@modelcontextprotocol/server-basic-vanillajs

Bootstraps a Model Context Protocol server using plain JavaScript (no frameworks or build tools) by instantiating the StdioServerTransport and Server classes, registering message handlers, and establishing bidirectional communication over stdin/stdout. The vanilla approach avoids dependency bloat and demonstrates the minimal surface area needed to implement MCP spec compliance without abstraction layers.

Registers callable tools by defining their names, descriptions, and input schemas using JSON Schema, then binding them to handler functions that receive validated arguments. The server validates incoming tool calls against the registered schema before invoking handlers, ensuring type safety and providing schema introspection to clients without runtime type checking overhead.

Exposes server-side resources (files, data, API responses) through a URI-based addressing scheme where clients request resources by URI and receive content with optional MIME type metadata. Resources are registered with read handlers that return content on demand, enabling lazy loading and dynamic content generation without pre-materializing all resources in memory.

Registers reusable prompt templates with named parameters that clients can instantiate with specific values, enabling prompt composition and reuse across multiple tool invocations. Templates are stored server-side and clients request them by name with argument bindings, reducing prompt duplication and enabling centralized prompt management without embedding prompts in client code.

Implements JSON-RPC 2.0 message routing over stdio transport where each request is assigned a unique ID, responses are correlated back to requests by ID, and both client and server can initiate requests. The transport layer handles message framing (newline-delimited JSON), serialization, and asynchronous request-response matching without blocking the event loop.

Advertises server capabilities (supported tools, resources, prompts) during the initialize handshake so clients can discover what the server offers before making requests. The server responds to the initialize request with a capabilities object listing all registered tools, resources, and prompts, enabling clients to adapt their behavior based on server features without trial-and-error.

Catches exceptions in tool handlers and resource readers, converts them to JSON-RPC error responses with error codes and messages, and returns them to clients without crashing the server. Error responses include structured error objects with code, message, and optional data fields, enabling clients to distinguish between different error types and handle them appropriately.