add signature_help tool

pkgs/dart_tooling_mcp_server/README.md

@@ -9,6 +9,7 @@ WIP. This package is still experimental and is likely to evolve quickly.
 | Tool Name | Feature Group | Description |
 | --- | --- | --- |
 | `analyze_files` | `static analysis` | Analyzes the entire project for errors. |
+| `signature_help` | `static_analysis` | Gets signature information for usage at a given cursor position. |
 | `resolve_workspace_symbol` | `static analysis` | Look up a symbol or symbols in all workspaces by name. |
 | `dart_fix` | `static tool` | Runs `dart fix --apply` for the given project roots. |
 | `dart_format` | `static tool` | Runs `dart format .` for the given project roots. |

pkgs/dart_tooling_mcp_server/lib/src/mixins/analyzer.dart

@@ -67,6 +67,7 @@ base mixin DartAnalyzerSupport
     if (unsupportedReasons.isEmpty) {
       registerTool(analyzeFilesTool, _analyzeFiles);
       registerTool(resolveWorkspaceSymbolTool, _resolveWorkspaceSymbol);
+      registerTool(signatureHelpTool, _signatureHelp);
     }
 
     // Don't call any methods on the client until we are fully initialized
@@ -173,6 +174,7 @@ base mixin DartAnalyzerSupport
                   textDocument: lsp.TextDocumentClientCapabilities(
                     publishDiagnostics:
                         lsp.PublishDiagnosticsClientCapabilities(),
+                    signatureHelp: lsp.SignatureHelpClientCapabilities(),
                   ),
                 ),
               ).toJson(),
@@ -272,6 +274,27 @@ base mixin DartAnalyzerSupport
     return CallToolResult(content: [TextContent(text: jsonEncode(result))]);
   }
 
+  /// Implementation of the [signatureHelpTool], get signature help for a given
+  /// position in a file.
+  Future<CallToolResult> _signatureHelp(CallToolRequest request) async {
+    final errorResult = await _ensurePrerequisites(request);
+    if (errorResult != null) return errorResult;
+
+    final uri = Uri.parse(request.arguments![ParameterNames.uri] as String);
+    final position = lsp.Position(
+      line: request.arguments![ParameterNames.line] as int,
+      character: request.arguments![ParameterNames.column] as int,
+    );
+    final result = await _lspConnection.sendRequest(
+      lsp.Method.textDocument_signatureHelp.toString(),
+      lsp.SignatureHelpParams(
+        textDocument: lsp.TextDocumentIdentifier(uri: uri),
+        position: position,
+      ).toJson(),
+    );
+    return CallToolResult(content: [TextContent(text: jsonEncode(result))]);
+  }
+
   /// Ensures that all prerequisites for any analysis task are met.
   ///
   /// Returns an error response if any prerequisite is not met, otherwise
@@ -382,6 +405,33 @@ base mixin DartAnalyzerSupport
     annotations: ToolAnnotations(title: 'Project search', readOnlyHint: true),
   );
 
+  @visibleForTesting
+  static final signatureHelpTool = Tool(
+    name: 'signature_help',
+    description:
+        'Get signature help for an API being used at a given cursor '
+        'position in a file.',
+    inputSchema: Schema.object(
+      properties: {
+        ParameterNames.uri: Schema.string(
+          description: 'The URI of the file to get signature help for.',
+        ),
+        ParameterNames.line: Schema.int(
+          description: 'The line number of the cursor position.',
+        ),
+        ParameterNames.column: Schema.int(
+          description: 'The column number of the cursor position.',
+        ),
+      },
+      required: [
+        ParameterNames.uri,
+        ParameterNames.line,
+        ParameterNames.column,
+      ],
+    ),
+    annotations: ToolAnnotations(title: 'Signature help', readOnlyHint: true),
+  );
+
   @visibleForTesting
   static final noRootsSetResponse = CallToolResult(
     isError: true,

pkgs/dart_tooling_mcp_server/lib/src/utils/cli_utils.dart

@@ -171,6 +171,7 @@ ListSchema rootsSchema({bool supportsPaths = false}) => Schema.list(
           title:
               'Paths to run this tool on. Must resolve to a path that is '
               'within the "root".',
+          items: Schema.string(),
         ),
     },
     required: [ParameterNames.root],

pkgs/dart_tooling_mcp_server/lib/src/utils/constants.dart

@@ -4,9 +4,12 @@
 
 /// A namespace for all the parameter names.
 extension ParameterNames on Never {
+  static const column = 'column';
   static const command = 'command';
+  static const line = 'line';
   static const packageName = 'packageName';
   static const paths = 'paths';
+  static const position = 'position';
   static const query = 'query';
   static const root = 'root';
   static const roots = 'roots';

pkgs/dart_tooling_mcp_server/test/tools/analyzer_test.dart

@@ -7,6 +7,7 @@ import 'dart:io';
 import 'package:dart_mcp/server.dart';
 import 'package:dart_tooling_mcp_server/src/mixins/analyzer.dart';
 import 'package:dart_tooling_mcp_server/src/utils/constants.dart';
+import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
 import 'package:test_descriptor/test_descriptor.dart' as d;
 
@@ -112,6 +113,37 @@ void main() {
       );
     });
 
+    test('can get signature help', () async {
+      final counterAppRoot = rootForPath(counterAppPath);
+      testHarness.mcpClient.addRoot(counterAppRoot);
+      await pumpEventQueue();
+
+      final result = await testHarness.callToolWithRetry(
+        CallToolRequest(
+          name: DartAnalyzerSupport.signatureHelpTool.name,
+          arguments: {
+            ParameterNames.uri: p.join(counterAppRoot.uri, 'lib', 'main.dart'),
+            ParameterNames.line: 16,
+            ParameterNames.column: 15,
+          },
+        ),
+      );
+      expect(result.isError, isNot(true));
+
+      expect(
+        result.content.single,
+        isA<TextContent>().having(
+          (t) => t.text,
+          'text',
+          allOf(
+            contains('Creates a MaterialApp'), // From the doc comment
+            contains('MaterialApp({Key? key,'), // The actual signature
+            contains('"label":"Key? key'), // Specific label for the key param
+          ),
+        ),
+      );
+    });
+
     test('cannot analyze without roots set', () async {
       final result = await testHarness.callToolWithRetry(
         CallToolRequest(name: DartAnalyzerSupport.analyzeFilesTool.name),