Render model and hooks

The render function runs on every turn, before every model step. It returns the instruction string for that step; hooks called during the render declare the step’s capabilities.

Hooks

All hooks throw HOOK_OUTSIDE_RENDER when called outside a render.

Hook	Effect
`useInput(): AgentInput`	the current turn’s input (`source`, `text`, `payload`, `live`, `sessionId`, `turn`, `alias`)
`useModel(id, options?)`	select the model for the next step; never calls it
`useTool(tool)`	declare a tool for the next step
`useMcpServer(ref)`	declare an MCP server
`useSubagent(id)`	declare a child agent the model may delegate to
`useSessionData(): Record<string, unknown>`	synchronous snapshot of durable session data taken before render
`useSecret(name): SecretRef`	opaque, write-only secret reference
`bearer(ref): HeaderValue`	bearer-auth header value from a secret ref

Per-render capability rebuild

Capabilities (tools, MCP servers, subagents) rebuild from empty on each render, so conditional hooks work naturally:

export default defineAgent(function Support() {
  const input = useInput();
  const data = useSessionData();
  useModel("openrouter/anthropic/claude-sonnet-4.6");
  if (data.escalated) useTool(pageOncall);       // exists only after escalation
  if (input.live) useTool(refundCustomer);       // live turns only
  return "Handle the support request. Escalate when policy requires it.";
});

Render guards

Render is strictly synchronous and free of I/O. During render:

fetch, setTimeout, setInterval, queueMicrotask, and process.env reads throw AGENT_RENDER_IO.
Returning a thenable throws AGENT_RENDER_ASYNC.
Non-string returns throw AGENT_RENDER_TYPE; a render that selects no model throws AGENT_NO_MODEL.

Side effects belong in tools, and tools that do CPU work run through sandbox.exec in the VM.

Determinism

zap agent render --agent <id> --input "..." --json executes a render with no model call and no spend. Identical input produces identical bytes — the exact instructions, model id, tool list, and referenced secret names (never values). Use it in CI to snapshot agent behavior.

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Render model and hooks

Hooks

Per-render capability rebuild

Render guards

Determinism

​Hooks

​Per-render capability rebuild

​Render guards

​Determinism

Hooks

Per-render capability rebuild

Render guards

Determinism