Antigravity

Google Antigravity can use Bifrost through a manual MCP server entry. Antigravity’s visible Add MCP flow is a curated marketplace, but the app also reads global MCP configuration from ~/.gemini/config/mcp_config.json and workspace-local configuration from <workspace>/.agents/mcp_config.json.

For Antigravity’s underlying host conventions, see the official MCP documentation.

Configure MCP

Install the release verified with this setup and record its absolute path:

cargo install brokk-bifrost --version 0.8.9 --locked --force
command -v bifrost
bifrost --version

The version check should print bifrost 0.8.9.

One fixed workspace

For a single fixed workspace, add a bifrost entry to the global ~/.gemini/config/mcp_config.json, using the absolute binary path reported by command -v bifrost:

{
  "mcpServers": {
    "bifrost": {
      "command": "/absolute/path/to/bifrost",
      "args": [
        "--root",
        "/absolute/path/to/workspace",
        "--mcp",
        "symbol|extended"
      ]
    }
  }
}

This global configuration is not dynamic. It always starts Bifrost for the same path, even when Antigravity opens another Project or Git worktree.

Git worktrees and multiple projects

Use workspace-local MCP configuration for worktrees and whenever you regularly switch projects. Create or merge a bifrost entry into <worktree>/.agents/mcp_config.json from that worktree:

{
  "mcpServers": {
    "bifrost": {
      "command": "/absolute/path/to/bifrost",
      "cwd": "/absolute/path/to/this/worktree",
      "args": [
        "--root",
        ".",
        "--mcp",
        "symbol|extended"
      ]
    }
  }
}

cwd makes --root . resolve to this exact worktree. Do not omit either field: Bifrost must not infer its analyzer root from Antigravity’s process working directory, plugin directory, or the selected Project. Antigravity does not currently provide Bifrost with a dynamic MCP workspace-root signal, so a rootless global server correctly remains unbound instead of following the active worktree.

The local file contains an absolute, machine-specific worktree path. Keep it uncommitted and create it in each new worktree. An installation helper may derive the path with git rev-parse --show-toplevel, but it must merge only the bifrost entry and must not overwrite other MCP servers in the file.

In Antigravity 2.x, create or select a Project that contains this same workspace: click the folder-plus icon beside Projects, choose New Project, and add the checkout. Use the Local environment when validating an existing checkout. New Worktree intentionally creates another checkout, so its expected analyzer root will differ.

Open Settings -> Customizations and click Refresh after creating or changing either MCP configuration. Refresh is normally enough; it restarts the MCP connection and reloads the tools without requiring a full Antigravity restart. Start a fresh conversation for the smoke test below. If Refresh still shows an old executable or workspace path, fully quit and reopen Antigravity so its MCP process and cached tool schemas are recreated. The Installed MCP Servers section should show bifrost with search_symbols and query_code.

Validate the Setup

For strong exact-checkout evidence, add a temporary declaration whose name is unique to this smoke, for example:

src/antigravity_bifrost_host_probe_4f6f2b7.rs

pub fn antigravity_bifrost_host_probe_4f6f2b7() {}

Start a fresh conversation under the Project you created above and use a prompt that requires two real MCP calls:

Use only Bifrost MCP tools for this verification; do not use terminal commands or built-in file-reading or search tools. Call search_symbols for antigravity_bifrost_host_probe_4f6f2b7, then call query_code with schema_version 1, languages ["rust"], match {"kind":"function","name":"antigravity_bifrost_host_probe_4f6f2b7"}, limit 10, and result_detail "full". PASS only if both real calls return src/antigravity_bifrost_host_probe_4f6f2b7.rs.

Antigravity should ask for permission to call bifrost/* the first time. You can save that rule for only this Project. A successful smoke shows real bifrost/search_symbols and bifrost/query_code calls, and both results name the temporary project-relative path. Remove the temporary file after retaining the evidence.

Avoid prompts that only ask about README.md or docs files; those can pass through ordinary file reading without proving the MCP server ran.

Apply the shared host-integration evidence contract: retain the permitted Bifrost tool event and structured result, verify the result belongs to the active project, and reject file-reading fallbacks.

Can My Agent Run RQL?

The configuration above uses symbol|extended. In a fresh Antigravity session, confirm that the enabled Bifrost tool list includes query_code, then call it with the inline JSON fields {"match":{"kind":"declaration"},"limit":1}. To validate saved RQL, check a workspace file named bifrost-smoke.rql containing (limit 1 (declaration)), then call query_code with {"query_file":"bifrost-smoke.rql"}.

The inline call is canonical JSON. MCP accepts RQL only from a workspace .rql file through query_file. See MCP query and RQL availability for the full surface matrix and Agent Result Safety before making completeness claims.