Elysia MCP Adapter — routes become protocol

HTTP routes become tools by default. Resources and prompts are explicit because they carry different discovery and invocation semantics.

Standalone resources

new Elysia()
  .use(mcp({ allowedRoutes: [] }))
  .mcpResource(
    'config://runtime',
    () => ({ environment: 'production' }),
    {
      name: 'runtime.config',
      title: 'Runtime configuration',
      mimeType: 'application/json'
    }
  )

Use URI templates for parameterized data:

.mcpResource(
  'users://{id}/profile',
  ({ variables }) => ({ id: variables.id, kind: 'profile' }),
  { name: 'users.profile', mimeType: 'application/json' }
)

Route-backed resources

.get('/users/:id/profile', getProfile, {
  params: t.Object({ id: t.String() }),
  mcp: {
    kind: 'resource',
    resource: {
      uriTemplate: 'users://{id}/profile',
      mimeType: 'application/json'
    }
  }
})

Template variables map to route params by default. Supply mapUriToInput when the route expects query or body values instead.

Prompts

new Elysia()
  .use(mcp({ allowedRoutes: [] }))
  .mcpPrompt(
    'debug-http-error',
    ({ status, body }) =>
      `Analyze this HTTP error:\n\nStatus: ${status}\n\n${body}`,
    {
      argsSchema: {
        type: 'object',
        properties: {
          status: { type: 'number' },
          body: { type: 'string' }
        },
        required: ['status', 'body'],
        additionalProperties: false
      }
    }
  )

A prompt handler may return a string, a list of MCP prompt messages, or a complete prompt result. Route-backed POST prompts receive arguments in the internal request body by default; GET prompts receive query parameters.

Resources and Prompts

Standalone resources

Route-backed resources

Prompts

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