What
New mcp_call subtype for MCP requests that enables directly invoking MCP tools via the subprocess MCP client without requiring a model turn.
Details
- Accepts a fully-qualified MCP tool name (e.g.,
mcp__server__tool_name) - No permission check — the control channel is trusted
- SDK-type MCP servers are rejected (they are caller-provided and should be invoked directly)
- Handles URL elicitation for tools that require browser authentication
- Result content passes through the same processing as model-turn MCP calls (large results may be truncated)
Evidence
MCP call handler (search for "mcp_call") — $K5 at line ~639274
Strings lifted out of the shipped bundle, so the claim above can be checked against them.