SeaShell

A terminal window manager. Run many shells side by side in one window — plain zsh, a dev server, or several AI coding agents — and keep a file explorer next to them. macOS is the reference platform; Windows and Linux builds are experimental.

Six panes in one window — and a Lookout card catching the session that stopped to ask a question.

⬇ Download the latest release — macOS · Windows · Linux

Built for running Claude Code. SeaShell works with any terminal program, but every design decision that had a tie-breaker was settled in favour of watching several long-running agent sessions at once. That is what it is for.

Agents work while you're not watching. So run six. A session only needs you for the ten seconds it stops to ask something. Tile six of them in one window, let the cards catch each question the moment it lands, and answering becomes your whole job — the agents do the rest. That's how one person ships like six.

Status: pre-alpha. Under active development.

Install

Download the .dmg from Releases, open it, and drag SeaShell to Applications. The build is universal — it runs natively on Apple Silicon and Intel, no Rosetta.

First launch: "SeaShell cannot be opened"

macOS will refuse to open it the first time, with either "cannot be opened because the developer cannot be verified" or "is damaged and can't be opened". The app is fine. That message means it was not signed with a paid Apple Developer ID, so macOS flagged it when your browser downloaded it.

Clear the flag:

xattr -dr com.apple.quarantine /Applications/SeaShell.app

Then open SeaShell normally. You only ever do this once.

Without using Terminal

Open System Settings → Privacy & Security, scroll down to the message about SeaShell, and click Open Anyway. Then launch the app again and confirm.

On macOS 15 (Sequoia) and later this is the only click-through route — Apple removed the old right-click → Open shortcut.

If you copy the app across by USB stick, scp, or a shared folder instead of downloading it, macOS never sets the flag and none of the above applies. It just opens.

Why

Terminal multiplexers make you choose. tmux and zellij are excellent but live inside a single terminal window and bring their own keybinding grammar. Tabbed terminal emulators give you tabs but only ever show you one thing at a time.

SeaShell is built around a narrower goal: watch several long-running sessions at once, and work with the files they mention without leaving the window.

Working with Claude Code

Running agents is not the same as running commands. An agent session lasts hours, spends most of that time either thinking or waiting on you, and prints paths you immediately want to look at. These exist because of that:

• Panes tell you when they want you. A pane breathes its border while its program sits waiting for input, and pulses briefly when a job finishes — never the pane you are already looking at. The distinction matters: an agent showing a spinner is emitting bytes constantly while doing nothing, so idle detection keys on what the foreground process is actually doing, not on output. An optional ping sounds the moment a pane starts asking.
• Sleep. One click in the tab bar stops every pane asking, glow and ping together, for when you need to concentrate on one of them.
• Panes name themselves. A pane takes the title the running program sets — Claude Code publishes its session summary that way — so six agents read as six pieces of work instead of six panes all labelled claude. Colour tags, automatic or hand-picked, tell them apart at a glance.
• Closing a pane really closes it. An escalating kill ladder reaps the pane's whole process tree, including anything that double-forked away from the shell. Forgotten agent sessions quietly eating a 16 GB machine is the problem this app was built to end; panes over 200 MB show their memory in the title bar.
• Paths and previews. Double-click a path an agent printed and the explorer reveals it. Inside a full-screen program that claims the mouse, hold ⌥. Open the file and it becomes a tiled pane with syntax highlighting, beside the agent that mentioned it.
• ⌘A selects your input line, not thousands of lines of scrollback — the difference between editing the prompt you are halfway through and selecting an entire transcript.
• Shift+Enter sends ESC CR, the multi-line-input binding agents expect, with no setup and without SeaShell touching your Terminal.app preferences.
• A pane is a real login shell. Your dotfiles, your PATH, your history. Nothing is wrapped or intercepted, and the environment a pane inherits is scrubbed of SeaShell's own identity so a nested launch cannot confuse the agent running inside it.

Lookout

Watching several agent panes means someone still has to notice the one that stopped to ask you something. Lookout does that noticing: the moment an unfocused pane goes idle on a question, a card appears in a rail beside the panes with the question on it, and a badge in the status bar counts every card waiting on you — the pane you are already looking at gets counted there too, just without a card of its own.

Two lanes feed the same stack:

• Detector, built in. SeaShell watches for a claude pane idle on a question and raises a card with canned replies — Continue / Yes / No, Go to pane, dismiss. No config, no tokens, no external process, on every platform.
• Drafted replies, via a plugin. With the c-assistant companion plugin installed, a Claude Code Stop hook drafts a one-line reply in your voice and pushes a richer card over the control socket — Approve ✓, Edit, Deny ✕. A pushed card replaces the detector's card for the same pane.

The safety model is one rule regardless of lane: external tools can only propose a card, and only your own click inside SeaShell ever submits it into the pane. A card on a pane showing a picker screen is look-only — there is no single word to send into a menu, so it shows the question and stops there.

Smart drafted cards require the c-assistant companion plugin:

/plugin marketplace add voidharbor/claude-plugins
/plugin install c-assistant@voidharbor

For the full skill set, install the voidharbor bundle instead — /plugin install voidharbor@voidharbor installs all of voidharbor's skills at once, Lookout hooks included, so the bundle alone is enough for drafted cards. Installing both is also fine: the hooks detect the duplicate and only one copy runs per session.

A master toggle in Settings turns Lookout off.

What it does

• Tiled panes. Each tab holds an auto-arranged grid of terminals — 1, then 2 side by side, then quadrants. Drag the borders to resize. Double-click a pane's title bar to zoom it to full tab, again to restore.
• Any command per pane. A pane is just a PTY. Run zsh, a build watcher, an SSH session, or a coding agent. Full-screen TUI programs render exactly as they do in a native terminal — alternate screen buffer, 24-bit color, correct glyph widths.
• Built-in file explorer. A lazy-loaded tree beside the panes. Drag a file into a terminal to paste its quoted path.
• Paths become clickable. Any filesystem path printed in a pane is detected and verified against disk. Double-click it and the explorer expands to that file and highlights it — nothing is opened behind your back. Opening is a separate, deliberate act from the tree. Bare URLs in output are clickable too, and open in your browser.
• Preview panes. Open a file and it becomes a tiled pane beside your terminals with syntax highlighting; point a web preview at a URL and watch a dev server render next to the pane running it. Previews are ordinary leaves in the layout tree, so they resize with the same dividers, zoom with the same ⌘⏎ and close with the same ⌘W as everything else.
• Resource visibility. Each pane reports the memory of its whole process subtree, and panes sitting idle say so. Long-lived agent sessions are easy to forget; this makes them visible.
• Closing a pane means it. Closing runs an escalating kill ladder across the pane's whole process tree — including background jobs and descendants that double-forked away from the shell. If anything survives, it says so rather than pretending the pane was clean.
• Zoom that stays sharp. ⌘+ / ⌘− scale the terminal text and the interface together. The font steps through only sizes whose advance lands near a device-pixel boundary, because the WebGL renderer floors charWidth * dpr and anything else drifts into misaligned borders.

Press ⌘/ for a short tutorial of the parts that aren't guessable.

Design

The full design specification lives in docs/superpowers/specs/. It covers the layout engine, PTY lifecycle, path detection, IPC contract, security posture, and build pipeline in enough detail to implement from.

Building

Requires macOS, Node 20+, and Xcode command line tools.

npm install
npm run dev            # run from source
npm run build          # compile to out/
npm run pack:mac       # SeaShell.app for this machine's architecture
npm run dist:mac       # universal .dmg for distribution

A universal build works from an Intel host with no cross-compiling. node-pty 1.1.0 is Node-API and already ships prebuilds for both darwin-x64 and darwin-arm64, so nothing needs recompiling — the loader picks the right one at runtime.

Two settings in electron-builder.yml are what make that true, and both are load-bearing:

• npmRebuild: false — it defaults to true, which rebuilds node-pty into build/Release for the host arch only. node-pty's loader prefers build/Release over prebuilds/, so a "universal" build made on an Intel Mac would ship an x64-only PTY layer: perfect locally, dead on every Apple Silicon machine.
• x64ArchFiles — @electron/universal aborts when it finds a non-universal Mach-O present identically in both slices. The per-arch node-pty prebuilds are exactly that, on purpose.

The app is ad-hoc signed (codesign -s -), which is what Apple Silicon requires in order to execute at all. It is not signed with a Developer ID and not notarized, which is why the quarantine step in Install exists.

Non-goals

SeaShell deliberately does not do these things:

• Edit files. File previews are read-only. Use your editor.
• Manage remote sessions. No SSH profile management, no session persistence across reboots.
• Support Windows or Linux. macOS only.
• Plugins or theming. No extension API, no theme editor.
• Replace your browser. The web preview exists for adjacency — a page next to the pane serving it. It saves no memory: Chromium costs about the same per page wherever it renders.

License

Apache-2.0.