ModelContextProtocol.jl

Julia server and client utilities for the Model Context Protocol (MCP).

The package supports the stateful MCP 2025-11-25 protocol and the stateless MCP 2026-07-28 protocol over Streamable HTTP. Clients default to 2025-11-25 for compatibility. The package includes discovery manifests, OAuth-protected resources, tools, prompts, resources, completions, logging, multi-round-trip results, subscriptions, and custom tool headers. It also ships first-class support for MCP Apps (SEP-1865) — interactive HTML widgets rendered inline by hosts like Cursor and Claude; see MCP-App-playbook.md for the end-to-end guide.

Installation

This JuliaServices package is currently unregistered. The General registry has a separate package with the same name, so Pkg.add("ModelContextProtocol") does not install this checkout.

Use a URL or local development checkout instead:

using Pkg
Pkg.develop(url="https://github.com/JuliaServices/ModelContextProtocol.jl")

Minimal Server

using ModelContextProtocol

server = MCPServer(MCPServerConfig(name="Calculator", version="0.1.0"))

register_tool!(
    server;
    name="add",
    description="Add numbers.",
    input_schema=Dict(
        "type" => "object",
        "properties" => Dict(
            "numbers" => Dict("type" => "array", "items" => Dict("type" => "number")),
        ),
        "required" => ["numbers"],
    ),
    handler=function (_context, args)
        total = sum(Float64.(args["numbers"]))
        return Dict(
            "content" => [Dict("type" => "text", "text" => string(total))],
            "structuredContent" => Dict("sum" => total),
        )
    end,
)

http_server = serve_mcp_http(server; host="127.0.0.1", port=3010)
wait(http_server.http)

The package also provides a specialized, tools-only server with a concrete request graph for JuliaC --trim=safe builds. Its API is namespaced and is not exported. See the trim-safe static server guide for the example and support limits.

Minimal Client

using ModelContextProtocol

discovery = discover_server("http://127.0.0.1:3010")
client = prepare_manual_client(discovery)
initialize_client!(client)

result = call_tool(client, "add"; arguments=Dict("numbers" => [1, 3, 4]))
terminate_session!(client)

Use the stateless protocol when the server supports MCP 2026-07-28:

client = prepare_manual_client(
    discovery;
    config=MCPClientConfig(
        protocol_version=ModelContextProtocol.PROTOCOL_VERSION_2026_07_28,
    ),
    capabilities=Dict("elicitation" => Dict()),
)
initialize_client!(client) # calls server/discover

See the MCP 2026-07-28 guide for capability checks, multi-round-trip results, custom headers, caching, and subscriptions.

Agentif Tools

When Agentif.jl is loaded, this package can register Agentif tools directly:

using Agentif
using ModelContextProtocol

tool = @tool "Echo text." echo(text::String) = text

server = MCPServer(MCPServerConfig(name="Agentif MCP", version="0.1.0"))
register_tool!(server, tool)

See examples/ for calculator servers with and without OAuth. The docs include a deeper walkthrough of the Auth0-federated calculator example.