AI agent skill · by @vellum-ai

Vellum Terminal Sessions

Manage persistent tmux terminal sessions: create, read, write to, list, and close named shell sessions. For Claude Code or Codex, use the ACP skill instead.

Archived 2026-10-23 · Source updated 2026-10-09 · License: MIT. Check the original repository for the latest version.

SKILL.md instructions

## Overview

This skill manages **persistent terminal sessions** via tmux. Two modes:

|                       | Sandbox Mode                                  | Host Mode                                                           |
| --------------------- | --------------------------------------------- | ------------------------------------------------------------------- |
| **Where tmux runs**   | In the assistant's sandbox                    | On the user's host machine                                          |
| **How user connects** | `vellum terminal attach <session-name>`       | User opens their own terminal                                       |
| **Assistant access**  | Direct (`bash` tool)                          | Via `host_bash`                                                     |
| **Best for**          | Long-running builds, servers, always-on tasks | Work needing host access (e.g. SwiftUI builds, host-local services) |

**Default:** Use **Sandbox Mode** unless the task specifically requires host access. Details: [sandbox-sessions.md](references/sandbox-sessions.md) · [host-sessions.md](references/host-sessions.md) · [tmux-best-practices.md](references/tmux-best-practices.md)

## Prerequisites

**Sandbox Mode:** No setup needed — tmux is available in the sandbox.

**Host Mode:** Requires tmux on the host — see [host-sessions.md](references/host-sessions.md).

## Workflow: Orchestrating Multiple Sessions

- **List** all sessions: `tmux list-sessions` to get names and current commands
- **Read** each: `tmux capture-pane -t NAME -p -S -50` (30-50 lines is usually enough)
- **Summarize** before acting: "Build done, tests at 73%, deploy waiting for confirmation"
- **Act** on the user's instructions