Provide output logs when the process launch fails.
gspencergoog Oct 6, 2025
5713879
Add tests
gspencergoog Oct 6, 2025
63314cb
Limit log output
gspencergoog Oct 6, 2025
f61dfda
Limit log line output
gspencergoog Oct 6, 2025
8a4d88f
Refine message
gspencergoog Oct 6, 2025
8c5912a
Update CHANGELOG.md
gspencergoog Oct 7, 2025
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2 changes: 2 additions & 0 deletions pkgs/dart_mcp_server/CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,8 @@
* Fix a bug in hot_reload ([#290](https://github.com/dart-lang/ai/issues/290)).
* Add the `list_devices`, `launch_app`, `get_app_logs`, and `list_running_apps`
tools for running Flutter apps.
* Add extra log output to failed launches, and allow AI to specify the maxLines
of log output.

# 0.1.0 (Dart SDK 3.9.0)

Expand Down
32 changes: 29 additions & 3 deletions pkgs/dart_mcp_server/lib/src/mixins/flutter_launcher.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@ library;
import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'dart:math' as math;

import 'package:dart_mcp/server.dart';

Expand Down Expand Up @@ -170,13 +171,21 @@ base mixin FlutterLauncherSupport
LoggingLevel.info,
'Flutter application ${process!.pid} exited with code $exitCode.',
);
_runningApps.remove(process.pid);
if (!completer.isCompleted) {
final logs = _runningApps[process.pid]?.logs ?? [];
// Only output the last 500 lines of logs.
final startLine = math.max(0, logs.length - 500);
final logOutput = [
if (startLine > 0) '[skipping $startLine log lines]...',
...logs.sublist(startLine),
];
completer.completeError(
'Flutter application exited with code $exitCode before the DTD '
'URI was found.',
'URI was found, with log output:\n${logOutput.join('\n')}',
);
}
_runningApps.remove(process.pid);

// Cancel subscriptions after all processing is done.
await stdoutSubscription.cancel();
await stderrSubscription.cancel();
Expand Down Expand Up @@ -360,6 +369,11 @@ base mixin FlutterLauncherSupport
'The process ID of the flutter run process running the '
'application.',
),
'maxLines': Schema.int(
description:
'The maximum number of log lines to return from the end of the '
'logs. Defaults to 500. If set to -1, all logs will be returned.',
),
},
required: ['pid'],
),
Expand All @@ -376,8 +390,9 @@ base mixin FlutterLauncherSupport

Future<CallToolResult> _getAppLogs(CallToolRequest request) async {
final pid = request.arguments!['pid'] as int;
var maxLines = request.arguments!['maxLines'] as int? ?? 500;
log(LoggingLevel.info, 'Getting logs for application with PID: $pid');
final logs = _runningApps[pid]?.logs;
var logs = _runningApps[pid]?.logs;

if (logs == null) {
log(
Expand All @@ -394,6 +409,17 @@ base mixin FlutterLauncherSupport
);
}

if (maxLines == -1) {
maxLines = logs.length;
}
if (maxLines > 0 && maxLines <= logs.length) {
final startLine = logs.length - maxLines;
logs = [
if (startLine > 0) '[skipping $startLine log lines]...',
...logs.sublist(startLine),
];
}

return CallToolResult(
content: [TextContent(text: logs.join('\n'))],
structuredContent: {'logs': logs},
Expand Down
279 changes: 279 additions & 0 deletions pkgs/dart_mcp_server/test/tools/flutter_launcher_test.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -197,6 +197,285 @@ void main() {
await client.shutdown();
},
);

test.test('launch_app tool fails when process exits early', () async {
final mockProcessManager = MockProcessManager();
mockProcessManager.addCommand(
Command(
[
Platform.isWindows
? r'C:\path\to\flutter\sdk\bin\cache\dart-sdk\bin\dart.exe'
: '/path/to/flutter/sdk/bin/cache/dart-sdk/bin/dart',
'language-server',
'--protocol',
'lsp',
],
stdout:
'''Content-Length: 145\r\n\r\n{"jsonrpc":"2.0","id":0,"result":{"capabilities":{"workspace":{"workspaceFolders":{"supported":true,"changeNotifications":true}},"workspaceSymbolProvider":true}}}''',
),
);
mockProcessManager.addCommand(
Command(
[
Platform.isWindows
? r'C:\path\to\flutter\sdk\bin\flutter.bat'
: '/path/to/flutter/sdk/bin/flutter',
'run',
'--print-dtd',
'--device-id',
'test-device',
],
stderr: 'Something went wrong',
exitCode: Future.value(1),
),
);
final serverAndClient = await createServerAndClient(
processManager: mockProcessManager,
fileSystem: fileSystem,
);
final server = serverAndClient.server;
final client = serverAndClient.client;

// Initialize
await client.initialize(
InitializeRequest(
protocolVersion: ProtocolVersion.latestSupported,
capabilities: ClientCapabilities(),
clientInfo: Implementation(name: 'test_client', version: '1.0.0'),
),
);
client.notifyInitialized();

// Call the tool
final result = await client.callTool(
CallToolRequest(
name: 'launch_app',
arguments: {'root': projectRoot, 'device': 'test-device'},
),
);

test.expect(result.isError, true);
final textOutput = result.content as List<TextContent>;
test.expect(
textOutput.map((context) => context.text).toList().join('\n'),
test.stringContainsInOrder([
'Flutter application exited with code 1 before the DTD URI was found',
'with log output',
'Something went wrong',
]),
);
await server.shutdown();
await client.shutdown();
});

test.test('stop_app tool stops a running app', () async {
final dtdUri = 'ws://127.0.0.1:12345/abcdefg=';
final processPid = 54321;
final mockProcessManager = MockProcessManager();
mockProcessManager.addCommand(
Command(
[
Platform.isWindows
? r'C:\path\to\flutter\sdk\bin\cache\dart-sdk\bin\dart.exe'
: '/path/to/flutter/sdk/bin/cache/dart-sdk/bin/dart',
'language-server',
'--protocol',
'lsp',
],
stdout:
'''Content-Length: 145\r\n\r\n{"jsonrpc":"2.0","id":0,"result":{"capabilities":{"workspace":{"workspaceFolders":{"supported":true,"changeNotifications":true}},"workspaceSymbolProvider":true}}}
''',
),
);
mockProcessManager.addCommand(
Command(
[
Platform.isWindows
? r'C:\path\to\flutter\sdk\bin\flutter.bat'
: '/path/to/flutter/sdk/bin/flutter',
'run',
'--print-dtd',
'--device-id',
'test-device',
],
stdout: 'The Dart Tooling Daemon is available at: $dtdUri\n',
pid: processPid,
),
);
final serverAndClient = await createServerAndClient(
processManager: mockProcessManager,
fileSystem: fileSystem,
);
final server = serverAndClient.server;
final client = serverAndClient.client;

// Initialize and launch the app
await client.initialize(
InitializeRequest(
protocolVersion: ProtocolVersion.latestSupported,
capabilities: ClientCapabilities(),
clientInfo: Implementation(name: 'test_client', version: '1.0.0'),
),
);
client.notifyInitialized();
await client.callTool(
CallToolRequest(
name: 'launch_app',
arguments: {'root': projectRoot, 'device': 'test-device'},
),
);

// Stop the app
final result = await client.callTool(
CallToolRequest(name: 'stop_app', arguments: {'pid': processPid}),
);

test.expect(result.isError, test.isNot(true));
test.expect(result.structuredContent, {'success': true});
test.expect(mockProcessManager.killedPids, [processPid]);
await server.shutdown();
await client.shutdown();
});

test.test('get_app_logs tool respects maxLines', () async {
final dtdUri = 'ws://127.0.0.1:12345/abcdefg=';
final processPid = 54321;
final mockProcessManager = MockProcessManager();
mockProcessManager.addCommand(
Command(
[
Platform.isWindows
? r'C:\path\to\flutter\sdk\bin\cache\dart-sdk\bin\dart.exe'
: '/path/to/flutter/sdk/bin/cache/dart-sdk/bin/dart',
'language-server',
'--protocol',
'lsp',
],
stdout:
'''Content-Length: 145\r\n\r\n{"jsonrpc":"2.0","id":0,"result":{"capabilities":{"workspace":{"workspaceFolders":{"supported":true,"changeNotifications":true}},"workspaceSymbolProvider":true}}}''',
),
);
mockProcessManager.addCommand(
Command(
[
Platform.isWindows
? r'C:\path\to\flutter\sdk\bin\flutter.bat'
: '/path/to/flutter/sdk/bin/flutter',
'run',
'--print-dtd',
'--device-id',
'test-device',
],
stdout:
'line 1\nline 2\nline 3\n'
'The Dart Tooling Daemon is available at: $dtdUri\n',
pid: processPid,
),
);
final serverAndClient = await createServerAndClient(
processManager: mockProcessManager,
fileSystem: fileSystem,
);
final server = serverAndClient.server;
final client = serverAndClient.client;

// Initialize and launch the app
await client.initialize(
InitializeRequest(
protocolVersion: ProtocolVersion.latestSupported,
capabilities: ClientCapabilities(),
clientInfo: Implementation(name: 'test_client', version: '1.0.0'),
),
);
client.notifyInitialized();
await client.callTool(
CallToolRequest(
name: 'launch_app',
arguments: {'root': projectRoot, 'device': 'test-device'},
),
);

// Get the logs
final result = await client.callTool(
CallToolRequest(
name: 'get_app_logs',
arguments: {'pid': processPid, 'maxLines': 2},
),
);

test.expect(result.isError, test.isNot(true));
test.expect(result.structuredContent, {
'logs': [
'[skipping 2 log lines]...',
'[stdout] line 3',
'[stdout] The Dart Tooling Daemon is available at: ws://127.0.0.1:12345/abcdefg=',
],
});
await server.shutdown();
await client.shutdown();
});

test.test('list_devices tool returns available devices', () async {
final mockProcessManager = MockProcessManager();
mockProcessManager.addCommand(
Command(
[
Platform.isWindows
? r'C:\path\to\flutter\sdk\bin\cache\dart-sdk\bin\dart.exe'
: '/path/to/flutter/sdk/bin/cache/dart-sdk/bin/dart',
'language-server',
'--protocol',
'lsp',
],
stdout:
'''Content-Length: 145\r\n\r\n{"jsonrpc":"2.0","id":0,"result":{"capabilities":{"workspace":{"workspaceFolders":{"supported":true,"changeNotifications":true}},"workspaceSymbolProvider":true}}}
''',
),
);
mockProcessManager.addCommand(
Command(
[
Platform.isWindows
? r'C:\path\to\flutter\sdk\bin\flutter.bat'
: '/path/to/flutter/sdk/bin/flutter',
'devices',
'--machine',
],
stdout: jsonEncode([
{'id': 'test-device-1'},
{'id': 'test-device-2'},
]),
),
);
final serverAndClient = await createServerAndClient(
processManager: mockProcessManager,
fileSystem: fileSystem,
);
final server = serverAndClient.server;
final client = serverAndClient.client;

// Initialize
await client.initialize(
InitializeRequest(
protocolVersion: ProtocolVersion.latestSupported,
capabilities: ClientCapabilities(),
clientInfo: Implementation(name: 'test_client', version: '1.0.0'),
),
);
client.notifyInitialized();

// List devices
final result = await client.callTool(
CallToolRequest(name: 'list_devices', arguments: {}),
);

test.expect(result.isError, test.isNot(true));
test.expect(result.structuredContent, {
'devices': ['test-device-1', 'test-device-2'],
});
await server.shutdown();
await client.shutdown();
});
});
}

Expand Down
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