Antigravity installer writes an ephemeral fnm per-shell path as the MCP command

[kevocodes]

Description

On macOS with fnm managing Node, installing the Antigravity target writes an MCP command that points into an ephemeral, per-shell directory. It's valid at install time but becomes dangling once the installing shell exits, so Antigravity can no longer spawn the codegraph MCP server.

Root cause

resolveCodegraphCommand() in src/installer/targets/antigravity.ts resolves the absolute path via command -v codegraph. Under fnm, an interactive shell resolves codegraph to ~/.local/state/fnm_multishells/<pid>_<ts>/bin/codegraph — a symlink directory fnm creates per shell and removes when that shell exits. That ephemeral path is written verbatim into ~/.gemini/config/mcp_config.json.

Steps to reproduce (macOS + fnm, codegraph 1.5.0)

1. command -v codegraph → ~/.local/state/fnm_multishells/77232_.../bin/codegraph
2. Install the Antigravity target — that path is written as the MCP command.
3. Close the shell; the fnm_multishells/77232_... directory is gone.
4. The command now points at a non-existent path → Antigravity can't launch the server.

Expected: a stable absolute path that survives shell exit.
Actual: an ephemeral per-shell path that dangles after the installing shell closes.

Proposed fix

Canonicalize the resolved path out of the ephemeral directory while keeping the codegraph binary name:

if (resolved && fs.existsSync(resolved)) {
  try {
    return path.join(fs.realpathSync(path.dirname(resolved)), path.basename(resolved));
  } catch {
    return resolved;
  }
}

For fnm this yields ~/.local/share/fnm/node-versions/<ver>/installation/bin/codegraph — stable, and path.basename stays codegraph (so agents that validate the command by basename still match). Happy to open a PR if this looks right.

Environment

• OS: macOS (darwin arm64)
• codegraph: 1.5.0
• Node manager: fnm
• Agent: Google Antigravity
• Shell: zsh