MCP Server Contract Tester — stdio + WebSocket Transports, JSON Schema 2020-12 Tool Validation

Protocol basis (initialize era, 2024-11-05 … 2025-11-25): the initialize request carries protocolVersion, client capabilities and clientInfo; the server answers serverInfo + capabilities; the client must then send notifications/initialized. The official stdio binding is newline-delimited JSON-RPC — no embedded newlines, logs on stderr, stdout reserved for MCP messages; WebSocket is a custom binding (spec § Custom Transports) carrying one JSON-RPC message per text frame. Every tool's inputSchema must be a JSON Schema with type:object at the root; this tester compiles each one under Ajv draft 2020-12. sampling is a server→client request: when the client declares the sampling capability, a server may send sampling/createMessage (messages[] + maxTokens), which the tester answers per spec and validates. The 2026-07-28 revision moves to a stateless core where servers no longer initiate requests; this tester targets the classic initialize era and notes that in the report. The default mode self-verifies the embedded, fully compliant reference server over an in-process stdio stream pair and a real loopback TCP WebSocket — deterministic and offline. For live targets, pick stdio-command (spawns your command) or websocket-url (connects to ws:// or wss://); connection failures become failed handshake checks on the report card rather than tool errors.