Signal Setup

Signal Setup

This skill provisions signal-cli and an account so the signal-mcp MCP server can run. It covers four phases: diagnose, install, provision an account (register or link), and configure. Stop at the first phase that resolves the user's blocker.

Phase 1 — Diagnose

Before installing anything, call the MCP tool to see exactly what's missing:

signal_check_setup()

Interpret the returned error field:

Error fragment	What's missing	Jump to
signal-cli not found / binary is null	signal-cli is not installed	Phase 2
SIGNAL_ACCOUNT environment variable is not set	env var for plugin	Phase 4
is not registered with signal-cli	account exists but not provisioned (or wrong number)	Phase 3
ok: true	nothing to do	stop

Run signal-cli --version directly only if the MCP tool itself is failing to launch (e.g., uv missing). For a one-shot diagnostic that doesn't need the MCP server at all, run scripts/check-install.sh from this skill — it prints PATH, signal-cli version, java version, and registered accounts.

Note on PATH: the MCP server inherits PATH from whatever shell launched Claude Code. If uv and signal-cli work in an interactive terminal but not in the MCP tool, ensure they're on PATH in the launching shell's profile (~/.zshrc, ~/.bashrc), not just in shell rc files.

Phase 2 — Install signal-cli

signal-cli requires Java 21+. Recommend the platform-native install path; fall back to the tarball when no package exists. See references/install-platforms.md for full per-OS detail.

Common one-liners:

• macOS (Homebrew): brew install signal-cli
• Debian/Ubuntu (apt + Eclipse Adoptium): install temurin-21-jre then download the official tarball
• Arch Linux: pacman -S signal-cli
• Nix/NixOS: nix-shell -p signal-cli
• Manual / generic Linux: download release tarball from GitHub, extract under /opt/signal-cli, symlink into /usr/local/bin

After installing, re-run signal_check_setup(). If signal-cli exists at a non-PATH location, set SIGNAL_CLI_BIN to its absolute path instead of putting it on PATH.

Phase 3 — Provision an account

Two flows. Ask the user which they want before running anything destructive.

Option A — Link as a secondary device (recommended)

If the user already has Signal on a phone and just wants Claude to send/receive on their behalf, link signal-cli as an additional device. No SMS, no captcha, no 7-day cooldowns. The primary phone stays in charge.

Steps:

1. Run signal-cli link -n "Claude (signal-mcp)" — prints a sgnl://linkdevice?... URI. With qrencode installed, pipe through it for a scannable QR (signal-cli link -n "..." | qrencode -t ansiutf8).
2. Open Signal on the user's phone → Settings → Linked Devices → Link New Device → scan the QR (or paste the URI).
3. Wait for signal-cli to print the registered phone number; that's the SIGNAL_ACCOUNT value for Phase 4.

Detail and troubleshooting in references/linking.md.

Option B — Register a new number (primary device)

Only when the user wants signal-cli to be the primary Signal device for a number — typically a fresh phone number used solely for automation. This will sign the user out of Signal on their phone if it's the same number; warn them.

Steps (full transcript and captcha link in references/registration.md):

1. signal-cli -a +<E164> register — Signal sends an SMS verification code. If rate-limited, ask the user to fetch a captcha token from https://signalcaptchas.org/registration/generate.html and re-run with --captcha "signalcaptcha://...".
2. signal-cli -a +<E164> verify <SMS_CODE> — completes registration. If a registration lock PIN is required, append -p <PIN>.
3. signal-cli -a +<E164> updateProfile --given-name "<Name>" — sets the profile name (Signal will warn on send if missing).

Phase 4 — Configure the plugin

The MCP server reads two env vars:

• SIGNAL_ACCOUNT (required) — E.164 phone number of the provisioned account, e.g. +14155551212.
• SIGNAL_CLI_BIN (optional) — absolute path to the binary if it's not on PATH.

Set these in the user's shell profile (~/.zshrc, ~/.bashrc) and export in the current shell so the MCP server picks them up on next launch:

export SIGNAL_ACCOUNT="+14155551212"

The plugin's .mcp.json already references these via ${SIGNAL_ACCOUNT} / ${SIGNAL_CLI_BIN}. Ask the user to restart Claude Code (or reload the plugin) so the MCP server picks up the new environment.

Verify:

signal_check_setup()  →  { ok: true, ... }

Then run a smoke test by asking the user for a username/number to send "hello" to, and call signal_send.

Decision tree (quick)

signal_check_setup
├── binary missing      → Phase 2 (install)
├── account not set     → Phase 4 (env var)
├── account not registered
│   ├── user has phone  → Phase 3A (link as secondary)
│   └── user has bare # → Phase 3B (register primary)
└── ok=true             → done

Common pitfalls

• signal-cli says "Config file is in use by another instance" — only one signal-cli process can hold the data store. If the MCP server is running, don't run signal-cli manually at the same time. The MCP server itself spawns signal-cli per call and releases between calls; serial calls are safe.
• "InvalidMessageException: invalid PreKey message" — incoming message couldn't be decrypted (sender's session got out of sync). Not a setup problem; the message is unrecoverable but later messages will work. Don't trigger setup flow.
• Linking finishes but listAccounts is empty — link sometimes silently fails on slow networks; re-run signal-cli link and re-scan.
• Unregistered phone number — signal_get_user_status returns isRegistered: false. Check the username flow or confirm the contact actually has Signal.

Additional Resources

Reference Files

• references/install-platforms.md — full install instructions for macOS, Debian/Ubuntu, RHEL/Fedora, Arch, Nix, and manual tarball.
• references/registration.md — register-as-primary flow including captcha and PIN handling.
• references/linking.md — link-as-secondary flow including QR generation and re-link.

Scripts

• scripts/check-install.sh — one-shot diagnostic: prints PATH, signal-cli version, java version, registered accounts. Run when the MCP tool is unavailable.