diff --git a/docs/API_REFERENCE.txt b/docs/API_REFERENCE.txt
index b08f84f..4e51b94 100644
--- a/docs/API_REFERENCE.txt
+++ b/docs/API_REFERENCE.txt
@@ -1850,6 +1850,8 @@ BOOKING APIS
   - 404 Not Found
   - 409 Conflict
       - Booking not checked in
+      - Primary guest missing / guest name missing / guest phone missing / guest signature missing
+      - checkOutAt must be after checkInAt
       - Room stay amount outside allowed range
       - Ledger mismatch tolerance failure
 
@@ -1878,7 +1880,11 @@ BOOKING APIS
   - 401 Unauthorized
   - 403 Forbidden
   - 404 Not Found (booking/stay)
-  - 409 Conflict (booking not checked in / amount or minimum duration invalid)
+  - 409 Conflict
+      - booking not checked in
+      - Primary guest missing / guest name missing / guest phone missing / guest signature missing
+      - checkOutAt must be after checkInAt
+      - amount or minimum duration invalid
 
 
 - Cancel booking API is this one:
diff --git a/src/main/kotlin/com/android/trisolarisserver/controller/booking/BookingFlow.kt b/src/main/kotlin/com/android/trisolarisserver/controller/booking/BookingFlow.kt
index f12ac3c..1870405 100644
--- a/src/main/kotlin/com/android/trisolarisserver/controller/booking/BookingFlow.kt
+++ b/src/main/kotlin/com/android/trisolarisserver/controller/booking/BookingFlow.kt
@@ -651,6 +651,7 @@ class BookingFlow(
         }
         val now = OffsetDateTime.now()
         val checkOutAt = parseOffset(request.checkOutAt) ?: now
+        validateCheckoutPrerequisites(booking, checkOutAt)
 
         val stays = roomStayRepo.findActiveByBookingId(bookingId)
         if (stays.any { !isCheckoutAmountValid(it) || !isMinimumStayDurationValid(it, checkOutAt) }) {
@@ -723,6 +724,7 @@ class BookingFlow(
 
         val now = OffsetDateTime.now()
         val checkOutAt = parseOffset(request.checkOutAt) ?: now
+        validateCheckoutPrerequisites(booking, checkOutAt)
         if (!isCheckoutAmountValid(stay)) {
             throw ResponseStatusException(HttpStatus.CONFLICT, "Room stay amount is outside allowed range")
         }
@@ -1060,6 +1062,29 @@ class BookingFlow(
         return java.time.Duration.between(stay.fromAt, checkOutAt).toMinutes() >= 60
     }
 
+    private fun validateCheckoutPrerequisites(
+        booking: com.android.trisolarisserver.models.booking.Booking,
+        checkOutAt: OffsetDateTime
+    ) {
+        val guest = booking.primaryGuest
+            ?: throw ResponseStatusException(HttpStatus.CONFLICT, "Primary guest required before checkout")
+        if (guest.name.isNullOrBlank()) {
+            throw ResponseStatusException(HttpStatus.CONFLICT, "Guest name required before checkout")
+        }
+        if (guest.phoneE164.isNullOrBlank()) {
+            throw ResponseStatusException(HttpStatus.CONFLICT, "Guest phone required before checkout")
+        }
+        if (guest.signaturePath.isNullOrBlank()) {
+            throw ResponseStatusException(HttpStatus.CONFLICT, "Guest signature required before checkout")
+        }
+
+        val checkInAt = booking.checkinAt ?: booking.expectedCheckinAt
+            ?: throw ResponseStatusException(HttpStatus.CONFLICT, "Check-in time missing for booking")
+        if (!checkOutAt.isAfter(checkInAt)) {
+            throw ResponseStatusException(HttpStatus.CONFLICT, "checkOutAt must be after checkInAt")
+        }
+    }
+
     private fun ensureLedgerToleranceForCheckout(
         bookingId: UUID,
         timezone: String?,