fix: Only append skills to system instruction if ListSkillsTool isn't available

This reduces the token usage of having skills appended to the instruction on every call

Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 910904229

Author: wukath

src/google/adk/tools/skill_toolset.py

@@ -900,11 +900,15 @@ async def process_llm_request(
       self, *, tool_context: ToolContext, llm_request: LlmRequest
   ) -> None:
     """Processes the outgoing LLM request to include available skills."""
-    skills = self._list_skills()
-    skills_xml = prompt.format_skills_as_xml(skills)
-    instructions = []
-    instructions.append(_DEFAULT_SKILL_SYSTEM_INSTRUCTION)
-    instructions.append(skills_xml)
+    instructions = [_DEFAULT_SKILL_SYSTEM_INSTRUCTION]
+
+    has_list_skills = any(isinstance(t, ListSkillsTool) for t in self._tools)
+
+    if not has_list_skills:
+      skills = self._list_skills()
+      skills_xml = prompt.format_skills_as_xml(skills)
+      instructions.append(skills_xml)
+
     llm_request.append_instructions(instructions)
 
 

tests/unittests/tools/test_skill_toolset.py

@@ -427,7 +427,7 @@ async def test_load_resource_process_llm_request_binary(
 
 
 @pytest.mark.asyncio
-async def test_process_llm_request(
+async def test_process_llm_request_with_list_skills_tool(
     mock_skill1, mock_skill2, tool_context_instance
 ):
   toolset = skill_toolset.SkillToolset([mock_skill1, mock_skill2])
@@ -437,6 +437,29 @@ async def test_process_llm_request(
       tool_context=tool_context_instance, llm_request=llm_req
   )
 
+  llm_req.append_instructions.assert_called_once_with(
+      [skill_toolset.DEFAULT_SKILL_SYSTEM_INSTRUCTION]
+  )
+
+
+@pytest.mark.asyncio
+async def test_process_llm_request_without_list_skills_tool(
+    mock_skill1, mock_skill2, tool_context_instance
+):
+  toolset = skill_toolset.SkillToolset([mock_skill1, mock_skill2])
+  # Manually remove ListSkillsTool from self._tools to simulate it not being available
+  toolset._tools = [
+      t
+      for t in toolset._tools
+      if not isinstance(t, skill_toolset.ListSkillsTool)
+  ]
+
+  llm_req = mock.create_autospec(llm_request_model.LlmRequest, instance=True)
+
+  await toolset.process_llm_request(
+      tool_context=tool_context_instance, llm_request=llm_req
+  )
+
   llm_req.append_instructions.assert_called_once()
   args, _ = llm_req.append_instructions.call_args
   instructions = args[0]