Summary

• Enables the MCP protocol's _meta parameter to be passed through to MCP servers when invoking tools
• Adds meta field to MCPToolCallRequest dataclass and _MCPToolCallRequestOverrides for interceptor support
• Extracts _meta from tool arguments and passes it to session.call_tool()

Related Issue

Proposal for #410

How It Works

When invoking an MCP tool, you can include a _meta field in the tool arguments. This metadata is extracted from the arguments and passed through to the MCP server via session.call_tool().

For example, to pass a correlation ID or user ID for tracing/auditing purposes:

# When invoking a tool, include _meta in the arguments
tool_args = {
    "query": "search term",
    "_meta": {
        "correlationId": "abc-123-xyz",
        "userId": "user_456",
        "requestSource": "web-app"
    }
}

The MCP server receives the _meta object as part of the CallToolRequest, allowing it to:

• Track requests across distributed systems using correlation IDs
• Implement user-specific logic or audit logging
• Route requests based on custom metadata in multi-tenant scenarios

Changes

• Added meta field to MCPToolCallRequest dataclass
• Added meta to _MCPToolCallRequestOverrides for interceptor support
• Extract _meta from tool arguments and pass to session.call_tool()
• Added tests for _meta passthrough functionality
• Added documentation for _meta usage in README

MCP Specification Reference

Per the MCP spec (2025-06-18), _meta is reserved to allow clients and servers to attach additional metadata to their interactions, including:

• Custom routing and context information
• Server-specific metadata for multi-tenant or federated scenarios