diff --git a/src/google/adk/cli/fast_api.py b/src/google/adk/cli/fast_api.py
index 199791f7da..7b18708064 100644
--- a/src/google/adk/cli/fast_api.py
+++ b/src/google/adk/cli/fast_api.py
@@ -307,8 +307,15 @@ def _has_parent_reference(path: str) -> bool:
     # Block any upload that contains an `args` key anywhere in the document.
     _BLOCKED_YAML_KEYS = frozenset({"args"})
 
+    # Fields that accept fully-qualified Python names and feed into
+    # importlib.import_module() at agent load time.  Dotted values in
+    # these fields cause module-level code execution from any installed
+    # package.  Builder uploads must use simple (non-dotted) names that
+    # resolve to ADK built-in types only.
+    _IMPORT_REFERENCE_KEYS = frozenset({"name", "agent_class", "code"})
+
     def _check_yaml_for_blocked_keys(content: bytes, filename: str) -> None:
-      """Raise if the YAML document contains any blocked keys."""
+      """Raise if the YAML document contains blocked keys or code refs."""
       import yaml
 
       try:
@@ -325,6 +332,17 @@ def _walk(node: Any) -> None:
                   f"The '{key}' field is not allowed in builder uploads "
                   "because it can execute arbitrary code."
               )
+            if (
+                key in _IMPORT_REFERENCE_KEYS
+                and isinstance(value, str)
+                and "." in value
+            ):
+              raise ValueError(
+                  f"Fully qualified Python reference in '{key}':"
+                  f" {value!r} in {filename!r} is not allowed in"
+                  " builder uploads. Only simple (non-dotted) names"
+                  " that resolve to ADK built-in types are permitted."
+              )
             _walk(value)
         elif isinstance(node, list):
           for item in node:
diff --git a/tests/unittests/cli/test_fast_api.py b/tests/unittests/cli/test_fast_api.py
index 3e63f31222..53f3377f22 100755
--- a/tests/unittests/cli/test_fast_api.py
+++ b/tests/unittests/cli/test_fast_api.py
@@ -2113,6 +2113,93 @@ def test_builder_save_rejects_nested_args_key(builder_test_client, tmp_path):
   assert "args" in response.json()["detail"]
 
 
+def test_builder_save_rejects_dotted_tool_name(builder_test_client, tmp_path):
+  """Uploading YAML with a dotted tool name is rejected (import prevention)."""
+  yaml_with_dotted_tool = b"""\
+name: my_agent
+tools:
+  - name: os.system
+"""
+  response = builder_test_client.post(
+      "/builder/save?tmp=true",
+      files=[(
+          "files",
+          ("app/root_agent.yaml", yaml_with_dotted_tool, "application/x-yaml"),
+      )],
+  )
+  assert response.status_code == 400
+  assert "os.system" in response.json()["detail"]
+  assert not (tmp_path / "app" / "tmp" / "app" / "root_agent.yaml").exists()
+
+
+def test_builder_save_rejects_dotted_agent_class(builder_test_client, tmp_path):
+  """Uploading YAML with a dotted agent_class is rejected."""
+  yaml_with_dotted_class = b"""\
+agent_class: evil.module.MyAgent
+name: my_agent
+"""
+  response = builder_test_client.post(
+      "/builder/save?tmp=true",
+      files=[(
+          "files",
+          ("app/root_agent.yaml", yaml_with_dotted_class, "application/x-yaml"),
+      )],
+  )
+  assert response.status_code == 400
+  assert "evil.module.MyAgent" in response.json()["detail"]
+
+
+def test_builder_save_rejects_dotted_code_ref(builder_test_client, tmp_path):
+  """Uploading YAML with a dotted code reference is rejected."""
+  yaml_with_dotted_code = b"""\
+name: my_agent
+sub_agents:
+  - code: evil.module.my_agent
+"""
+  response = builder_test_client.post(
+      "/builder/save?tmp=true",
+      files=[(
+          "files",
+          ("app/root_agent.yaml", yaml_with_dotted_code, "application/x-yaml"),
+      )],
+  )
+  assert response.status_code == 400
+  assert "evil.module.my_agent" in response.json()["detail"]
+
+
+def test_builder_save_allows_simple_tool_name(builder_test_client, tmp_path):
+  """Uploading YAML with a simple (non-dotted) tool name is allowed."""
+  yaml_with_simple_tool = b"""\
+name: my_agent
+tools:
+  - name: google_search
+"""
+  response = builder_test_client.post(
+      "/builder/save?tmp=true",
+      files=[(
+          "files",
+          ("app/root_agent.yaml", yaml_with_simple_tool, "application/x-yaml"),
+      )],
+  )
+  assert response.status_code == 200
+
+
+def test_builder_save_allows_simple_agent_class(builder_test_client, tmp_path):
+  """Uploading YAML with a simple agent_class (e.g. LlmAgent) is allowed."""
+  yaml_with_simple_class = b"""\
+agent_class: LlmAgent
+name: my_agent
+"""
+  response = builder_test_client.post(
+      "/builder/save?tmp=true",
+      files=[(
+          "files",
+          ("app/root_agent.yaml", yaml_with_simple_class, "application/x-yaml"),
+      )],
+  )
+  assert response.status_code == 200
+
+
 def test_builder_get_rejects_non_yaml_file_paths(builder_test_client, tmp_path):
   """GET /builder/app/{app_name}?file_path=... rejects non-YAML extensions."""
   app_root = tmp_path / "app"