MCP — Model Context Protocol server framework for SWI-Prolog

This pack provides a framework for writing Model Context Protocol servers in SWI-Prolog, plus a set of ready-made server modules that an LLM coding assistant can use to introspect Prolog code, drive a real SWI toplevel as a “target”, and read source / git history from configured directories.

The framework rides on top of library(json_rpc_server) and library(json_schema) that ship with SWI-Prolog. Per-tool argument validation against the supplied JSON Schema is handled by the JSON Schema layer.

Status

Experimental. Implements the 2024-11-05 revision of the MCP specification.

Transports:

• stdio — library(mcp/stdio); newline-delimited JSON-RPC over stdin/stdout, the form used by the official mcp CLI and by Claude Code’s local-server configuration.
• HTTP + SSE — library(mcp/http); long-lived SSE stream for server-initiated notifications plus POSTs for client requests. Sessions are keyed by the Mcp-Session-Id HTTP header. Intended for hosting on www.swi-prolog.org (behind a reverse proxy handling TLS and auth).

MCP capabilities

• Tools (tools/list, tools/call, notifications/tools/list_changed)
• Resources (resources/list, resources/read, resources/templates/list)
• Prompts (prompts/list, prompts/get)

Progress notifications (notifications/progress and the params._meta.progressToken intake on requests) are not implemented. Tool calls reply with a single buffered JSON-RPC response; there is no intra-call streaming. On the HTTP transport the mcp_notify/2 path reaches only clients holding a GET /sse stream — the streamable-HTTP POST /mcp reply is sent as application/json, not as an SSE event stream, so a tool cannot interleave progress events into its own response.

Writing a server

A minimal hello server:

:- module(hello_server, []).
:- use_module(library(mcp/server)).
:- use_module(library(mcp/stdio)).

:- mcp_server_info(#{name:"hello", version:"0.1.0"}).

:- mcp_tool(echo,
            "Echo the input message",
            #{ properties: #{ msg: #{type: string} },
               required: [msg]
             }).

echo(#{msg: M}, M).

:- initialization(mcp_stdio_run, main).

Run it as swipl hello_server.pl. The server speaks MCP on stdin/stdout.

mcp_tool/3 is the only registration primitive: a name, a one-line description shown to the LLM, and a JSON Schema for the arguments. The matching Prolog predicate is Name(+Args, -Result), where Args is the validated input dict and Result is either a string (auto- wrapped as a text content item), a dict (a single content item) or a list of content items.

Included server modules

Each of these is :- use_module/1-able as library(mcp/servers/). They all register their tools through the same multifile tool/4 predicate, so a single server can compose