Elysia MCP Adapter — routes become protocol

mcp({
  server: {
    name: 'example-api',
    version: '1.0.0',
    title: 'Example API',
    instructions: 'Prefer read-only tools unless mutation is requested.'
  },
  path: '/mcp',
  allowedRoutes: '*',
  excludedRoutes: ['/admin/*'],
  methods: ['GET', 'POST', 'PUT', 'PATCH', 'DELETE'],
  defaultRouteKind: 'tool',
  onNameCollision: 'error',
  inputMode: 'envelope',
  includeHiddenRoutes: false,
  headers: {
    passThroughFromMcpRequest: ['authorization', 'cookie', 'x-api-key'],
    allowFromToolInput: []
  },
  marshal: {
    maxTextBytes: 262144,
    maxStructuredBytes: 262144,
    includeHttpMetadata: true,
    binary: 'error'
  },
  transport: {
    validateOrigin: true,
    allowedOrigins: [],
    enableGetSse: false,
    enableDeleteSession: false,
    protocolVersion: '2025-11-25'
  },
  diagnostics: {
    failOnMissingSchema: false
  }
})

Core options

Option	Default	Purpose
`path`	`/mcp`	HTTP path for the MCP transport
`allowedRoutes`	`'*'`	Route matchers eligible for discovery
`excludedRoutes`	`[]`	Additional matchers excluded from discovery
`methods`	GET, POST, PUT, PATCH, DELETE	Eligible HTTP methods
`defaultRouteKind`	`'tool'`	Default route-backed MCP primitive
`onNameCollision`	`'error'`	Error or suffix duplicate registry names
`inputMode`	`'envelope'`	Preserve or flatten params, query, body, and headers
`includeHiddenRoutes`	`false`	Include routes hidden by Elysia metadata

Set diagnostics.failOnMissingSchema to true when every exposed route must provide an inferable input schema. mapJsonSchema(schema, context) can normalize schemas for a specific MCP client after route inference or explicit schema selection.

Per-route marshaling

A route-level mcp.marshal object overrides the plugin defaults for that route. This lets a deliberate binary or large-text tool opt in without broadening every tool.

Configuration

Core options

Schema diagnostics

Per-route marshaling

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