Elysia MCP Adapter — routes become protocol

Standalone primitives are useful when behavior belongs to the MCP surface but not to your public HTTP API.

Tools

new Elysia()
  .use(mcp({ allowedRoutes: [] }))
  .mcpTool(
    'math.add',
    ({ a, b }: { a: number; b: number }) => ({ sum: a + b }),
    {
      title: 'Add numbers',
      description: 'Add two finite numbers.',
      inputSchema: {
        type: 'object',
        properties: {
          a: { type: 'number' },
          b: { type: 'number' }
        },
        required: ['a', 'b'],
        additionalProperties: false
      },
      outputSchema: {
        type: 'object',
        properties: { sum: { type: 'number' } },
        required: ['sum']
      },
      annotations: {
        readOnlyHint: true,
        idempotentHint: true
      }
    }
  )

Handlers may return a plain JSON-compatible value or a complete MCP tool result with content, structuredContent, isError, and _meta.

Install methods before the endpoint

withMcpMethods() installs the fluent registration methods without exposing /mcp yet:

import { mcp, withMcpMethods } from '@mwillbanks/elysia-mcp-adapter'

new Elysia()
  .use(withMcpMethods())
  .mcpTool('internal.health', () => ({ ok: true }))
  .use(mcp())

This can help compose feature plugins before the application decides where and how to expose MCP.

Standalone Primitives

Tools

Install methods before the endpoint

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