Add error checks for accessing Request members

pkgs/dart_mcp/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 0.2.3-wip
+
+- Added error checking to required fields of all `Request` subclasses so that
+  they will throw helpful errors when accessed and not set.
+
 ## 0.2.2
 
 - Refactor `ClientImplementation` and `ServerImplementation` to the shared

pkgs/dart_mcp/lib/src/api/completions.dart

@@ -27,11 +27,22 @@ extension type CompleteRequest.fromMap(Map<String, Object?> _value)
   ///
   /// In the case of a [ResourceReference], it must refer to a
   /// [ResourceTemplate].
-  Reference get ref => _value['ref'] as Reference;
+  Reference get ref {
+    final ref = _value['ref'] as Reference?;
+    if (ref == null) {
+      throw ArgumentError('Missing ref field in $CompleteRequest.');
+    }
+    return ref;
+  }
 
   /// The argument's information.
-  CompletionArgument get argument =>
-      (_value['argument'] as Map).cast<String, Object?>() as CompletionArgument;
+  CompletionArgument get argument {
+    final argument = _value['argument'] as CompletionArgument?;
+    if (argument == null) {
+      throw ArgumentError('Missing argument field in $CompleteRequest.');
+    }
+    return argument;
+  }
 }
 
 /// The server's response to a completion/complete request

pkgs/dart_mcp/lib/src/api/initialization.dart

@@ -30,10 +30,21 @@ extension type InitializeRequest._fromMap(Map<String, Object?> _value)
   ProtocolVersion? get protocolVersion =>
       ProtocolVersion.tryParse(_value['protocolVersion'] as String);
 
-  ClientCapabilities get capabilities =>
-      _value['capabilities'] as ClientCapabilities;
+  ClientCapabilities get capabilities {
+    final capabilities = _value['capabilities'] as ClientCapabilities?;
+    if (capabilities == null) {
+      throw ArgumentError('Missing capabilities field in $InitializeRequest.');
+    }
+    return capabilities;
+  }
 
-  Implementation get clientInfo => _value['clientInfo'] as Implementation;
+  Implementation get clientInfo {
+    final clientInfo = _value['clientInfo'] as Implementation?;
+    if (clientInfo == null) {
+      throw ArgumentError('Missing clientInfo field in $InitializeRequest.');
+    }
+    return clientInfo;
+  }
 }
 
 /// After receiving an initialize request from the client, the server sends

pkgs/dart_mcp/lib/src/api/logging.dart

@@ -26,8 +26,18 @@ extension type SetLevelRequest.fromMap(Map<String, Object?> _value)
   ///
   /// The server should send all logs at this level and higher (i.e., more
   /// severe) to the client as notifications/message.
-  LoggingLevel get level =>
-      LoggingLevel.values.firstWhere((level) => level.name == _value['level']);
+  LoggingLevel get level {
+    final levelName = _value['level'];
+    final foundLevel = LoggingLevel.values.firstWhereOrNull(
+      (level) => level.name == levelName,
+    );
+    if (foundLevel == null) {
+      throw ArgumentError(
+        "Invalid level field in $SetLevelRequest: didn't find level $levelName",
+      );
+    }
+    return foundLevel;
+  }
 }
 
 /// Notification of a log message passed from server to client.

pkgs/dart_mcp/lib/src/api/prompts.dart

@@ -49,7 +49,13 @@ extension type GetPromptRequest.fromMap(Map<String, Object?> _value)
   });
 
   /// The name of the prompt or prompt template.
-  String get name => _value['name'] as String;
+  String get name {
+    final name = _value['name'] as String?;
+    if (name == null) {
+      throw ArgumentError('Missing name field in $GetPromptRequest.');
+    }
+    return name;
+  }
 
   /// Arguments to use for templating the prompt.
   Map<String, Object?>? get arguments =>

pkgs/dart_mcp/lib/src/api/resources.dart

@@ -80,7 +80,13 @@ extension type ReadResourceRequest.fromMap(Map<String, Object?> _value)
 
   /// The URI of the resource to read. The URI can use any protocol; it is
   /// up to the server how to interpret it.
-  String get uri => _value['uri'] as String;
+  String get uri {
+    final uri = _value['uri'] as String?;
+    if (uri == null) {
+      throw ArgumentError('Missing uri field in $ReadResourceRequest.');
+    }
+    return uri;
+  }
 }
 
 /// The server's response to a resources/read request from the client.
@@ -128,7 +134,13 @@ extension type SubscribeRequest.fromMap(Map<String, Object?> _value)
 
   /// The URI of the resource to subscribe to. The URI can use any protocol;
   /// it is up to the server how to interpret it.
-  String get uri => _value['uri'] as String;
+  String get uri {
+    final uri = _value['uri'] as String?;
+    if (uri == null) {
+      throw ArgumentError('Missing uri field in $SubscribeRequest.');
+    }
+    return uri;
+  }
 }
 
 /// Sent from the client to request cancellation of resources/updated
@@ -146,7 +158,13 @@ extension type UnsubscribeRequest.fromMap(Map<String, Object?> _value)
       UnsubscribeRequest.fromMap({'uri': uri, if (meta != null) '_meta': meta});
 
   /// The URI of the resource to unsubscribe from.
-  String get uri => _value['uri'] as String;
+  String get uri {
+    final uri = _value['uri'] as String?;
+    if (uri == null) {
+      throw ArgumentError('Missing uri field in $UnsubscribeRequest.');
+    }
+    return uri;
+  }
 }
 
 /// A notification from the server to the client, informing it that a resource

pkgs/dart_mcp/lib/src/api/roots.dart

@@ -33,7 +33,13 @@ extension type ListRootsResult.fromMap(Map<String, Object?> _value)
         if (meta != null) '_meta': meta,
       });
 
-  List<Root> get roots => (_value['roots'] as List).cast<Root>();
+  List<Root> get roots {
+    final roots = _value['roots'] as List?;
+    if (roots == null) {
+      throw ArgumentError('Missing roots field in $ListRootsResult.');
+    }
+    return roots.cast<Root>();
+  }
 }
 
 /// Represents a root directory or file that the server can operate on.

pkgs/dart_mcp/lib/src/api/sampling.dart

@@ -39,8 +39,13 @@ extension type CreateMessageRequest.fromMap(Map<String, Object?> _value)
   });
 
   /// The messages to send to the LLM.
-  List<SamplingMessage> get messages =>
-      (_value['messages'] as List).cast<SamplingMessage>();
+  List<SamplingMessage> get messages {
+    final messages = _value['messages'] as List?;
+    if (messages == null) {
+      throw ArgumentError('Missing messages field in $CreateMessageRequest.');
+    }
+    return messages.cast<SamplingMessage>();
+  }
 
   /// The server's preferences for which model to select.
   ///
@@ -69,7 +74,13 @@ extension type CreateMessageRequest.fromMap(Map<String, Object?> _value)
   /// The maximum number of tokens to sample, as requested by the server.
   ///
   /// The client MAY choose to sample fewer tokens than requested.
-  int get maxTokens => _value['maxTokens'] as int;
+  int get maxTokens {
+    final maxTokens = _value['maxTokens'] as int?;
+    if (maxTokens == null) {
+      throw ArgumentError('Missing maxTokens field in $CreateMessageRequest.');
+    }
+    return maxTokens;
+  }
 
   /// Note: This has no documentation in the specification or schema.
   List<String>? get stopSequences =>

pkgs/dart_mcp/lib/src/api/tools.dart

@@ -80,7 +80,13 @@ extension type CallToolRequest._fromMap(Map<String, Object?> _value)
   });
 
   /// The name of the method to invoke.
-  String get name => _value['name'] as String;
+  String get name {
+    final name = _value['name'] as String?;
+    if (name == null) {
+      throw ArgumentError('Missing name field in $CallToolRequest');
+    }
+    return name;
+  }
 
   /// The arguments to pass to the method.
   Map<String, Object?>? get arguments =>

pkgs/dart_mcp/lib/src/shared.dart

@@ -79,10 +79,15 @@ base class MCPBase {
   void registerRequestHandler<T extends Request?, R extends Result?>(
     String name,
     FutureOr<R> Function(T) impl,
-  ) => _peer.registerMethod(
-    name,
-    (Parameters p) => impl((p.value as Map?)?.cast<String, Object?>() as T),
-  );
+  ) => _peer.registerMethod(name, (Parameters p) {
+    if (p.value != null && p.value is! Map) {
+      throw ArgumentError(
+        'Request to $name must be a Map or null. Instead, got '
+        '${p.value.runtimeType}',
+      );
+    }
+    return impl((p.value as Map?)?.cast<String, Object?>() as T);
+  });
 
   /// Registers a notification handler named [name] on this server.
   void registerNotificationHandler<T extends Notification?>(

pkgs/dart_mcp/pubspec.yaml

@@ -1,5 +1,5 @@
 name: dart_mcp
-version: 0.2.2
+version: 0.2.3-wip
 description: A package for making MCP servers and clients.
 repository: https://github.com/dart-lang/ai/tree/main/pkgs/dart_mcp
 issue_tracker: https://github.com/dart-lang/ai/issues?q=is%3Aissue+is%3Aopen+label%3Apackage%3Adart_mcp
@@ -10,7 +10,7 @@ environment:
 dependencies:
   async: ^2.13.0
   collection: ^1.19.1
-  json_rpc_2: '>=3.0.3 <5.0.0'
+  json_rpc_2: ">=3.0.3 <5.0.0"
   meta: ^1.16.0
   stream_channel: ^2.1.4
   stream_transform: ^2.1.1

pkgs/dart_mcp/test/api/api_test.dart

@@ -58,4 +58,51 @@ void main() {
       false,
     );
   });
+
+  group('API object validation', () {
+    test('throws when required fields are missing', () {
+      final empty = <String, Object?>{};
+
+      // Initialization
+      expect(
+        () => (empty as InitializeRequest).capabilities,
+        throwsArgumentError,
+      );
+      expect(
+        () => (empty as InitializeRequest).clientInfo,
+        throwsArgumentError,
+      );
+
+      // Tools
+      expect(() => (empty as CallToolRequest).name, throwsArgumentError);
+
+      // Resources
+      expect(() => (empty as ReadResourceRequest).uri, throwsArgumentError);
+      expect(() => (empty as SubscribeRequest).uri, throwsArgumentError);
+      expect(() => (empty as UnsubscribeRequest).uri, throwsArgumentError);
+
+      // Roots
+      expect(() => (empty as ListRootsResult).roots, throwsArgumentError);
+
+      // Prompts
+      expect(() => (empty as GetPromptRequest).name, throwsArgumentError);
+
+      // Completions
+      expect(() => (empty as CompleteRequest).ref, throwsArgumentError);
+      expect(() => (empty as CompleteRequest).argument, throwsArgumentError);
+
+      // Logging
+      expect(() => (empty as SetLevelRequest).level, throwsArgumentError);
+
+      // Sampling
+      expect(
+        () => (empty as CreateMessageRequest).messages,
+        throwsArgumentError,
+      );
+      expect(
+        () => (empty as CreateMessageRequest).maxTokens,
+        throwsArgumentError,
+      );
+    });
+  });
 }