Instructions may be null

[domesticmouse]

While attempting to run my mcp_client against @modelcontextprotocol/server-filesystem I hit an issue:

$ dart run bin/mcp_client.dart \
  npx -y @modelcontextprotocol/server-filesystem \
  ../../colorist_ui
connecting to server
server started
initializing server
[StdErr from server npx]: Secure MCP Filesystem Server running on stdio
[StdErr from server npx]: Allowed directories: [ '/Users/brett/Documents/GitHub/colorist_ui' ]
Unhandled exception:
type 'Null' is not a subtype of type 'String' in type cast
#0      InitializeResult.instructions (package:dart_mcp/src/api/initialization.dart:68:53)
#1      main (file:///Users/brett/Documents/GitHub/developer_assistant_mcp/mcp_client/bin/mcp_client.dart:38:36)
<asynchronous suspension>

The core issue is that the dart_mcp initialization code assumes that the server always supplies instructions.

Here's my code, for reference:

// 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.

import 'package:dart_mcp/client.dart';

void main(List<String> args) async {
  if (args.isEmpty) {
    print('Usage: dart mcp_client.dart <mcp server command>');
    return;
  }

  final client = MCPClient(
    ClientImplementation(name: 'example dart client', version: '0.1.0'),
  );
  print('connecting to server');
  final server = await client.connectStdioServer(
    args.first,
    args.length > 1 ? args.sublist(1) : [],
  );
  print('server started');

  print('initializing server');
  final initializeResult = await server.initialize(
    InitializeRequest(
      protocolVersion: protocolVersion,
      capabilities: client.capabilities,
      clientInfo: client.implementation,
    ),
  );
  print('''
Initialized:
  Protocol Version: ${initializeResult.protocolVersion}
  Capabilities: ${initializeResult.capabilities}
  Server Info:
    Name: ${initializeResult.serverInfo.name}
    Version: ${initializeResult.serverInfo.version}
  Instructions: ${initializeResult.instructions}
''');
  if (initializeResult.protocolVersion != protocolVersion) {
    throw StateError(
      'Protocol version mismatch, expected $protocolVersion, '
      'got ${initializeResult.protocolVersion}',
    );
  }

  if (initializeResult.capabilities.tools == null) {
    await server.shutdown();
    throw StateError('Server doesn\'t support tools!');
  }

  server.notifyInitialized(InitializedNotification());
  print('sent initialized notification');

  print('Listing tools from server: ');
  final toolsResult = await server.listTools(ListToolsRequest());
  for (final tool in toolsResult.tools) {
    print('  - Tool: ${tool.name}');
    final description = tool.description;
    if (description != null) print('    Description: $description');
  }

  await client.shutdown();
}

[jakemac53]

closed by #98