{"rule_id":"AB-021","category":"machine_readable","icon":"📞","title":"MCP Tool Calls","short_description":"Your MCP server responds when agents try to call a tool.","user_value":"Listing tools isn't enough — agents need to actually call them. If calls fail, your MCP server is useless.","wrong_example":"MCP server lists tools but doesn't respond when agents try to use them.","right_example":"MCP server processes tool calls and returns results.","effort_hint":"moderate","estimated_cost":"$10-50","severity":"high","pillar":"discovery","version":"1.0.0","counted_in_score":true,"check":{"type":"json_rpc","sources":["mcp"],"target":"/mcp"},"fix":{"eligible":false,"type":"none","note":"Implement MCP tools/call JSON-RPC method"},"checklist_anchor":"#AB-021","checklist_url":"/agent-readiness-checklist#AB-021","html_url":"/rules/AB-021","json_url":"/rules/AB-021.json","related_rules":[{"rule_id":"AB-010","title":"Machine-Readable Pricing","category":"machine_readable"},{"rule_id":"AB-011","title":"Rate Limits Declared","category":"machine_readable"},{"rule_id":"AB-012","title":"Structured Error Responses","category":"machine_readable"},{"rule_id":"AB-020","title":"MCP Tools List","category":"machine_readable"},{"rule_id":"AB-022","title":"MCP Streaming Support","category":"machine_readable"},{"rule_id":"AB-109","title":"Agent Card version 1.0.0+","category":"machine_readable"},{"rule_id":"AB-163","title":"Skill.json (JSON-LD) availability","category":"machine_readable"},{"rule_id":"AB-164","title":"Error catalog endpoint","category":"machine_readable"}]}