fix(spawn): make Cursor local mode work on macOS (and any non-Linux OS)

[aulorbe]

Problem

spawn cursor local fails on macOS with four cascading errors:

chmod: /usr/local/bin/caddy: No such file or directory
bash: line 4: caddy: command not found
bash: /etc/hosts: Permission denied
bash: line 24: setsid: command not found

These occur in packages/cli/src/shared/cursor-proxy.ts (setupCursorProxy and startCursorProxy). Even though local/agents.ts wires runLocal as the runServer callback, the proxy scripts it constructs were written exclusively for Linux:

1. Caddy downloads the wrong binary — setupCursorProxy calls https://caddyserver.com/api/download?os=linux&arch=amd64 unconditionally. On macOS this downloads a Linux ELF binary that immediately fails to execute.
2. Caddy install path is root-owned on macOS — the script writes to /usr/local/bin/caddy. On macOS this directory either doesn't exist (no Homebrew) or is owned by a system process, so chmod +x immediately fails without sudo.
3. /etc/hosts is read-only without root on macOS — the domain-spoofing step runs echo "127.0.0.1 ..." >> /etc/hosts directly, which fails with Permission denied because /etc/hosts is only writable by root on macOS (unlike some Linux setups where the invoking user may own it).
4. setsid is Linux-only — the non-systemd fallback in startCursorProxy calls setsid to detach backend processes. setsid is a util-linux command; it doesn't ship on macOS (or BSD, or Windows). nohup … & is the portable equivalent.
   The downstream symptom of all four: Caddy never starts, the proxy is never listening, and Cursor's auth exchange fails — which surfaces as "The provided API key is invalid" even when the key is valid.

Fix

• OS/arch detection — query process.platform and process.arch (already available in Bun) to select the correct Caddy download (os=darwin, arch=arm64 | amd64 for macOS; os=linux for Linux).
• User-writable install path — install Caddy to ~/.local/bin/caddy (guaranteed writable, always exists after mkdir -p) instead of /usr/local/bin/. Add ~/.local/bin to PATH in the execution environment.
• sudo for /etc/hosts — prefix the sed and echo host entries with sudo. Wrap in a try/catch with a clear fallback message if sudo isn't available (some sandboxed environments).
• Replace setsid with nohup — swap setsid $NODE ... for nohup $NODE ... < /dev/null >> /tmp/cursor-proxy.log 2>&1 & in the non-systemd branch. nohup is POSIX and present on macOS, Linux, and BSD.

Affected code

• packages/cli/src/shared/cursor-proxy.ts — installCaddy shell fragment, hosts-spoofing fragment, startCursorProxy non-systemd branch
• No changes to the CloudRunner interface, tests, or other agents

Testing

Tested on macOS (Apple Silicon, M3) with SPAWN_CLI_DIR pointing at a local build:

• spawn cursor local completes all setup steps without errors
• Caddy starts on port 443, unary backend on 18644, bidi backend on 18645
• Cursor authenticates and routes completions through OpenRouter

[aulorbe]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(cursor-proxy): support model selection via CURSOR_MODEL env var #3423
• fix(spawn): make Cursor local mode work on macOS (and any non-Linux OS) #3422 👈 (View in Graphite)
• fix(cursor-proxy): skip proxy setup when running on local cloud #3424
• fix(tests): relax flaky digitalocean-token callCount assertion #3425
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[louisgv]

a bit of a weird diff. (?)..