# SWI-Prolog MCP server and service

**URL:** <https://swi-prolog.discourse.group/t/swi-prolog-mcp-server-and-service/9716>\
**Category:** Announce\
**Created:** [June 30, 2026, 2:05pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-mcp-server-and-service/9716 "2026-06-30T14:05:13Z")\
**Posts on this page:** 1\
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**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\
**Post date:** [June 30, 2026, 2:05pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-mcp-server-and-service/9716/1 "2026-06-30T14:05:13Z")

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I have created [GitHub - JanWielemaker/mcp: SWI-Prolog MCP server framework with Prolog manual, introspection and toplevel servers. · GitHub](https://github.com/JanWielemaker/mcp), also available as the pack [mcp](https://www.swi-prolog.org/pack/list?p=mcp). It provides

- A implementation of an MCP server framework for SWI-Prolog with both `stdio` JSON-RPC transport and HTTP transport.
- A good collection of Prolog specific _tools_, split over four groups:
  - **prolog\_help** tools provides access to the SWI-Prolog manual, including (apropos) search and using introspection tools to find additional information on predicates, modules, files, etc.
  - **source** tools allow the LLM to list, search and extract fragments of the entire SWI-Prolog source code.
  - **git** tools allows the LLM to inspect the GIT history and search using `git grep`.
  - **kernel** tools find the location of foreign predicates.
  - **target\_toplevel** allows the LLM to run a Prolog instance and interact with it. It also provides access to the tracer, linter, profiler and coverage tools. This only makes sense when using the `stdio` transport. The Prolog process runs under the same sandbox as the coding agent (make sure it runs in sandbox!). In theory you could also the the HTTP transport as long as you ensure the server is properly sandboxed.

There is a public server on [https://ai.swi-prolog.org/mcp](https://ai.swi-prolog.org/mcp). I have connected that to the web version of Claude and asked

> Can you tell me how SWI-Prolog implements atom\_codes/2?

It nicely started using the MCP. Creates a long story and this summary:

> > **The short version**
> > 
> > `atom_codes/2` itself contains essentially no logic; it sets `X_ATOM` and hands off to `x_chars()`, which is a generic bidirectional atomic↔text converter. The two modes collapse to: try `PL_get_text` on arg1 and `PL_unify_text(...PL_CODE_LIST)` on arg2; if arg1 is a variable, read arg2’s code list and `PL_unify_text(...PL_ATOM)` back onto arg1. All the encoding handling lives inside `PL_get_text`/`PL_unify_text`, and the number-parsing complexity in `x_chars` is dead code for the atom case.
> > 
> > One consequence worth noting: because arg1 is fetched with `CVT_ATOMIC`, `atom_codes/2` in SWI-Prolog is more permissive than strict ISO on the forward mode — e.g. `atom_codes(42, X)` succeeds, since a number is atomic and its text is taken directly.

That is pretty accurate 🙂

I think (hope) that connecting this to your web interface or coding agent should improve its ability to generate and reason about SWI-Prolog code. For coding

- Install the pack. First update SWI-Prolog to the latest GIT version (or wait for version 10.1.11) using `swipl pack install mcp`
- Next, create `.mcp.json` in your project directory holding

```prolog
{
  "mcpServers": {
    "swipl": {
      "command": "swipl",
      "args": [
        "mcp"
      ]
    }
  }
}

```

Use `swipl mcp --help` for some options.

## Writing your MCP servers

There are several examples in the `examples` directory to help you write your own Prolog based MCP server.

## Limitations

Currently there is no way to have progress events in HTTP servers. Adding that requires some extensions to SWI-Prolog’s HTTP server framework.

## Finally

Please try and report experience and/or suggests how we can improve this.

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_[View the full topic](https://swi-prolog.discourse.group/t/swi-prolog-mcp-server-and-service/9716)._