MCP JSON-RPC validator
Paste one JSON message copied from MCP Inspector, an SDK test, or a client/server log. The validator explains whether it found a request, response, tools list, or tool definition and points to missing core fields.
Copy one JSON message from MCP Inspector, an SDK test, or a client/server debug log. You can also paste a single tool definition from a tools/list response. Remove secrets before using any debugging tool.
How it works
The dropdown contains four small examples. Use them to identify the message you have before replacing the example with your own JSON.
- 01
Choose an example to learn the expected shape, or copy one JSON message from MCP Inspector, your SDK test output, or a debug log.
- 02
The report identifies the message type and lists structural errors or warnings as you edit.
- 03
Before release, run the same message through the exact SDK and MCP protocol version used by your client or server.
Messages it recognizes
- JSON-RPC 2.0 request and response envelopes
- MCP tools/list requests and results
- Tool names, descriptions, inputSchema, and optional outputSchema
- Clear pass, warning, and error lines that can be copied
What still needs an SDK test
- This is a focused structural check, not a complete implementation of every versioned MCP schema.
- It does not connect to a server, execute a tool, inspect transport messages, or verify authorization.
- Custom extensions can be valid even when this validator does not interpret them.
Technical reference
The checks use the JSON-RPC 2.0 envelope rules and the MCP tools specification linked here.