Core concepts
These are the terms that show up everywhere in Sentinel.
Workspace
A workspace is the project container.
It usually points at a local folder on disk.
Thread
A thread is the unit of work.
It keeps messages and the rest of the state around the task.
Engine
An engine is the runtime layer for a thread.
Sentinel supports sentinel, codex, claude, copilot, cursor, opencode, grok, antigravity, pi, and acp (ACP Registry agents).
Engine instance
An engine instance is one configured copy of an agent, with its own label, binary, environment, and, for some agents, its own home directory. Threads and automations run on an instance.
Chat mode
Chat mode is the normal back-and-forth thread mode.
Plan mode
Plan mode stores a plan document, tasks, and question sets as thread state.
Worktree
A worktree is a separate repo checkout path tied to a thread.
Checkpoint
A checkpoint is saved repo state tied to thread history.
Approval
An approval is a pause before a tool or tool group runs.
Sub-agent
A sub-agent is delegated work stored as child thread state.
Skill
A skill is an installable package of behavior or instructions.
MCP server
An MCP server is an external tool server connected over stdio or http.
Integration
An integration is a built-in connection to an outside system.