diff --git a/docs/channels/groups.md b/docs/channels/groups.md
index 353baa9d319e..588d47ed5a48 100644
--- a/docs/channels/groups.md
+++ b/docs/channels/groups.md
@@ -311,11 +311,11 @@ Group messages require a mention unless overridden per group. Defaults live per
 
 Supported implicit mention facts are channel-specific:
 
-| Fact                  | Current built-in producers                       |
-| --------------------- | ------------------------------------------------ |
-| Reply to the bot      | Discord, Microsoft Teams, QQBot, Slack, Telegram |
-| Quote of the bot      | WhatsApp, Zalo personal                          |
-| Bot joined the thread | Mattermost, Slack, Tlon                          |
+| Fact                  | Current built-in producers                                   |
+| --------------------- | ------------------------------------------------------------ |
+| Reply to the bot      | Discord, Mattermost, Microsoft Teams, QQBot, Slack, Telegram |
+| Quote of the bot      | WhatsApp, Zalo personal                                      |
+| Bot joined the thread | Mattermost, Slack, Tlon                                      |
 
 Each fact defaults to enabled when the channel produces it. Set the corresponding `implicitMentions` flag to `false` to stop that fact from bypassing mention gating; native explicit mentions remain unaffected. A flag has no effect on channels that do not produce that fact.
 
diff --git a/docs/channels/mattermost.md b/docs/channels/mattermost.md
index 4ee43c541f10..958ee7ebd97d 100644
--- a/docs/channels/mattermost.md
+++ b/docs/channels/mattermost.md
@@ -169,6 +169,7 @@ Notes:
 - `onchar` still responds to explicit @mentions.
 - `channels.mattermost.requireMention` is still honored, but `chatmode` is preferred. Per-channel `groups.<channelId>.requireMention` settings win over both.
 - After the bot sends a visible reply in a channel thread, later messages in that same thread are answered without a new @mention or `onchar` prefix, so multi-turn thread conversations keep flowing. Participation is remembered for 7 days after the bot last replied in that thread and persists across gateway restarts. Threads the bot has only observed are unaffected; start a new top-level message to require an explicit mention again.
+- Replies in a thread whose **root post the bot authored** also auto-engage, even before the bot's first reply in that thread — for example replying to a bot announcement or to a non-threaded bot answer. Unlike remembered participation this needs no prior reply, so it works for old threads and across restarts (parity with Slack and Telegram reply-to-bot handling). On by default; set `channels.mattermost.implicitMentions.replyToBot` to `false` to disable.
 - Set `channels.mattermost.implicitMentions.threadParticipation: false` to stop participated-thread follow-ups from bypassing mention gating. Account overrides use `channels.mattermost.accounts.<id>.implicitMentions`. Mattermost does not currently produce `replyToBot` or `quotedBot` facts, so those flags have no effect here.
 
 ## Threading and sessions