Hooks

Hooks

A hook is a script crickets runs at a fixed point in an agent session — before a tool call, at session start, when a turn ends — to observe or steer what the agent does. Each plugin declares its hooks in a single hooks.json (Claude Code: <plugin>/hooks/hooks.json; Antigravity: <plugin>/hooks.json), and the host runs them straight from the plugin. Installing the plugin is the whole setup — nothing lands in .claude/hooks/.

⚡ The hooks

Hook	Plugin	Event	What it does	Details
harness-context	developer-workflows	SessionStart	surfaces .harness/PLAN.md + progress.md at boot so the agent reads the plan first	spec
kill-switch	developer-safety	PreToolUse	halts the next tool call when .harness/STOP exists	how to use
steer	developer-safety	PreToolUse	injects .harness/STEER.md into context, then archives it	how to use
commit-on-stop	developer-safety	Stop	saves a dirty tree to an auto-save/<ts> branch for crash recovery	how to use
evidence-tracker	code-review	PreToolUse	blocks flipping a PLAN.md task to [x] until evidence for it is recorded (default-FAIL /work gate)	spec

Hosts: harness-context and evidence-tracker are Claude-only. The developer-safety trio install on both hosts but run observe-only on Antigravity (no veto/inject) — see Compatibility.

The Details links point at how each hook operates. Today the developer-safety trio point at Operator-control hooks and the rest at their hook spec; when each plugin gets its own page, these repoint to that page's hooks section.

How hooks function

• Events. A hook registers on one host event — SessionStart (session boot), PreToolUse (before every tool call), or Stop (end of each turn). The host fires every hook registered on that event.
• Effect. A PreToolUse hook can block the tool call (a non-zero exit — e.g. kill-switch) or inject text into context (stdout — e.g. steer). A Stop hook runs as a side effect (e.g. commit-on-stop); a SessionStart hook seeds context.
• Ordering. When several hooks share an event, they run in the order the plugin's hooks.json declares them — developer-safety registers kill-switch before steer, so a halt always pre-empts a steer.
• Per-host effectiveness. On Antigravity, plugin hooks run observe/side-effect-only: any hook whose value depends on a veto (exit code) or an inject (stdout) is Claude-only-effective. See Compatibility.

See also

• Operator-control hooks — how to use the developer-safety trio (halt / redirect / recover).
• Compatibility — per-host hook effectiveness.
• Customization Types — what kind: hook is, and where hook sources live.
• Modify a crickets plugin — edit a hook script and dogfood it.