setup-metals-mcp

Configures metals-mcp for the current Scala project so the same setup works in both:

• the Claude Code CLI (claude in a terminal), and
• "Code" inside the Claude Desktop app (embedded Claude Code — same engine as the CLI).

This does not target the Claude Desktop app's plain chat (that reads a separate global claude_desktop_config.json and has no project concept). This skill is only about Claude Code's project-level .mcp.json.

Why a wrapper script instead of $PWD

metals-mcp needs the absolute workspace path (--workspace); it does not auto-detect the project. The obvious approach — sh -c 'metals-mcp --workspace "$PWD" ...' — is not reliable, because of a confirmed open bug (claude-code#75266):

• When Claude Code runs embedded in the Claude Desktop app, it launches stdio MCP servers with cwd = $HOME instead of the project root. So $PWD (and any relative command path) resolves to the home directory, and metals opens the wrong workspace. The CLI is unaffected, but we want one config that works in both.
• ${CLAUDE_PROJECT_DIR} does not help in the command/args position: Claude Code does not populate it in the server-launch environment there. claude mcp list reports Missing environment variables: CLAUDE_PROJECT_DIR if you try. (Verified against Claude Code 2.1.x.)

The robust fix is a self-locating wrapper script committed inside the project. The wrapper derives the project root from its own on-disk location, which is independent of the launcher's cwd — so it computes the correct --workspace in both the CLI and the embedded app. The only piece that must be cwd-independent is the path to the wrapper itself, so .mcp.json points at it by absolute path (see the portability note below).

Files this skill manages

1. <project>/.claude/metals-mcp.sh — committed, portable, self-locating wrapper.
2. <project>/.mcp.json — machine-local (absolute path inside), gitignored, regenerated per machine by re-running this skill.
3. <project>/.gitignore — ensure .mcp.json is ignored.

Wrapper script: .claude/metals-mcp.sh

POSIX sh, no readlink -f (BSD readlink on older macOS lacks -f); uses pwd -P to resolve the script's directory:

#!/bin/sh
# Self-locate the project root from this script's own path, independent of cwd.
# .claude/metals-mcp.sh  ->  project root is one level up from .claude/
scriptdir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd -P)
root=$(CDPATH= cd -- "$scriptdir/.." && pwd -P)
exec metals-mcp --workspace "$root" --transport stdio

--transport stdio is still required: metals-mcp defaults to HTTP (Undertow) and Claude Code expects stdio (it times out after 30s otherwise).

.mcp.json entry (absolute command)

The skill fills in the real absolute path to the wrapper at run time:

"metals-mcp": {
  "type": "stdio",
  "command": "/absolute/path/to/project/.claude/metals-mcp.sh"
}

Steps

1. Determine the absolute path of the current project root (the directory .mcp.json will live in).
2. Write .claude/metals-mcp.sh with the wrapper above. Create .claude/ if needed. chmod +x it.
3. Read/parse .mcp.json if it exists (start from {"mcpServers": {}} otherwise). Set mcpServers["metals-mcp"] to the entry above with the absolute wrapper path filled in. Preserve all other mcpServers entries; overwrite an existing metals-mcp entry and note it. Write back with 2-space indentation and a trailing newline.
4. Ensure .mcp.json is gitignored: if <project>/.gitignore doesn't already ignore it, append a .mcp.json line. (The wrapper under .claude/ stays committed — it's portable.)
5. Tell the user:
   • That .claude/metals-mcp.sh (portable, commit it) and .mcp.json (machine-local, gitignored) were written, and that teammates/other machines re-run this skill once to generate their own .mcp.json.
   • To restart their Claude Code session (CLI) or reopen the "Code" panel in Desktop for the change to take effect.
   • That this works in both the CLI and "Code" in the Desktop app, but not Desktop plain chat.

Notes

• Requires metals-mcp v1.6.7+ (cs install metals-mcp via Coursier) and Java on PATH. Because the embedded app launches with a minimal environment, if metals-mcp can't find java, add JAVA_HOME/PATH via an env block on the .mcp.json entry, or set them inside the wrapper.
• Why absolute path in .mcp.json (and hence gitignored): the path to the wrapper must not depend on cwd, or bug #75266 breaks the embedded app. An absolute path is the only cwd-independent way to find the wrapper; the wrapper itself is portable and committed. The .mcp.json is cheap to regenerate — just re-run this skill.
• If a global metals-mcp entry exists in ~/.claude.json with no --workspace, it conflicts (fails with --workspace is required). Suggest: claude mcp remove --scope user metals-mcp.