LemmaBase MCP

LemmaBase MCP

LemmaBase MCP lets you connect an AI assistant to LemmaBase.com so it can work with published rules, author new specs in chat, and publish them to repositories you own. The catalog has nine tools: list, search, show, run, evaluate_inline, source, create_repo, publish, and guide.

MCP requires a personal access key or OAuth. Creating or writing repositories needs a personal key with write permissions. Organization access keys can read org repositories but cannot create or write.

Connecting

Add this URL in your MCP client settings. LemmaBase MCP is stateless Streamable HTTP (no session). It accepts classic initialize for 2025-03-26 / 2025-11-25 (Unsloth, many editors), and modern per-request _meta for 2026-07-28 (optional server/discover). 

https://lemmabase.com/mcp

The LemmaBase MCP endpoint requires authentication on every request. Use a personal access key (Bearer header) or OAuth where your client supports it (Claude Desktop / claude.ai Connectors, Unsloth remote MCP). Create and manage keys under Dashboard → Connectors. Organization keys are read-only and can be scoped to specific repositories. See Using your specs for the key format.

What your assistant can do

Registry tools read published snapshots only. Chat drafts never hit LemmaBase until publish publishes them.

  • list: repositories your key can access, each as @handle/slug with published spec names underneath
  • search: find public owners, repositories, and specs
  • show: data catalog and rule types for a published spec (repository + spec)
  • run: run a published spec with object data (repository + spec; optional rules)
  • evaluate_inline: run pasted Lemma source in a throwaway engine (never persists)
  • source: formatted published Lemma for a repo or one spec
  • create_repo: create an empty repository when the user explicitly asks for a new one (name / slug; personal Bearer token)
  • publish: upsert path + code into an existing repository=@handle/slug and publish a new version (personal Bearer token; never creates a repo)
  • guide: Lemma language / intake guide with a short hosted playbook

Authoring and publishing

Author in the conversation with guide (topic full) and evaluate_inline in multiple rounds. Those calls are request-local: nothing is stored. When the user explicitly asks to publish, call list for an existing @handle/slug and publish with repository= that path. For a brand-new repository, call create_repo first, then publish. publish always creates a new published version so run can run it. Then call source for user verify.

Resource templates expose published source, schema, and OpenAPI at lemma://registry/... URIs. Language guides and examples are the same lemma://guide and lemma://examples resources as the CLI.

If a rule changed on a particular date, pass effective on run or show. The syntax for temporal rules is on Lemma.run (opens in new tab) .

VS Code / Antigravity / Cursor

In your editor’s MCP settings, add a server entry like this: 

{
  "mcpServers": {
    "lemmabase": {
      "url": "https://lemmabase.com/mcp",
      "headers": {
        "Authorization": "Bearer lmb_YOUR_TOKEN"
      }
    }
  }
}

Create a personal access key under Dashboard → Connectors and put it in the headers block. Clients that support OAuth (Claude Connectors) can skip the headers and complete browser sign-in instead.

Claude Desktop / claude.ai Connectors

In Claude Desktop or claude.ai, add a custom connector with URL https://lemmabase.com/mcp. Claude should detect that sign-in is required and prefer Anthropic’s hosted client metadata (CIMD). Dynamic client registration remains available for other hosts. After you approve access in the browser (email or Google / Microsoft / GitHub), Claude receives a personal access key automatically (visible under Connectors).

Localhost URLs do not work in Claude Connectors: Anthropic’s cloud must reach a public https endpoint. Use production LemmaBase.com, or a tunnel to your local server.

Gemini CLI

Configure mcpServers in your Gemini CLI settings with the LemmaBase.com URL and a Bearer access key (or use an OAuth-capable client).

Public and private specs

The LemmaBase MCP server has access to all Lemma specs on the platform that your account can see. Public specs are available once you are authenticated. With a personal access key, your assistant can also reach your private repositories, create repositories, and write published specs. Organization access keys can read organization repositories they are scoped for.

Use it when you want your assistant to reference what others have published: look up a spec someone shared, read how it is structured, run it with sample input, or compare it with rules you are writing. Everything you evaluate from the registry comes from published versions on LemmaBase.com. Authoring stays in chat via evaluate_inline until publish publishes.

That makes it useful when you build on existing work. Your assistant can find a VAT or pricing spec in the registry, inspect its inputs and rules, and help you wire it into your own repository or explain what a published result means.

LLMs explains the llms.txt fallback for clients that do not support MCP.

Registry search lets you browse public rules in the browser.