JSON-RPC is used in the Model Context Protocol (MCP) as the communication layer to enable structured interactions between models, services, and clients. The protocol leverages JSON-RPC’s lightweight, text-based format to define remote procedure calls (RPCs) for tasks like model inference, configuration, or data processing. For example, a client might send a JSON-RPC request with a method like model/predict, including parameters such as input data and model settings. The server processes the request, executes the method, and returns a response containing results or errors. This approach standardizes communication, making it easier for systems to integrate models regardless of their underlying implementation.
The structure of JSON-RPC messages in MCP follows a predictable pattern. A typical request includes fields like method (the operation to perform), params (input data or configuration), and id (a unique identifier for tracking responses). For instance, a request to a text-generation model might specify method: "model/generate" with params: {text: "Hello", max_length: 50}. The server responds with a JSON object containing result (the generated text) or error (e.g., invalid parameters). MCP often extends JSON-RPC by defining custom error codes or metadata, such as model version or processing time. This consistency allows developers to build clients that work across different model implementations, as long as they adhere to the protocol’s method and parameter conventions.
From a practical standpoint, MCP’s use of JSON-RPC simplifies integration for developers. A client could be written in Python using the jsonrpcclient library, while the server runs a model in a language like Rust. For example, a developer might send an HTTP POST request to an MCP-compliant endpoint with a JSON payload like {"jsonrpc": "2.0", "method": "model/classify", "params": {"image": "base64_data"}, "id": 1}. The server parses this, runs the classification model, and returns {"jsonrpc": "2.0", "result": "cat", "id": 1}. Error handling is straightforward: if the image parameter is malformed, the response might include {"code": -32602, "message": "Invalid params"}. Since JSON-RPC is stateless and transport-agnostic, MCP can be used over HTTP, WebSockets, or message queues, offering flexibility in deployment. This design ensures interoperability while keeping the implementation simple for developers working with diverse tools and frameworks.