MCP responses use structuredContent instead of content, breaking Claude Code compatibility

[Merugokhale]

Description

The agentmail-mcp package returns tool results using the structuredContent field instead of the content field. This causes compatibility issues with Claude Code and other MCP clients that expect results in the content field.

Error

When calling any tool (e.g., list_inboxes, get_inbox), Claude Code returns:

MCP error -32602: Invalid tools/call result: [
  {
    "expected": "record",
    "code": "invalid_type",
    "path": ["structuredContent"],
    "message": "Invalid input: expected record, received string"
  }
]

Environment

• agentmail-mcp version: 0.2.1
• Client: Claude Code CLI
• MCP server shows as "Connected" via claude mcp list

Expected Behavior

Tool results should be returned in a format compatible with the MCP specification that Claude Code can parse.

Related Issues

This appears to be a known issue affecting multiple MCP adapter libraries:

• langchain-mcp-adapters ignores structuredContent field from MCP tool responses langchain-ai/langchain-mcp-adapters#283
• Python: MCP CallToolResult.structuredContent field is not parsed, causing tool results to return None microsoft/agent-framework#3313

Workaround

Currently using the AgentMail REST API directly as a workaround.