diff --git a/11-Features.md b/11-Features.md
index 71d4640..0f80b22 100644
--- a/11-Features.md
+++ b/11-Features.md
@@ -3643,3 +3643,26 @@ does not recognise — it rethrows it unchanged, and a test pins that. Closing t
 the exception are different things, and only the first was missing. Do not "simplify" this by
 mapping the error to `PeerUnreachableException`; that reintroduces the exact behaviour #176 wrote
 `failFastOnGoneBackend` to keep distinct.
+
+## A reply with no content is refused instead of resolving the waiter
+
+**What.** `POST /sessions/{id}/reply` read the `content` field with a silent default, so a body that
+omitted the field became an empty reply. That empty reply resolved the lead's waiter and the turn
+completed. The required-content check now lives in `MessageService.reply`, which both doors call, so
+the REST door returns `400 bad_request` and the MCP tool returns a tool error. Blank and
+whitespace-only content are treated the same as missing.
+
+**On.** Always on.
+
+**Why it exists.** The lead could not tell an empty reply from a member that genuinely said nothing.
+This project already has several real conditions that look like that — a clipped completion scrape,
+a backend that died mid-turn — so the bad case hid among them. The guard was written in one handler
+instead of in the thing both handlers call, which is the same drift shape as #284 and #297.
+
+**One thing to know for maintenance.** `fleet_reply` had a smaller version of the same hole:
+its guard checked `content == null`, not blank, so whitespace went through. Moving the check into
+`MessageService.reply` closed that too, but it also made the shared method throw. `replyHandler` in
+`FleetMcp` is a bare `BiFunction` with no try/catch, so that throw would have escaped the tool
+handler as an uncaught exception. The tool's own guard was widened to `isBlank` for that reason, and
+`FleetMcpTest.replyWithBlankContentIsACleanToolErrorNotAnUncaughtException` pins it. If you ever move
+the check again, check the handler between the door and the service, not only the two ends.