MCP: get_tool_schema should return output schema + support batch fetch

[barockok]

Context

Audit vs Composio COMPOSIO_GET_TOOL_SCHEMAS, which returns input + output schema and accepts multiple slugs per call. Workbench get_tool_schema returns input only, one tool per call.

Gap

• Agent can't predict result shape → must execute to discover output, or guess downstream parsing.
• One round-trip per schema lookup when prepping a multi-tool plan.

Proposal

1. Extend get_tool_schema to accept tool: string | string[]; return a keyed map for arrays.
2. Surface output schema where plugins define one. Add optional outputSchema to the plugin tool type; return alongside schema (input). Tools without it return input-only (no breakage).

Acceptance

• Array input returns { [tool]: { input, output? } }.
• Single string input keeps current shape (back-compat) or documented change.
• Plugin SDK type gains optional outputSchema.
• Tests: single, batch, unknown-tool-in-batch, tool with/without output schema.

Source

packages/server/src/mcp/meta-tools.ts (get_tool_schema, lines ~77-85).