detect VM service connections and return a nicer error (#316)

Closes #256

Detect VM Service connections by calling `getVM` - if that _succeeds_ then we return a better error instead of the misleading error about needing a newer SDK.

If it failes with a method not found exception, we continue.

Author: jakemac53

pkgs/dart_mcp_server/CHANGELOG.md

@@ -3,6 +3,8 @@
 - Add `--tools=dart|all` argument to allow enabling only vanilla Dart tools for
   non-flutter projects.
 - Include the device name and target platform in the list_devices tool.
+- Fix erroneous SDK version error messages when connecting to a VM Service
+  instead of DTD URI.
 
 # 0.1.1 (Dart SDK 3.10.0)
 

pkgs/dart_mcp_server/lib/src/mixins/dtd.dart

@@ -8,6 +8,7 @@ import 'dart:convert';
 import 'package:dart_mcp/server.dart';
 import 'package:dds_service_extensions/dds_service_extensions.dart';
 import 'package:dtd/dtd.dart';
+import 'package:json_rpc_2/json_rpc_2.dart';
 import 'package:meta/meta.dart';
 import 'package:unified_analytics/unified_analytics.dart' as ua;
 import 'package:vm_service/vm_service.dart';
@@ -231,9 +232,23 @@ base mixin DartToolingDaemonSupport
     }
 
     try {
-      _dtd = await DartToolingDaemon.connect(
+      final dtd = _dtd = await DartToolingDaemon.connect(
         Uri.parse(request.arguments![ParameterNames.uri] as String),
       );
+      try {
+        await dtd.call(null, 'getVM');
+        // If the call above succeeds, we were connected to the vm service, and
+        // should error.
+        await _resetDtd();
+        return _gotVmServiceUri;
+      } on RpcException catch (e) {
+        // Double check the failure was a method not found failure, if not
+        // rethrow it.
+        if (e.code != RpcErrorCodes.kMethodNotFound) {
+          await _resetDtd();
+          rethrow;
+        }
+      }
       unawaited(_dtd!.done.then((_) async => await _resetDtd()));
 
       await _listenForServices();
@@ -1098,6 +1113,20 @@ base mixin DartToolingDaemonSupport
     isError: true,
   )..failureReason = CallToolFailureReason.flutterDriverNotEnabled;
 
+  static final _gotVmServiceUri = CallToolResult(
+    content: [
+      Content.text(
+        text:
+            'Connected to a VM Service but expected to connect to a Dart '
+            'Tooling Daemon service. When launching apps from an IDE you '
+            'should have a "Copy DTD URI to clipboard" command pallete option, '
+            'or when directly launching apps from a terminal you can pass the '
+            '"--print-dtd" command line option in order to get the DTD URI.',
+      ),
+    ],
+    isError: true,
+  );
+
   static final runtimeErrorsScheme = 'runtime-errors';
 
   static const _defaultTimeoutMs = 5000;

pkgs/dart_mcp_server/test/test_harness.dart

@@ -158,11 +158,16 @@ class TestHarness {
     await AppDebugSession.kill(session.appProcess, session.isFlutter);
   }
 
-  /// Connects the MCP server to the dart tooling daemon at the `dtdUri` from
-  /// [fakeEditorExtension] using the "connectDartToolingDaemon" tool function.
+  /// Connects the MCP server to the dart tooling daemon at the [dtdUri] using
+  /// the "connectDartToolingDaemon" tool function.
+  ///
+  /// By default the DTD Uri will come from the [fakeEditorExtension].
   ///
   /// This mimics a user using the "copy DTD Uri from clipboard" action.
-  Future<void> connectToDtd() async {
+  Future<CallToolResult> connectToDtd({
+    String? dtdUri,
+    bool expectError = false,
+  }) async {
     final tools = (await mcpServerConnection.listTools()).tools;
 
     final connectTool = tools.singleWhere(
@@ -172,11 +177,11 @@ class TestHarness {
     final result = await callToolWithRetry(
       CallToolRequest(
         name: connectTool.name,
-        arguments: {ParameterNames.uri: fakeEditorExtension.dtdUri},
+        arguments: {ParameterNames.uri: dtdUri ?? fakeEditorExtension.dtdUri},
       ),
+      expectError: expectError,
     );
-
-    expect(result.isError, isNot(true), reason: result.content.join('\n'));
+    return result;
   }
 
   /// Helper to send [request] to [mcpServerConnection].

pkgs/dart_mcp_server/test/tools/dtd_test.dart

@@ -794,6 +794,25 @@ void main() {
       expect(log.characters, 10);
     });
   });
+
+  test('connect_to_dtd will reject a vm service URI', () async {
+    final testHarness = await TestHarness.start(inProcess: true);
+    final debugSession = await testHarness.startDebugSession(
+      dartCliAppsPath,
+      'bin/infinite_wait.dart',
+      isFlutter: false,
+    );
+    final connectResult = await testHarness.connectToDtd(
+      dtdUri: debugSession.vmServiceUri,
+      expectError: true,
+    );
+    expect(
+      (connectResult.content.first as TextContent).text,
+      contains('Connected to a VM Service'),
+    );
+    final retryResult = await testHarness.connectToDtd();
+    expect(retryResult.isError, isNot(true));
+  });
 }
 
 extension on Iterable<Resource> {

pkgs/dart_mcp_server/test_fixtures/dart_cli_app/bin/infinite_wait.dart

@@ -0,0 +1,9 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+void main() async {
+  while (true) {
+    await Future.delayed(const Duration(seconds: 1));
+  }
+}