Filter 18: mcp_tool_resolve

[leseb]

Parent

Part of #93

Purpose

Resolve declared MCP tool references into concrete tool definitions by calling upstream MCP servers. Runs after tool_parse (#42), gated on tool_parse.has_mcp == true.

The client declares "I want tools from this MCP server" without specifying which tools. This filter resolves those declarations by calling tools/list on each MCP server and building a tool map for downstream dispatch by mcp_tool (#27).

Scope

• New filter: mcp_tool_resolve
• Uses rmcp SDK (StreamableHttpClientTransport) for outbound MCP calls
• For each MCP tool entry (from tool_parse metadata):
  • Resolve server_url from the MCP tool entry
  • Check responses.previous_tools for reusable listings (keyed on server_label + allowed_tools)
  • If not reusable: call MCP server tools/list via rmcp and cache results
  • Reject duplicate tool names across MCP servers
  • Apply allowed_tools filter
  • Build mcp_tool_map: tool_name → server config
• Write responses.mcp_tool_map to ResponsesState
• Short-circuit with FilterAction::Continue when tool_parse.has_mcp is false
• Tests for MCP tool listing, caching, dedup, and filtering

Dependencies

• Depends on tool_parse: parse and register function/built-in tools #42 (tool_parse: classification + opaque MCP entries)
• MCP servers (HTTP, Streamable HTTP)

Reference

• Proposal: Responses API Filters — Filter 4 (MCP listing portion)