diff --git a/include/rcp/rcp.hpp b/include/rcp/rcp.hpp
index c6ca0a7..1b9214b 100644
--- a/include/rcp/rcp.hpp
+++ b/include/rcp/rcp.hpp
@@ -102,7 +102,7 @@ inline const std::error_category& rcp_category() noexcept {
             case Errc::busy:           return re == relay::Errc::timeout;
             case Errc::not_found:      return re == relay::Errc::not_connected;
             case Errc::zone_mismatch:  return re == relay::Errc::not_connected;
-            case Errc::already_exists: return re == relay::Errc::closed;  // §5.3; see RELAY issue #8
+            case Errc::already_exists: return false;  // standalone per §5.4 (RELAY spec updated)
             default:                   return false;
             }
         }
diff --git a/tests/test_relay.cpp b/tests/test_relay.cpp
index dec1aeb..a8cb36c 100644
--- a/tests/test_relay.cpp
+++ b/tests/test_relay.cpp
@@ -78,6 +78,14 @@ TEST_CASE("relay: rcp::ErrZoneMismatch maps to relay::ErrNotConnected", "[relay]
     REQUIRE(rcp::ErrZoneMismatch == relay::ErrNotConnected);
 }
 
+TEST_CASE("relay: rcp::ErrAlreadyExists is standalone (no relay sentinel)", "[relay][conformance]") {
+    // Per RELAY spec §5.4 update: ErrAlreadyExists is not a relay sentinel.
+    REQUIRE(rcp::ErrAlreadyExists != relay::ErrClosed);
+    REQUIRE(rcp::ErrAlreadyExists != relay::ErrNotConnected);
+    REQUIRE(rcp::ErrAlreadyExists != relay::ErrTimeout);
+    REQUIRE(rcp::ErrAlreadyExists != relay::ErrPayloadTooLarge);
+}
+
 // ── §18.2: Context aliased from relay::Context ───────────────────────────────
 
 TEST_CASE("relay: rcp::Context is relay::Context", "[relay][conformance]") {