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VSCode OCaml Platform

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Visual Studio Code extension for OCaml and relevant tools.

❗️ You are encouraged to read the Getting started section. The rest of the document assumes you have read it. You may also find Important concepts useful.

If you have issues with the extension and you have read the "Getting Started" section, see Debugging the extension and FAQ below.

Getting started

Installation

Below we first install the extension dependencies and then the extension itself. You can reverse the order; it's just that the extension will not work to its full without all of its dependencies.

1. Installing extension dependencies

   This VS Code for most of its OCaml language support functionality requires OCaml Language Server (often called ocaml-lsp or ocamllsp). Install ocaml-lsp-server package as usual with a package manager of your choice: OPAM or esy. Installation instructions by package manager are available here.

   Make sure to install the packages in the sandbox (usually, OPAM switch or esy sandbox) you use for compiling your project.

   Optionally:

   • Install ocamlformat package if you want source file formatting support.

     Note: Formatting support requires having .ocamlformat file in your project root directory.

   • When you hover the cursor over OCaml code, the extension shows you the type of the code. Install ocamlformat to get nicely formatted types.

2. Install this extension from the VSCode Marketplace. VSCode extension installations instructions are available here.

Setting up the extension for your project

1. Open your OCaml/ReasonML project (File > Add Folder to Workspace...).

2. Configure the extension to use the desired sandbox (usually, OPAM switch or esy sandbox). You can pick it by

   • either calling VSCode command "OCaml: Select a Sandbox for this Workspace" (one can do this from VSCode Command Palette - <kbd>Ctrl</kbd>+<kbd>P</kbd> or on MacOS <kbd>Cmd</kbd>+<kbd>Shift</kbd>+<kbd>P</kbd>)

   • or clicking on the package icon at the bottom of VSCode window and picking your sandbox from the menu

     

   What's a sandbox? In short, the main purpose of a sandbox is to specify how this extension should invoke its dependencies such as ocaml-lsp-server or ocamlformat. For more information on what a sandbox is, see "Sandbox" subsection.

3. Build your project with Dune to get go-to-definition, auto-completion, etc.

   Important note: OCaml Language Server has its information about the files from the last time your built your project.

   Caveat 1: Because of the note above, during active development of your project, we advise building your project with dune in a polling mode using the option --watch. This rebuilds your project whenever a file is changed in your project. For example, run dune build --watch --terminal-persistence=clear-on-rebuild in your VSCode integrated terminal.

   Caveat 2: Save the currently open file to get latest diagnostics (error and warning squiggly underlining). For example, if you created a module A in some file, and you still get an error that it's "unbound" (i.e., not found) in the current file, save the file to get up-to-date diagnostics, assuming you built your project after adding A or are running build in a polling mode, and make sure that error isn't a stale error.

By this point, you should have a working OCaml development editor ready.

Dune Package Management (DPM)

This extension now supports Dune Package Management (DPM) as a sandbox for your OCaml projects.

DPM becomes available when your system has:

• Dune version ≥ 3.20.0, or
• A Dune preview build dated 2025-07-30 or later.

A final required feature was implemented on 2025-07-29, enabling full support for Dune development tools (dune lock, dune tools exec, and tools like ocamlformat) inside the ocamllsp sandbox environment.

Automatic Detection

When a compatible Dune version is detected:…