MCP adapter

@eddy-works/never-rest/mcp maps a ContractDef onto MCP tools. It does not authorize. Put definePolicy on createDispatcher; the adapter only translates tools/list and tools/call.

This page describes the current module. It is not a spec.

When to use

Use this when a host already speaks MCP (stdio, an editor, an agent runtime) and you want the same operations as ./local and HTTP. Do not use it as a second permission system.

@modelcontextprotocol/sdk is an optional peer. toMcpTools does not import it. registerContract does.

Rules

• Each exposed operation must declare zero or one of params, query, body, headers. Two sources throw ContractConfigurationError.
• Tool schemas set additionalProperties: false. Extra keys become validation_error before dispatch.
• GET routes set readOnlyHint: true.
• Success goes in structuredContent. Failures set isError with the disclosed RailError.
• The MCP request AbortSignal is signal. resolvePrincipal supplies the principal.
• The module writes nothing to stdout.

Example

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
import { createDispatcher } from '@eddy-works/never-rest/local';
import { registerContract } from '@eddy-works/never-rest/mcp';
import { definePolicy } from '@eddy-works/never-rest/policy';

const policy = definePolicy(contract, {
  operations: {
    health: 'public',
    getItem: ({ principal, args }) => canRead(principal, args),
  },
});

const dispatcher = createDispatcher(contract, handlers, { policy });
const server = new McpServer({ name: 'app', version: '1.0.0' });

registerContract(server, dispatcher, {
  contract,
  resolvePrincipal: () => principalFromLocalConfig(),
});

await server.connect(new StdioServerTransport());

Runnable copy: examples/mcp-stdio. Spec: specs/mcp-adapter.spec.md.