This PR was co-authored with Claude Code (Anthropic). Claude helped investigate the root cause, write the fix, and draft the PR description.

Summary

Fix LSP server crash when the client sends only rootUri without workspaceFolders in InitializeParams.

The bug

When Biome's LSP server starts, it registers file watchers (for biome.json, .editorconfig, etc.) in setup_capabilities(). There are two code paths:

Path 1: client sends workspaceFolders (works):

if let Some(folders) = self.session.get_workspace_folders() {
    // Uses folder.uri directly — already a proper URI like "file:///Users/foo/project"
    base_uri: OneOf::Left(folder.clone()),
}

VSCode always sends workspaceFolders in the init request, so this path always runs for VSCode users and the bug was never caught.

Path 2: client sends only rootUri (crashes):

} else if let Some(base_path) = self.session.base_path() {
    // base_path() converts rootUri → filesystem path: "/Users/foo/project"
    // Then tries to parse that bare path as a URI — panics
    base_uri: OneOf::Right(Uri::from_str(base_path.as_str()).unwrap()),
}

session.base_path() takes the rootUri (file:///Users/foo/project) and converts it to a filesystem path (/Users/foo/project). Then the code tries to parse /Users/foo/project back as a URI. But a bare filesystem path is not a valid URI; URIs require a scheme like file://. So Uri::from_str fails at the first character, and .unwrap() panics:

Source Location: crates/biome_lsp/src/server.rs:182:90
Thread Name: tokio-runtime-worker
Message: called `Result::unwrap()` on an `Err` value: ParseError { index: 0, kind: UnexpectedChar }

Context

The LSP specification defines workspaceFolders in InitializeParams as optional. rootUri is the primary way to identify the workspace, workspaceFolders was added later for multi-root support.

Emacs lsp-mode only includes workspaceFolders in InitializeParams when the LSP client is registered as multi-root. Clients like lsp-biome don't opt into multi-root, so the init request contains rootUri but no workspaceFolders. This is spec-compliant. VSCode always sends workspaceFolders, so the fallback path was never exercised there.

The first path (with workspaceFolders) uses URI objects directly, which works. The second path (without workspaceFolders) round-trips URI -> filesystem path -> URI, and the conversion back is missing the file:// scheme.

The fix

Instead of the broken round-trip, added session.base_uri() which returns the original rootUri as-is, no conversion needed. This mirrors what the first path already does with folder.uri.

Changes

• crates/biome_lsp/src/session.rs: Add base_uri() method that returns the original rootUri from the client.
• crates/biome_lsp/src/server.rs: Use base_uri() for the RelativePattern base URI in the fallback path. Remove unused use std::str::FromStr import.
• crates/biome_lsp/src/server.tests.rs: Add regression test initialize_without_workspace_folders_does_not_panic that initializes the server with didChangeWatchedFiles.dynamicRegistration: true and workspace_folders: None, then calls initialized to trigger setup_capabilities.

Test Plan

• Add regression test that reproduces the exact panic (ParseError { index: 0, kind: UnexpectedChar }), fails without the fix, passes with it
• Manually tested with Emacs lsp-mode + lsp-biome (which does not send workspaceFolders in InitializeParams): editing, saving, and formatting .ts files all work without errors after the fix