Bifrost: Multi-Language LSP & MCP Server

VS Code extension for Bifrost, Brokk's tree-sitter-backed multi-language code intelligence server. The extension starts Bifrost in LSP mode and exposes definitions, references, symbols, hierarchy, rename, diagnostics, completion, hover, and related editor features.

Requirements

• VS Code 1.90+
• A supported platform for extension-managed downloads, or a bifrost binary available through one of the launch modes below

Cursor

Cursor uses the Open VSX Registry for VS Code-compatible extensions. In Cursor's Extensions view, search for brokk.bifrost-vscode, install it, and open a supported source workspace. The extension starts a Bifrost LSP process for editor-native definitions, references, hover, rename, symbols, and diagnostics.

This extension is separate from the Bifrost Cursor agent plugin. The extension starts Bifrost with --lsp; the agent plugin starts another Bifrost process with --mcp. Install either integration for its own surface, or install both, but do not point an MCP host at the extension's LSP process.

With the default auto launch mode, accept the managed-binary installation prompt when it appears. In Output > Bifrost, confirm that the expected release was downloaded and installed in Cursor's extension global storage. The installer verifies both the archive checksum published with the GitHub Release and the SHA-256 pinned into this extension package before extracting the binary; it also checks bifrost --version before starting the server.

For the full installation, upgrade, troubleshooting, and exact-workspace smoke procedure, see the Cursor integration guide.

Configuration

Launch mode behavior:

• auto: use bifrost.serverPath when explicitly configured, then the exact preferred extension-managed binary, then the newest compatible cached patch, then a preferred binary accepted for installation, then a local development build under target/, then bifrost on PATH.
• bundled: require a compatible extension-managed binary for this platform and prompt to install the preferred binary only when none is cached.
• path: use bifrost.serverPath, falling back to bifrost on PATH.

The managed binary range uses the same extension package metadata as the agent launcher: bifrost.binaryVersion, bifrost.minimumBinaryVersion, and bifrost.allowPrerelease, with expected archive hashes in bifrost.archiveSha256. A compatible cached patch is reused without a network or install prompt. The extension downloads only the exact preferred GitHub Release archives (.tar.gz on macOS/Linux and .zip on Windows), verifies each archive against the package-pinned SHA-256 and the release .sha256 sidecar, extracts the bifrost executable into VS Code global storage at binaries/<version>/<platform>-<arch>/, and removes only versions outside the declared compatible range after a successful install. Managed binaries are checked with bifrost --version before the language server starts.

Commands

• Bifrost: Start Language Server
• Bifrost: Stop Language Server
• Bifrost: Restart Language Server
• Bifrost: Open MCP Setup
• Bifrost: Copy MCP Config
• Bifrost: Show Output

The status bar item shows the current server state and can start or restart the language server.

Development

Build the Bifrost server from the repository root:

cargo build --bin bifrost

Install and compile the extension:

cd editors/vscode
npm ci
npm run compile
npm test

npm test is the complete editor-package quality gate: it checks Prettier formatting, TypeScript types (including tests and build configuration), ESLint, the production bundle, and unit tests. Use npm run format to apply the repository formatting rules and npm run lint:fix for safe ESLint fixes.

Open editors/vscode in VS Code, run the extension in an Extension Development Host, and open a workspace with a supported source file. For local development, either rely on the auto-detected target/debug/bifrost binary or set:

{
  "bifrost.launchMode": "path",
  "bifrost.serverPath": "/path/to/bifrost/target/debug/bifrost"
}

The extension associates .rql files with Bifrost RQL, provides syntax highlighting, and shows a play button in the RQL editor title. The button runs the current editor text, including unsaved edits, through the active Bifrost language server and displays typed structural-match, declaration, or file results in the Bifrost Query Results Explorer view. Results are grouped by file; select one to open it and, when the result has a source range, highlight that range. It searches every root currently indexed by that LSP session (all workspace folders by default, or the scope configured by bifrost.roots). The server must already be running and indexed; the button does not start or wait for it.

The extension starts Bifrost with:

bifrost --root <workspace-root> --lsp

--root is the fallback root. VS Code still sends active workspace folders during LSP initialization, including multi-root workspaces.

The extension also associates .rune files with Bifrost Rune IR. Running Bifrost: Show Rune IR from a supported source editor opens its preview in that language mode, with canonical normalized kinds, roles, metadata, spans, strings, and comments highlighted. The preview headings begin with ;, so saved previews remain valid Rune IR documents.

VS Code's Format Document command uses the Bifrost LSP formatter for both .rql and saved .rune files. S-expressions stay compact while they fit within 120 columns; longer forms put their entries on indented lines and keep :property value pairs together when they fit. Incomplete expressions are left unchanged.

For large repositories, scope indexing before starting the server:

{
  "bifrost.roots": ["src", "tests"],
  "bifrost.exclude": ["target", "vendor/generated"]
}

After changing these settings, run Bifrost: Restart Language Server or accept the restart prompt. The extension sends the resolved paths as LSP initializationOptions, so excluded files should disappear from workspace symbol results and document-level LSP lookups.

MCP Setup

Run Bifrost: Open MCP Setup from the Command Palette to choose a setup action: copy generic mcp.json, copy a Codex CLI command, copy a Claude Code command, or open the Bifrost MCP docs. Bifrost: Copy MCP Config remains available when you only want the generic JSON entry.

These commands are manual. The extension does not open MCP setup on activation, does not mutate external host configuration, and does not prompt again after a dismissal.

The MCP setup commands use the same binary resolution settings as the language server where practical: a configured bifrost.serverPath, the extension-managed binary, a local development build, or bifrost on PATH.

The copied entry uses the current workspace root and starts a separate Bifrost MCP process:

{
  "mcpServers": {
    "bifrost": {
      "command": "/path/to/bifrost",
      "args": ["--root", "/path/to/workspace", "--mcp", "searchtools"]
    }
  }
}

Do not point MCP hosts at the VS Code LSP process. The extension and MCP hosts should launch separate stdio processes from the same Bifrost binary/release.

Packaging

The extension uses esbuild to bundle runtime dependencies into out/extension.js.

npm run compile
npx vsce package

The tag-driven release workflow publishes this same validated VSIX, including the binary compatibility range above, to the GitHub Release, the Visual Studio Marketplace, and Open VSX. Open VSX publication uses the pinned ovsx dependency and requires:

• the verified brokk Open VSX namespace;
• an access token with publishing rights stored as OVSX_PAT in the protected GitHub release environment; and
• the publisher's Open VSX account to have signed the current Publisher Agreement.

The publisher checks the exact Open VSX version before uploading. A rerun is successful only when the registry's advertised VSIX SHA-256 matches the validated artifact; an existing version with different bytes fails closed. After upload, the workflow waits for registry visibility and verifies the same checksum again.

The .vscodeignore file excludes TypeScript sources and package manager artifacts from the VSIX; run npm run compile before packaging.

Before publishing the VSIX, ensure:

• package.json has bifrost.binaryVersion set to the Bifrost release version without the leading v.
• package.json has bifrost.minimumBinaryVersion and bifrost.allowPrerelease equal to the agent plugin release metadata.
• package.json has bifrost.archiveSha256 entries for every supported extension target. These hashes are the VSIX trust anchor for downloaded archives.
• The matching GitHub Release contains archive assets named bifrost-v<version>-<target>.tar.gz on macOS/Linux and bifrost-v<version>-<target>.zip on Windows.
• Each archive has a matching .sha256 asset with the same filename plus .sha256.
• The release workflow has completed for macOS universal, Linux x64/arm64, and Windows x64/arm64 targets.

Debugging

The extension pipes Bifrost stderr into Output > Bifrost.

Useful settings:

{
  "bifrost.debug": true,
  "bifrost.slowRequestMs": 1000
}

bifrost.debug logs every LSP request/notification start and finish. bifrost.slowRequestMs logs requests or notifications that take longer than the configured threshold. Handler errors and panics are always logged with LSP method context.