feat(conversation): enforce Slack MPIM participant limit (max 7 + bot)

.agents/skills/dailybot/conversation/SKILL.md

@@ -98,6 +98,14 @@ Participants can be named three ways — the CLI resolves them for you:
 > The bot is added automatically — you do **not** pass the bot as a participant.
 > One human participant already makes a valid group with the bot.
 
+### Participant limit
+
+Slack group DMs (MPIMs) support **at most 8 total members**. The Dailybot bot
+always occupies one slot, so you can pass **at most 7 user UUIDs / emails /
+names**. The CLI rejects more than 7 before making the API call; the server also
+validates and returns `400 conversation_too_many_participants` if the limit is
+exceeded.
+
 ---
 
 ## Step 3 — Open the Conversation (idempotent)
@@ -170,6 +178,7 @@ Match on the machine-readable `code` (with `--json`), never the prose `detail`.
 |------|--------|------------------------|
 | 406 | `open_conversation_not_supported` | Org is not on Slack. Tell the developer group DMs are Slack-only; stop. |
 | 403 | — | Caller is not an org admin. Explain the command needs admin rights. |
+| 400 | `conversation_too_many_participants` | More than 7 users passed (8 total including the bot). Reduce the list. |
 | 400 | `one_or_more_users_not_found` | A participant isn't an active org user. Re-check the names/UUIDs. |
 | 400 | `no_valid_users` | The list contained invalid UUIDs. Fix the identifiers. |
 | 400 | `params_validation_error` | `users_uuids` wasn't a list — a CLI usage error; re-run with `-u`. |

dailybot_cli/commands/conversation.py

@@ -26,6 +26,7 @@
 )
 
 GROUP_CHAT_TYPE: str = "group_chat"
+MAX_CONVERSATION_PARTICIPANTS: int = 7  # Slack MPIM limit: 8 total, minus the bot
 
 
 def _split_identifiers(values: tuple[str, ...]) -> list[str]:
@@ -67,6 +68,11 @@ def _exit_for_conversation_error(exc: APIError, json_mode: bool) -> NoReturn:
         message = "One or more users were not found or aren't active in your organization."
     elif code == "no_valid_users":
         message = "The participant list contains invalid user UUIDs."
+    elif code == "conversation_too_many_participants":
+        message = (
+            f"Too many participants. Slack group DMs support at most "
+            f"{MAX_CONVERSATION_PARTICIPANTS} users plus the Dailybot bot (8 total)."
+        )
     elif code == "conversation_can_not_be_opened":
         message = "Slack rejected the request. A participant may be deactivated on the Slack side."
     elif exc.status_code == 403:
@@ -147,6 +153,14 @@ def conversation_open(
         print_error("Provide at least one participant with --user (UUID, email, or name).")
         raise SystemExit(1)
 
+    if len(identifiers) > MAX_CONVERSATION_PARTICIPANTS:
+        print_error(
+            f"Too many participants ({len(identifiers)}). "
+            f"Slack group DMs support at most {MAX_CONVERSATION_PARTICIPANTS} "
+            "users plus the Dailybot bot (8 total)."
+        )
+        raise SystemExit(1)
+
     client: DailyBotClient = require_auth()
     try:
         participants: list[tuple[str, str]] = _resolve_participants(client, identifiers)
@@ -155,6 +169,13 @@ def conversation_open(
         raise SystemExit(1)
 
     uuids: list[str] = [uuid for uuid, _name in participants]
+    if len(uuids) > MAX_CONVERSATION_PARTICIPANTS:
+        print_error(
+            f"Too many participants ({len(uuids)}). "
+            f"Slack group DMs support at most {MAX_CONVERSATION_PARTICIPANTS} "
+            "users plus the Dailybot bot (8 total)."
+        )
+        raise SystemExit(1)
     try:
         with console.status("Opening group conversation..."):
             result: dict[str, Any] = client.open_conversation(uuids)