Building MCP servers

ligero-mcp turns a Ligero app into a Model Context Protocol server: you expose your application's capabilities as tools that an LLM (Claude, etc.) can discover and call. It speaks JSON-RPC 2.0 over the Streamable HTTP transport and mounts as a normal middleware — so an MCP server is just a route in your app.

Dependency

implementation "com.ligeroframework:ligero-mcp:$ligeroVersion"

Quick start

var app = new Ligero();

McpServer mcp = McpServer.create("weather", "1.0.0")
    .tool("get_forecast", "Get the forecast for a city",
          McpServer.objectSchema(
              Map.of("city", McpServer.stringParam("City name")), "city"),
          args -> weather.forecast((String) args.get("city")));

app.use(mcp.http("/mcp"));   // POST /mcp now speaks MCP
app.start();

Point an MCP client (Claude Desktop, the MCP Inspector, …) at http://localhost:8080/mcp and get_forecast shows up as a callable tool.

Registering tools

McpServer.create("calc", "1.0.0")
    .tool("add", "Add two numbers",
          McpServer.objectSchema(Map.of(
              "a", McpServer.numberParam("first"),
              "b", McpServer.numberParam("second")), "a", "b"),
          args -> String.valueOf(
              ((Number) args.get("a")).doubleValue() + ((Number) args.get("b")).doubleValue()));

Method	What it does
create(name, version)	Start a server with its serverInfo.
tool(name, description, schema, handler)	Register a callable tool. schema is JSON Schema for the arguments; handler maps parsed arguments → a text result.
objectSchema(props, required...)	Build an object JSON Schema.
stringParam(desc) / numberParam(desc)	Property schemas for the common cases.
http(path)	A middleware serving the server at path.

A tool handler that throws is reported to the client as an MCP error result (isError: true) — the connection stays healthy.

What's implemented

• The initialize handshake (protocol version, serverInfo, tools capability).
• ping.
• tools/list — every tool with its JSON Schema.
• tools/call — runs the handler and returns text content.
• Notifications (notifications/*) are accepted with 202 Accepted.

One JSON-RPC message per POST is supported (the common client behaviour). Batching, SSE server-streaming, and resources / prompts are on the roadmap.

Why Ligero for MCP

An MCP server is an HTTP + JSON service, which is exactly what Ligero is — so you get the reflection-free Beans container, middleware (auth, rate limiting) and devtools for free around your tools. Put an MCP server and your regular REST API in the same app, sharing the same services.