Transports

The server registry is independent of delivery. Choose stdio when an MCP client launches your executable, Streamable HTTP for a network endpoint, or in-process transport when two Nim components share a process.

Stdio

For local MCP servers, call:

server.serveStdio()

Use serveStdioAsync inside an existing async application. Both functions read one JSON-RPC message per line and write one response per line. The default message limit is 1 MiB and the default nesting limit is 64 levels:

server.serveStdio(
  maxMessageBytes = 2 * 1024 * 1024,
  maxNestingDepth = 64,
  securityLimits = newMcpSecurityLimits(maxLineBytes = 2 * 1024 * 1024))

Do not write human-readable logs to stdout. Stdio clients treat that stream as protocol data.

Built-in HTTP server

The standard library adapter is enough for a small server:

import std/[asyncdispatch, nativesockets]
import nimwire

let app = mcpServer("http-example", "1.0.0"):
  discard

let http = newMcpHttpServer(app, newMcpHttpConfig(
  endpoint = "/mcp",
  host = "127.0.0.1",
  port = Port(8080),
  allowedHosts = @["127.0.0.1"]))

waitFor http.serveHttp()

newMcpHttpServer uses Nim’s asynchttpserver. Call http.shutdown() from your application when it needs to stop accepting requests. The adapter cancels active requests and closes subscriptions during shutdown.

Framework-neutral HTTP

Web frameworks can adapt their request object to McpHttpRequest, call handleHttpRequest, then write the returned McpHttpResponse. Populate the request’s method, path, headers, body, cancellation signal, and optional stream callbacks. This keeps framework-specific routing outside nimwire.

POST requests need Content-Type: application/json, an Accept header containing both application/json and text/event-stream, and headers that match the JSON body:

MCP-Protocol-Version: 2026-07-28
Mcp-Method: tools/call
Mcp-Name: echo

Tool arguments can opt into additional Mcp-Param-* header checks with x-mcp-header in their schema. The header value must match the body value.

HTTP limits and origins

Configure maxBodyBytes, maxNestingDepth, requestTimeoutMs, and maxConcurrentRequests in newMcpHttpConfig. allowedHosts protects the Host header. allowedOrigins controls CORS responses, and an empty list allows no cross-origin Origin value.

preferSse = true enables the standard adapter’s event-stream response shape. Use a streamWriter when a framework adapter needs to send notifications or streamed responses as they arrive.

The built-in adapter does not terminate TLS. Put a public HTTP server behind TLS and a trusted reverse proxy. The repository includes a minimal reverse proxy configuration.

In-process transport

Link a server without a process or socket:

import nimwire

let upstream = newMcpServer("upstream", "1.0.0")
let peer = newMcpInProcessPeer(upstream)
let discovery = peer.request("server/discover")
echo discovery.fields
peer.close()

The in-process transport carries the same validated JSON-RPC values as other transports, which makes it useful for tests and composition.

Related: Security and the HTTP API reference.