diff --git a/tools/delegate_tool.py b/tools/delegate_tool.py
index 5e1875b5198..1be02f240e0 100644
--- a/tools/delegate_tool.py
+++ b/tools/delegate_tool.py
@@ -130,6 +130,12 @@ _SUBAGENT_TOOLSETS = sorted(
 _TOOLSET_LIST_STR = ", ".join(f"'{n}'" for n in _SUBAGENT_TOOLSETS)
 
 _DEFAULT_MAX_CONCURRENT_CHILDREN = 3
+# One-shot guard: the high-concurrency cost advisory is emitted at most once
+# per process. _get_max_concurrent_children() runs on every get_definitions()
+# schema rebuild (via _build_top_level_description / _build_tasks_param_description),
+# so without this flag a config of max_concurrent_children>10 spams the log on
+# every turn / agent spawn even when delegate_task is never called.
+_HIGH_CONCURRENCY_WARNED = False
 MAX_DEPTH = 1  # flat by default: parent (0) -> child (1); grandchild rejected unless max_spawn_depth raised.
 # Configurable depth cap consulted by _get_max_spawn_depth; MAX_DEPTH
 # stays as the default fallback and is still the symbol tests import.
@@ -374,11 +380,14 @@ def _get_max_concurrent_children() -> int:
         try:
             result = max(1, int(val))
             if result > 10:
-                logger.warning(
-                    "delegation.max_concurrent_children=%d: each child consumes API tokens "
-                    "independently. High values multiply cost linearly.",
-                    result,
-                )
+                global _HIGH_CONCURRENCY_WARNED
+                if not _HIGH_CONCURRENCY_WARNED:
+                    _HIGH_CONCURRENCY_WARNED = True
+                    logger.warning(
+                        "delegation.max_concurrent_children=%d: each child consumes API tokens "
+                        "independently. High values multiply cost linearly.",
+                        result,
+                    )
             return result
         except (TypeError, ValueError):
             logger.warning(