docs(chat): the turn an action requests carries trigger action-turn

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AxuSksX18bj1yhnLpkcQ6a

Author: ericallam

docs/ai-chat/actions.mdx

@@ -75,7 +75,7 @@ onAction: async ({ action }) => {
 }
 ```
 
-`run()` receives the edited history with no incoming user message, the same shape as a `regenerate-message` turn, and its `trigger` is `"action"`. Returning anything other than `chat.turn()` or nothing is an error; a response can no longer be returned from `onAction` directly.
+`run()` receives the edited history with no incoming user message, the same shape as a `regenerate-message` turn, and its `trigger` is `"action-turn"`, so a `run()` that returns early on `"action"` (the pre-May behaviour, when actions invoked `run()` directly) still answers. Returning anything other than `chat.turn()` or nothing is an error; a response can no longer be returned from `onAction` directly.
 
 ### Actions and persistence
 

docs/ai-chat/reference.mdx

@@ -105,7 +105,7 @@ The payload passed to the `run` function.
 | `streamText`   | `typeof streamText`                        | The AI SDK's `streamText` with this agent's managed options already applied: the prompt, skill tools, telemetry, and the `prepareStep` that delivers steering, compaction and injected context. Prefer it over importing `streamText` from `ai`. See [The managed streamText](/ai-chat/backend#the-managed-streamtext). |
 | `chatId`       | `string`                                   | Your conversation ID (the session's `externalId`)                    |
 | `sessionId`    | `string`                                   | Friendly ID of the backing Session (`session_*`). Use with `sessions.open()` for advanced cases. Always set — every chat.agent run is bound to a Session. |
-| `trigger`      | `"submit-message" \| "regenerate-message"` | What triggered the request                                           |
+| `trigger`      | `"submit-message" \| "regenerate-message" \| "action-turn"` | What triggered the request; `"action-turn"` is a turn requested by `chat.turn()` |
 | `messageId`    | `string \| undefined`                      | Message ID (for regenerate)                                          |
 | `clientData`   | Typed by `clientDataSchema`                | Custom data from the frontend (typed when schema is provided)        |
 | `continuation` | `boolean`                                  | Whether this run is continuing an existing chat (previous run ended) |