Generic Glean LSP Server

glean-lsp is a generic LSP server that provides a minimal set of LSP features using Glean data. A Glean-based LSP is particularly suited for navigating very large codebases, where it wouldn't be practical to use a traditional language service that parses all the source files on the fly.

The Glean LSP server works for any language supported by Glean and Glass, and currently provides the following LSP features:

• Go to Definition
• Go to References
• Type on Hover
• Outline
• Symbol Search

More features may be added in the future.

The LSP server is purely static, in that it will not re-index the source files if they are edited. To update the Glean data, currently you have to manually re-run the indexer yourself. This can be done without restarting VS Code or the LSP server, though.

Building & installing

cabal install glean-lsp

Using it with VS Code

First index your source code into a Glean DB. This will be something like

cd SRC
glean index LANGUAGE --db-root GLEANDB --db NAME/HASH .

where

• SRC is the directory containing your source code
• LANGUAGE is the language you want to index, see the Glean manual for available languages
• GLEANDB is where you want to store your DBs
• NAME' is the name for the DB, and HASH` can just be 0 or you could use the git hash of the repository if you want to index multiple versions of the code.

See the Glean manual for any language-specific options that you might need.

To use this LSP server with VS Code you need a generic LSP client such as Generic LSP Client (v2). Using that client, add the following to .vscode/settings.json

[
    "glean-lsp": {
        "repo": "NAME"
    },
    "glspc.server.command": "GLEAN-LSP"
    "glspc.server.commandArguments": ["--db-root", "GLEANDB"],
    "glspc.server.languageId": [
      "LANGUAGE-ID"
    ]
}

where

• GLEAN-LSP is the path to the glean-lsp binary, which might be somethihng like $HOME/.cabal/bin/glean-lsp if you installed it with cabal install glean-lsp.
• GLEANDB is the path to your Glean DBs (same as when running the indexer above).
• NAME is the name of the Glean DB you want to use. Don't add HASH here, glean-lsp will always use the most recent version of the DB.
• LANGUAGE-ID is the language you want to enable this extension for. Make sure you also disable any other extensions you have for the same language, you can do that locally for this workspace in VS Code.

Now if you open this workspace in VS Code and open one of the source files that was indexed, you should have go-to-definition and the other features available.

Example: browse the React codebase with Glean

Clone a copy of React:

$ git clone --depth 1 https://github.com/facebook/react.git
$ cd react

Start a Glean server:

$ glean-server --db-root .glean --port 1234 &

If you don't have Flow installed:

$ sudo npm install -g flow-bin

Index the code into a DB called react/0:

$ grep -v '^%' scripts/flow/config/flowconfig >.flowconfig
$ glean index flow . --write-root "" --service localhost:1234 --db react/0

Put the following into /tmp/react/.vscode/settings:

{
    "glean-lsp": {
        "repo": "react"
    },
    "glspc.server.command": "glean-lsp",
    "glspc.server.commandArguments": ["--service", "localhost:1234"],
    "glspc.server.languageId": [
      "typescript", "javascript"
    ]
}

Start VS Code, and open the folder react that you cloned. You probably want to disable the built-in TypeScript extensions to avoid conflicts: Ctrl-Shift-X and filter on built-in extensions (Show Built-in Extensions in the ... dropdown), then disable the extensions for TypeScript for this workspace only.

Now open a .js file, and you should have navigation features provided by glean-lsp: try right-click and "Go to Definition" or "Go to References". Check that the Outline view is populated. Try symbol search with Ctrl-T.

Troubleshooting

In the "Output" pane, select the output for the LSP client in the dropdown on the right to see the debugging output.

A common problem is the file paths not matching up, which will result in the LSP server starting up and appearing to be working, but go-to-definition just doesn't work. Make sure that in the Glean DB the file paths are all relative to the root of the workspace. To check this you can open the DB in the shell and query for src.File _. For example when I look at my DB stackage/1 which contains all the Haskell packages in Stackage:

$ glean shell --db-root ~/gleandb --db stackage/1
Glean Shell, built on 2025-07-14 13:39:35.711312749 UTC, from rev <unknown>
Using local DBs from rocksdb:/home/simon/gleandb
type :help for help.
stackage> :limit 5
stackage> src.File _
{ "id": 66477, "key": "AC-Angle-1.0/Data/Angle.hs" }
{ "id": 19929124, "key": "ANum-0.2.0.2/src/Data/ANum.hs" }
{ "id": 7772678, "key": "Agda-2.6.4/src/full/Agda/Auto/Auto.hs" }
{ "id": 15530734, "key": "Agda-2.6.4/src/full/Agda/Auto/CaseSplit.hs" }
{ "id": 11442673, "key": "Agda-2.6.4/src/full/Agda/Auto/Convert.hs" }

5 results, 5 facts, 151.82ms, 208003368 bytes, 584 compiled bytes
results truncated (current limit 5, use :limit <n> to change it)
Use :more to see more results
stackage> 

Check that the paths look reasonable. If not, you may need to use language-specific options to get the indexer to generate the right paths.

If the paths look OK, things should work to the extent that support for the language is implemented in Glean's codemarkup layer and the Glass server. Some languages might be missing certain features, but the basics should work for most languages. If you're still having trouble try raising an issue on the Glean issue tracker or asking on Discord, details of which are in Glean's README.