diff --git a/src/google/adk/agents/base_agent.py b/src/google/adk/agents/base_agent.py
index ed721ffa3c..916bfdc9dd 100644
--- a/src/google/adk/agents/base_agent.py
+++ b/src/google/adk/agents/base_agent.py
@@ -737,20 +737,26 @@ def validate_sub_agents_unique_names(
     seen_names: set[str] = set()
     duplicates: set[str] = set()
 
-    for sub_agent in value:
-      name = sub_agent.name
+    def visit(agent: BaseAgent) -> None:
+      name = agent.name
       if name in seen_names:
         duplicates.add(name)
       else:
         seen_names.add(name)
+      for child in agent.sub_agents:
+        visit(child)
+
+    for sub_agent in value:
+      visit(sub_agent)
 
     if duplicates:
       duplicate_names_str = ', '.join(
           f'`{name}`' for name in sorted(duplicates)
       )
       logger.warning(
-          'Found duplicate sub-agent names: %s. '
-          'All sub-agents must have unique names.',
+          'Found duplicate agent names: %s. '
+          'All agents in the tree must have unique names. '
+          'find_agent() returns the first match in a depth-first walk.',
           duplicate_names_str,
       )
 
diff --git a/tests/unittests/agents/test_base_agent.py b/tests/unittests/agents/test_base_agent.py
index 1dbe7e4bae..9995e90c7d 100644
--- a/tests/unittests/agents/test_base_agent.py
+++ b/tests/unittests/agents/test_base_agent.py
@@ -1248,7 +1248,7 @@ def test_validate_sub_agents_unique_names_single_duplicate(
         name=f'{request.function.__name__}_parent',
         sub_agents=[sub_agent_1, sub_agent_2],
     )
-  assert f'Found duplicate sub-agent names: `{duplicate_name}`' in caplog.text
+  assert f'Found duplicate agent names: `{duplicate_name}`' in caplog.text
 
 
 def test_validate_sub_agents_unique_names_multiple_duplicates(
@@ -1328,6 +1328,35 @@ def test_validate_sub_agents_unique_names_no_duplicates(
   assert parent.sub_agents[2].name == f'{request.function.__name__}_sub_agent_3'
 
 
+def test_validate_sub_agents_unique_names_across_parents(
+    request: pytest.FixtureRequest,
+    caplog: pytest.LogCaptureFixture,
+):
+  """Cousins under different parents that share a name must warn."""
+  shared = f'{request.function.__name__}_shared'
+  left = _TestingAgent(
+      name=f'{request.function.__name__}_left',
+      sub_agents=[_TestingAgent(name=shared)],
+  )
+  right = _TestingAgent(
+      name=f'{request.function.__name__}_right',
+      sub_agents=[_TestingAgent(name=shared)],
+  )
+
+  with caplog.at_level(logging.WARNING):
+    root = _TestingAgent(
+        name=f'{request.function.__name__}_root',
+        sub_agents=[left, right],
+    )
+
+  assert f'Found duplicate agent names: `{shared}`' in caplog.text
+  assert 'unique names' in caplog.text
+  # First DFS match wins; the other cousin is unreachable by name.
+  found = root.find_agent(shared)
+  assert found is not None
+  assert found is left.sub_agents[0]
+
+
 def test_validate_sub_agents_unique_names_empty_list(
     request: pytest.FixtureRequest,
 ):