docs(ai-chat): AI Agents documentation for v4.5 (#3226)

## Summary

Lands the full AI Agents documentation surface alongside the v4.5
release candidate of `@trigger.dev/sdk`. Covers `chat.agent` end to end
— defining agents, lifecycle hooks, the frontend transport, sub-agents,
recovery from cancel/crash/OOM, AI Prompts integration — and the
Sessions primitive that backs it.

## Coverage

- **Conceptual**: Overview, Quick Start, How it works.
- **Building agents**: Backend (`chat.agent` / `chat.createSession` /
raw primitives), Lifecycle hooks, Frontend transport, Server-side
`AgentChat`, Sessions reference, `chat.local` state primitive,
TypeScript types.
- **Features**: AI Prompts integration, Fast starts (Preload + Head
Start), Compaction, Pending Messages (steering), Background Injection
(`chat.inject` + `chat.defer`), Actions (undo / regenerate / edit),
Error handling.
- **Patterns (13)**: Sub-agents, Branching conversations, Code sandbox,
Database persistence, Persistence and replay, HITL, Tool result
auditing, Large payloads, Agent skills, OOM resilience, Recovery boot,
Trusted edge signals, Version upgrades.
- **Reference**: API Reference, Client Protocol (wire format), Testing
harness (`mockChatAgent`), MCP server tools, Upgrade guide, Changelog.

## Structure changes

- Top-level nav: AI → **Agents**, with sub-groups for *Building agents /
Features / Patterns / Reference*.
- New RC banner snippet on every page links to the supported AI SDK
versions table on the API Reference.
- All examples use Anthropic with `stopWhen: stepCountIs(15)`.

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ericallam

docs/ai-chat/actions.mdx

@@ -0,0 +1,115 @@
+---
+title: "Actions"
+sidebarTitle: "Actions"
+description: "Custom commands sent from the frontend that mutate chat state without consuming a turn — undo, rollback, edit, regenerate."
+---
+
+import RcBanner from "/snippets/ai-chat-rc-banner.mdx";
+
+<RcBanner />
+
+## Overview
+
+Custom actions let the frontend send structured commands (undo, rollback, edit, regenerate) that modify the conversation state. **Actions are not turns**: they fire `hydrateMessages` (if set) and `onAction` only. No turn lifecycle hooks (`onTurnStart` / `prepareMessages` / `onBeforeTurnComplete` / `onTurnComplete`), no `run()`, no turn-counter increment. The trace span is named `chat action`.
+
+Actions wake the agent from suspension the same way a new message does, run their handler against the latest accumulator state, and emit a `trigger:turn-complete` chunk so the frontend's `useChat` knows the action has been applied.
+
+## Defining an action handler
+
+Define an `actionSchema` for validation and an `onAction` handler that uses [`chat.history`](/ai-chat/backend#chat-history) to modify state:
+
+```ts
+import { z } from "zod";
+
+export const myChat = chat.agent({
+  id: "my-chat",
+  actionSchema: z.discriminatedUnion("type", [
+    z.object({ type: z.literal("undo") }),
+    z.object({ type: z.literal("rollback"), targetMessageId: z.string() }),
+    z.object({ type: z.literal("edit"), messageId: z.string(), text: z.string() }),
+  ]),
+
+  onAction: async ({ action }) => {
+    switch (action.type) {
+      case "undo":
+        chat.history.slice(0, -2); // Remove last user + assistant exchange
+        break;
+      case "rollback":
+        chat.history.rollbackTo(action.targetMessageId);
+        break;
+      case "edit":
+        chat.history.replace(action.messageId, {
+          id: action.messageId,
+          role: "user",
+          parts: [{ type: "text", text: action.text }],
+        });
+        break;
+    }
+    // returning void → side-effect-only, no model call
+  },
+
+  run: async ({ messages, signal }) => {
+    return streamText({ model: anthropic("claude-sonnet-4-5"), messages, abortSignal: signal });
+  },
+});
+```
+
+**Lifecycle flow:** Wake → parse action against `actionSchema` → `hydrateMessages` (if set) → **`onAction`** → apply `chat.history` mutations → emit `trigger:turn-complete` → wait for next message.
+
+## Returning a model response from an action
+
+`onAction` can return a `StreamTextResult`, `string`, or `UIMessage` to produce a response. The returned stream is auto-piped to the frontend just like a normal turn, but the rest of the turn machinery (`onTurnStart`, `onTurnComplete`, etc.) still does not fire.
+
+```ts
+onAction: async ({ action, messages }) => {
+  if (action.type === "regenerate") {
+    chat.history.slice(0, -1); // drop the last assistant
+    return streamText({
+      model: anthropic("claude-sonnet-4-5"),
+      messages,
+      stopWhen: stepCountIs(15),
+    });
+  }
+  // other actions return void → side-effect only
+}
+```
+
+This is useful for actions that both mutate state and want a fresh model response (regenerate-from-here, retry-with-different-style). Persistence is your responsibility inside `onAction` itself; you have access to the streamed response object.
+
+## Gating actions on HITL state
+
+If you have a [human-in-the-loop](/ai-chat/patterns/human-in-the-loop) tool waiting on `addToolOutput`, you usually want to refuse competing actions like `regenerate` until the answer arrives. [`chat.history.getPendingToolCalls()`](/ai-chat/backend#chat-history) gives you exactly that signal:
+
+```ts
+onAction: async ({ action, messages, signal }) => {
+  if (action.type === "regenerate") {
+    if (chat.history.getPendingToolCalls().length > 0) return; // gated
+    chat.history.slice(0, -1);
+    return streamText({ model: anthropic("claude-sonnet-4-5"), messages, abortSignal: signal });
+  }
+},
+```
+
+## Sending actions from the frontend
+
+```ts
+// Browser — TriggerChatTransport
+const stream = await transport.sendAction(chatId, { type: "undo" });
+
+// Server — AgentChat
+const stream = await agentChat.sendAction({ type: "rollback", targetMessageId: "msg-3" });
+```
+
+The action payload is validated against `actionSchema` on the backend; invalid actions throw and surface as a stream error. The `action` parameter in `onAction` is fully typed from the schema.
+
+<Note>
+  For silent state changes that should never appear as a turn (e.g. injecting background context), use [`chat.inject()`](/ai-chat/background-injection) instead. Actions are explicit user-driven mutations; injections are agent-side context updates.
+</Note>
+
+## See also
+
+- [`chat.history`](/ai-chat/backend#chat-history) — the imperative API actions use to mutate state
+- [Sending actions from the frontend](/ai-chat/frontend#sending-actions) — `transport.sendAction` ergonomics
+- [`hydrateMessages`](/ai-chat/lifecycle-hooks#hydratemessages) — fires before `onAction` when set
+- [Branching conversations](/ai-chat/patterns/branching-conversations) — pairs action handlers with backend-controlled history
+- [Human-in-the-loop](/ai-chat/patterns/human-in-the-loop) — gating fresh actions while a tool is waiting