A Model Context Protocol (MCP) server library for C3. Build MCP tool servers that integrate with Claude Code, Claude Desktop, and other MCP clients.

Add mcp.c3l to your project's dependency search path:

{
  "dependency-search-paths": ["dependencies"],
  "dependencies": ["mcp"]
}

module my_server;

import mcp;

fn ToolResult handle_greet(String tool_name, String args, void* user_data)
{
    return mcp::text_result("Hello from C3!");
}

fn void main()
{
    Server s = mcp::new_server({
        .name = "my-server",
        .version = "1.0.0",
    });

    s.add_tool({
        .name = "greet",
        .description = "Say hello",
        .input_schema = `{"type":"object","properties":{}}`,
        .handler = &handle_greet,
    });

    s.serve();
}

Build and run:

c3c build

• mcp::new_server(ServerInfo{...}) -- Create a new server
• Server.add_tool(ToolDef{...}) -- Register a tool (max 32)
• Server.serve() -- Start the stdio event loop

ToolDef {
    String name;            // Tool name
    String description;     // Shown to the LLM
    String input_schema;    // JSON Schema for parameters
    ToolHandlerFn handler;  // fn ToolResult(String tool_name, String args, void* user_data)
    void* user_data;        // Optional context pointer
}

• mcp::text_result(String text) -- Return text content
• mcp::error_result(String message) -- Return an error
• mcp::image_result(String base64_data, String mime_type) -- Return an image

The library includes lightweight JSON helpers for parsing tool arguments:

• mcp::extract_string_value(String s) -- Extract a quoted string value
• mcp::extract_object_value(String s) -- Extract a {...} object
• mcp::extract_array_value(String s) -- Extract a [...] array
• mcp::skip_whitespace(String s) -- Skip leading whitespace
• mcp::json_escape_into(DString* out, String s) -- Escape a string for JSON output

The library implements MCP over stdio using newline-delimited JSON-RPC 2.0. It handles initialize, tools/list, and tools/call messages automatically.