diff --git a/tools/delegate_tool.py b/tools/delegate_tool.py
index 2cdce9cae64..f3a037c4341 100644
--- a/tools/delegate_tool.py
+++ b/tools/delegate_tool.py
@@ -1836,9 +1836,13 @@ def _run_single_child(
 
     finally:
         # Stop the heartbeat thread so it doesn't keep touching parent activity
-        # after the child has finished (or failed).
+        # after the child has finished (or failed).  Guard the join: .start()
+        # now lives inside the try block, so if it raised (OS thread
+        # exhaustion) the thread was never started and Thread.join() would
+        # raise RuntimeError.  ident is None until start() succeeds.
         _heartbeat_stop.set()
-        _heartbeat_thread.join(timeout=5)
+        if _heartbeat_thread.ident is not None:
+            _heartbeat_thread.join(timeout=5)
 
         # Drop the TUI-facing registry entry.  Safe to call even if the
         # child was never registered (e.g. ID missing on test doubles).