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Lean Theorem Prover

STDIO

MCP server for agentic interaction with Lean theorem prover via LSP

lean-lsp-mcp

Lean Theorem Prover MCP

PyPI version last update license

MCP that allows agentic interaction with the Lean theorem prover via the Language Server Protocol using leanclient. This server provides a range of tools for LLM agents to understand, analyze and interact with Lean projects.

Currently beta testing: Please help us by submitting bug reports and feature requests!

Key Features

  • Rich Lean Interaction: Access diagnostics, goal states, term information, hover documentation and more.
  • Agent-Focused Toolset: Includes tools for theorem search (leansearch.net), code completion, and project builds.
  • Easy Setup: Simple configuration for various clients, including VSCode, Cursor and Claude Code.

Setup

Overview

  1. Install uv, a Python package manager.
  2. Make sure your Lean project builds quickly by running lake build manually.
  3. Add JSON configuration to your IDE/Setup and configure LEAN_PROJECT_PATH.

1. Install uv

Install uv for your system.

E.g. on Linux/MacOS:

curl -LsSf https://astral.sh/uv/install.sh | sh

2. Run lake build

lean-lsp-mcp will run lake build in the project root upon startup. Some clients (e.g. Cursor) might timeout during this process. Therefore, it is recommended to run lake build manually before starting the MCP. This ensures a faster startup time and avoids timeouts.

E.g. on Linux/MacOS:

cd /path/to/lean/project
lake build

Note: Your build does not necessarily need to be successful, some errors or warnings (e.g. declaration uses 'sorry') are OK.

3. a) VSCode Setup

VSCode and VSCode Insiders are supporting MCPs in agent mode. For VSCode you might have to enable Chat > Agent: Enable in the settings.

  1. One-click config setup:

Install in VS Code

Install in VS Code Insiders

OR manually add config to settings.json (global):

{
    "mcp": {
        "servers": {
            "lean-lsp": {
                "command": "uvx",
                "args": ["lean-lsp-mcp"],
                "env": {
                    "LEAN_PROJECT_PATH": "/path/to/lean/project"
                }
            }
        }
    }
}

  1. Next change the env variable LEAN_PROJECT_PATH to point to the root of your Lean project. This is required for the MCP to work. You can also remove this from the config and set this env variable differently.

  2. Click "Start" above server config, open a Lean file, change to agent mode in the chat and run e.g. "auto proof" to get started:

3. b) Cursor Setup

  1. Open MCP Settings (File > Preferences > Cursor Settings > MCP)

  2. "+ Add a new global MCP Server" > ("Create File")

  3. Paste the server config into mcp.json file and adjust the LEAN_PROJECT_PATH to point to the root of your Lean project:

{
    "mcpServers": {
        "lean-lsp": {
            "command": "uvx",
            "args": ["lean-lsp-mcp"],
            "env": {
                "LEAN_PROJECT_PATH": "/path/to/lean/project"
            }
        }
    }
}

3. c) Claude Code

Run one of these commands in the root directory of your Lean project (where lakefile.toml is located):

# Local-scoped MCP server
claude mcp add lean-lsp uvx lean-lsp-mcp -e LEAN_PROJECT_PATH=$PWD

# OR project-scoped MCP server (creates or updates a .mcp.json file in the current directory)
claude mcp add lean-lsp -s project uvx lean-lsp-mcp -e LEAN_PROJECT_PATH=$PWD

You can find more details about MCP server configuration for Claude Code here.

Other Setups

Other setups, such as Claude Desktop, OpenAI Agent SDK, Windsurf or Goose should work with similar configs.

Tools

Lean LSP MCP currently provides various tools to interact with the Lean theorem prover:

Core interactions

lean_diagnostic_messages

Get all diagnostic messages for a Lean file. This includes infos, warnings and errors.

Example output

l20c42-l20c46, severity: 1
simp made no progress

l21c11-l21c45, severity: 1
function expected at h_empty term has type T ∩ compl T = ∅

...

lean_goal

Get the proof goal at a specific location (line or line & column) in a Lean file.

Example output (line) Before:
S : Type u_1
inst✝¹ : Fintype S
inst✝ : Nonempty S
P : Finset (Set S)
hPP : ∀ T ∈ P, ∀ U ∈ P, T ∩ U ≠ ∅
hPS : ¬∃ T ∉ P, ∀ U ∈ P, T ∩ U ≠ ∅
compl : Set S → Set S := fun T ↦ univ \ T
hcompl : ∀ T ∈ P, compl T ∉ P
all_subsets : Finset (Set S) := Finset.univ
h_comp_in_P : ∀ T ∉ P, compl T ∈ P
h_partition : ∀ (T : Set S), T ∈ P ∨ compl T ∈ P
⊢ P.card = 2 ^ (Fintype.card S - 1)
After:
no goals

lean_term_goal

Get the term goal at a specific position (line & column) in a Lean file.

lean_hover_info

Retrieve hover information (documentation) for symbols, terms, and expressions in a Lean file (at a specific line & column).

Example output (hover info on a `sorry`) The `sorry` tactic is a temporary placeholder for an incomplete tactic proof,
closing the main goal using `exact sorry`.

This is intended for stubbing-out incomplete parts of a proof while still having a syntactically correct proof skeleton.
Lean will give a warning whenever a proof uses sorry, so you aren't likely to miss it,
but you can double check if a theorem depends on sorry by looking for sorryAx in the output
of the #print axioms my_thm command, the axiom used by the implementation of sorry.

lean_declaration_file

Get the file contents where a symbol or term is declared.

lean_completions

Code auto-completion: Find available identifiers or import suggestions at a specific position (line & column) in a Lean file.

lean_multi_attempt

Attempt multiple lean code snippets on a line and return goal state and diagnostics for each snippet. This tool is useful to screen different proof attempts before using the most promising one.

Example output (attempting `rw [Nat.pow_sub (Fintype.card_pos_of_nonempty S)]` and `by_contra h_neq`) rw [Nat.pow_sub (Fintype.card_pos_of_nonempty S)]:
S : Type u_1
inst✝¹ : Fintype S
inst✝ : Nonempty S
P : Finset (Set S)
hPP : ∀ T ∈ P, ∀ U ∈ P, T ∩ U ≠ ∅
hPS : ¬∃ T ∉ P, ∀ U ∈ P, T ∩ U ≠ ∅
⊢ P.card = 2 ^ (Fintype.card S - 1)

l14c7-l14c51, severity: 1
unknown constant 'Nat.pow_sub'

by_contra h_neq:
S : Type u_1
inst✝¹ : Fintype S
inst✝ : Nonempty S
P : Finset (Set S)
hPP : ∀ T ∈ P, ∀ U ∈ P, T ∩ U ≠ ∅
hPS : ¬∃ T ∉ P, ∀ U ∈ P, T ∩ U ≠ ∅
h_neq : ¬P.card = 2 ^ (Fintype.card S - 1)
⊢ False

...

lean_leansearch

Search for theorems in Mathlib using leansearch.net (natural language search).

Example output (query by LLM: "finite set, subset, complement, cardinality, half, partition")
{"module_name": ["Mathlib", "Data", "Fintype", "Card"], "kind": "theorem", "name": ["Finset", "card_compl"], "signature": " [DecidableEq \u03b1] [Fintype \u03b1] (s : Finset \u03b1) : #s\u1d9c = Fintype.card \u03b1 - #s", "type": "\u2200 {\u03b1 : Type u_1} [inst : DecidableEq \u03b1] [inst_1 : Fintype \u03b1] (s : Finset \u03b1), s\u1d9c.card = Fintype.card \u03b1 - s.card", "value": ":=\n Finset.card_univ_diff s", "docstring": null, "informal_name": "Cardinality of Complement Set in Finite Type", "informal_description": "For a finite type $\\alpha$ with decidable equality and a finite subset $s \\subseteq \\alpha$, the cardinality of the complement of $s$ equals the difference between the cardinality of $\\alpha$ and the cardinality of $s$, i.e.,\n$$|s^c| = \\text{card}(\\alpha) - |s|.$$"}

...
More answers like above
...

lean_proofs_complete

Check if all proofs in a file are complete. This is currently very simple and will be improved in the future.

lean_file_contents

Get the contents of a Lean file, optionally with line number annotations.

Project-level tools

lean_build

Rebuild the Lean project and restart the Lean LSP server.

Disabling Tools

Many clients allow the user to disable specific tools manually (e.g. lean_build).

VSCode: Click on the Wrench/Screwdriver icon in the chat.

Cursor: In "Cursor Settings" > "MCP" click on the name of a tool to disable it (strikethrough).

Example Uses

Here are a few example prompts and interactions to try. All examples use VSCode (Agent Mode) and Gemini 2.5 Pro (Preview).

Use tools to assist with a proof

After installing the MCP, tools are automatically available to the agent. E.g. Open a Lean file with a sorry and run the following prompt: "Solve this sorry" The agent should use various tools such as lean_goal to understand and create a proof. You can also ask the agent to use tools explicitly, e.g. "Help me write this proof using tools." or "Use tools to analyze the goal and hover information, then write a proof."

Analyze a theorem

Open Algebra/Lie/Abelian.lean. Example prompt:

"Analyze commutative_ring_iff_abelian_lie_ring thoroughly using various tools such as goal, term goal, hover info. Explain the key proof steps in english.".

Analyzing a theorem in chat

Design proof approaches

Open an incomplete proof such as putnam 1964 b2. Example prompt:

"First analyze the problem statement by checking the goal, hover info and looking up key declarations. Next use up to three queries to leansearch to design three different approaches to solve this problem. Very concisely present each approach and its key challenge."

Designing proof approaches

Notes on MCP Security

There are many valid security concerns with the Model Context Protocol (MCP) in general!

This MCP server is meant as a research tool and is currently in beta. While it does not handle any sensitive data such as passwords or API keys, it still includes various security risks:

  • Access to your local file system.
  • No rate limiting on tool calls.
  • No input or output validation.

Please be aware of these risks. Feel free to audit the code and report security issues!

For more information, you can use Awesome MCP Security as a starting point.

Related Projects

License & Citation

MIT licensed. See LICENSE for more information.

Citing this repository is highly appreciated but not required by the license.

@software{lean-lsp-mcp,
  author = {Oliver Dressler},
  title = {{Lean LSP MCP: Tools for agentic interaction with the Lean theorem prover}},
  url = {https://github.com/oOo0oOo/lean-lsp-mcp},
  month = {3},
  year = {2025}
}

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