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Adding Projects

It is possible to add additional projects to the web editor which can be selected from the dropdown or through the URL.

The default directory for projects is ./Projects/ but this can be changed by setting the environment variable PROJECTS_BASE_PATH. This needs to be set for the build process (npm run build:server) and for the production mode (npm run prod / ecosystem.config.cjs) and it contains a relativ path from the project's root directory.

Inside this folder, new Lean projects can be created containing 2 special files:

• leanweb-config.json (mandatory)
• leanweb-build.sh (optional)

The name of the folder is simultaneously the key used in the URL, e.g. a project in the folder MyCoolProject can be accessed via lean.your.website.com/#project=MyCoolProject.

You might want to look at the provided MathlibDemo project for comparison.

project structure

Any lake project can be used as a project. However, same features make some assumptions:

• In order for lake to use any leanOptions specified in the project's lakefile, you must make sure there is a file Projects/MyCoolProject/MyCoolProject.lean where folder name and file name coincide.
• The download button creates a ZIP where the editor content is placed in MyCoolProject/MyCoolProject.lean and the files lean_toolchain, lake-manifest.json, lakefile.lean (or lakefile.toml) are copied over. In particular, if the lakefile defines other targets, the downloaded lake project might not work as there might be missing these files.

project config

The file leanweb-config.json takes the following form:

{
  "name": "Display name",
  "default": false,
  "hidden": false,
  "sortOrder": 0,
  "examples": [
    { "file": "MathlibDemo/Bijection.lean", "name": "Example's display name" },
    ...
  ]
}

• name: The display name of the project as shown in the dropdown menu. The substrings _Vers_ and _LeanVers_ in the project name will be replaced by indicators of which version is being used:
  • If the toolchain is in a recognized format (a regular release like v4.30.0-rc1 or nightly release like 2026-04-04), then _Vers_ will be replaced by the version number (e.g. v4.30.0-rc1) and _LeanVers_ will be replaced by "Lean" followed by the version number (e.g. Lean v4.30.0-rc1)
  • If the toolchain is not in a recognized format, both _Vers_ and _LeanVers_ will be replaced by "Lean".
• default: There must be exactly one project with this set to true. This is the project loaded by default and when no project is specified in the url.
• hidden: If set to true, then the project does not appear in the dropdown and can only be accessed via direct link.
• sortOrder(non-negative number): sort order of the projects in the dropdown. The default always comes first, then projects with higher sortOrder value. Ties are resolved alphabetically.
• examples: list of examples. The path is relativ to the project's directory, e.g. {PROJECTS_BASE_PATH}/{PROJECT_FOLDER}/{EXAMPLE_PATH.lean}

automatic builds

If the project contains a file leanweb-build.sh, it will be executed as part of npm run build:server. This can be used to automatically update and build the project. A typical build script could look like this:

#!/usr/bin/env bash

# Operate in the directory where this file is located
cd $(dirname \$0)

# add code here to update the project correctly

lake build

If no build script exists, you must build your Lean projects manually.