fix(slack): rotate long native streams before expiry

.changeset/fresh-ducks-stream.md

@@ -0,0 +1,5 @@
+---
+"@chat-adapter/slack": minor
+---
+
+Rotate long-running native Slack streams before Slack expires them. Once a stream segment passes `streamSegmentMaxAgeMs` (default four minutes) the adapter finalizes it at the next paragraph break and continues the reply in a new message, closing and reopening code fences, repeating table headers, replaying open task cards and the plan title, and keeping the agent session in `processing`. A segment Slack already expired during an idle gap is recovered the same way instead of failing the reply.

apps/docs/content/adapters/official/slack.mdx

@@ -164,6 +164,12 @@ bot.onNewMention(async (thread, message) => {
       description:
         "Use Slack's native streaming API for streamed posts. Set false to always stream via post-and-edit.",
     },
+    streamSegmentMaxAgeMs: {
+      type: "number",
+      default: "240000",
+      description:
+        "Finalize and continue native streams in a new message after this many milliseconds to stay below Slack's roughly five-minute stream expiry. Rotation waits up to 30 seconds for a paragraph break. Set Infinity to disable.",
+    },
     feedbackButtons: {
       type: "boolean | SlackFeedbackButtonsOptions",
       description:
@@ -828,6 +834,8 @@ Threads without streaming context fall back to post-and-edit (`chat.update` delt
 const slack = createSlackAdapter({ nativeStreaming: false });
 ```
 
+Slack expires a native stream after roughly five minutes. A reply that streams longer than that is finalized and continued in a new message: once a segment is four minutes old (`streamSegmentMaxAgeMs`, default `240000`; `Infinity` disables rotation), the adapter rotates at the next paragraph break, or after at most 30 more seconds if none arrives. Across the boundary an open code fence is closed and reopened, a table that continues gets its header repeated, the plan title and any task cards still in progress are replayed so later updates land on them, and with `agentView` the session stays in `processing`. The finalized message keeps its task cards in their last state, and a list split across the boundary restarts its numbering. The `SentMessage` returned by `thread.post()` refers to the last message of the reply. If Slack expires a segment during a long idle gap anyway, the adapter continues in a new message with any text Slack had not confirmed rather than failing the reply.
+
 #### Feedback buttons
 
 Slack's agent UX guidance recommends native thumbs up/down feedback on agent replies (a `context_actions` block with a `feedback_buttons` element). Configure `feedbackButtons` and the adapter appends them to every streamed reply when the stream finishes:

packages/adapter-slack/AGENTS.md

@@ -273,7 +273,19 @@ Fallback to post-and-edit (`chat.update` deltas, throttled by
   skipped and text streaming continues.
 
 Failures after native content has rendered still propagate — mixing the
-two surfaces would duplicate output.
+two surfaces would duplicate output. The one exception is Slack expiring
+a stream (`message_not_in_streaming_state`): the adapter continues in a
+new message with any text Slack had not confirmed.
+
+Slack expires a native stream after roughly five minutes, so `stream()`
+works in segments: once a segment is `streamSegmentMaxAgeMs` old (the
+clock starts at the first call Slack accepts, not at construction), the
+next paragraph break (or, after a 30 s grace, the next line break)
+finalizes it and a new `chatStream` continues the reply. Rotation closes
+and reopens an open code fence, repeats a table header, replays the plan
+and open task cards, and keeps `session_status: "processing"` with
+`agentView`. Tests drive this with a mocked `Date.now()`; see the
+"native stream rotation" describe block.
 
 ## Socket Mode