diff --git a/slime/ray/rollout.py b/slime/ray/rollout.py
index 0bd4c0fa0..ec043385d 100644
--- a/slime/ray/rollout.py
+++ b/slime/ray/rollout.py
@@ -156,7 +156,7 @@ def start_engines(self, port_cursors: dict[int, int] | None = None) -> tuple[lis
         else:
             # Compute base_port from the maximum cursor across all nodes that
             # this group's engines may land on (conservative: just use global max).
-            base_port = max(port_cursors.values()) if port_cursors else 15000
+            base_port = max(port_cursors.values()) if port_cursors else 15000 + random.randint(0, 20000)
             addr_and_ports, port_cursors = _allocate_rollout_engine_addr_and_ports_normal(
                 args=self.args,
                 rollout_engines=rollout_engines,
@@ -864,7 +864,9 @@ def port(consecutive=1):
                         consecutive=consecutive,
                     )
                 )
-                start_port = port + consecutive
+                # Advance cursor past the allocated range with a safety gap
+                # to reduce TOCTOU collisions with other processes.
+                start_port = port + consecutive + 5
                 node_port_cursor[node_idx] = start_port
                 return port
 
diff --git a/slime/utils/http_utils.py b/slime/utils/http_utils.py
index ede851f6b..f3db802a6 100644
--- a/slime/utils/http_utils.py
+++ b/slime/utils/http_utils.py
@@ -26,17 +26,21 @@ def find_available_port(base_port: int):
 
 
 def is_port_available(port):
-    """Return whether a port is available."""
-    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+    """Return whether a port is available.
+
+    Checks both IPv4 and IPv6 to avoid false positives from SO_REUSEADDR.
+    Does NOT set SO_REUSEADDR so the check reflects actual availability.
+    """
+    for family in (socket.AF_INET, socket.AF_INET6):
         try:
-            s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
-            s.bind(("", port))
-            s.listen(1)
-            return True
+            with socket.socket(family, socket.SOCK_STREAM) as s:
+                s.bind(("" if family == socket.AF_INET else "::", port))
+                s.listen(1)
         except OSError:
             return False
         except OverflowError:
             return False
+    return True
 
 
 def get_host_info():