fix(planner): Fixes the thought field handling in _planning.py

Before this change: `thought` flags was incorrectly removed if the current agent enables BuiltInPlanner.

After this change:

- When it's BuiltInPlanner, keep the thought flag in content history, so that model has full context of its previous thinking.
- When it's PlanReactPlanner, removes the `thought` flag in content history, so that model sees as-is when the content was generated.

PiperOrigin-RevId: 802737130

src/google/adk/flows/llm_flows/_nl_planning.py

@@ -17,7 +17,6 @@
 from __future__ import annotations
 
 from typing import AsyncGenerator
-from typing import Generator
 from typing import Optional
 from typing import TYPE_CHECKING
 
@@ -35,7 +34,6 @@ if TYPE_CHECKING:
   from ...models.llm_request import LlmRequest
   from ...models.llm_response import LlmResponse
   from ...planners.base_planner import BasePlanner
-  from ...planners.built_in_planner import BuiltInPlanner
 
 
 class _NlPlanningRequestProcessor(BaseLlmRequestProcessor):
@@ -52,14 +50,13 @@ class _NlPlanningRequestProcessor(BaseLlmRequestProcessor):
 
     if isinstance(planner, BuiltInPlanner):
       planner.apply_thinking_config(llm_request)
+    elif isinstance(planner, PlanReActPlanner):
+      if planning_instruction := planner.build_planning_instruction(
+          ReadonlyContext(invocation_context), llm_request
+      ):
+        llm_request.append_instructions([planning_instruction])
 
-    planning_instruction = planner.build_planning_instruction(
-        ReadonlyContext(invocation_context), llm_request
-    )
-    if planning_instruction:
-      llm_request.append_instructions([planning_instruction])
-
-    _remove_thought_from_request(llm_request)
+      _remove_thought_from_request(llm_request)
 
     # Maintain async generator behavior
     if False:  # Ensures it behaves as a generator
@@ -75,6 +72,8 @@ class _NlPlanningResponse(BaseLlmResponseProcessor):
   async def run_async(
       self, invocation_context: InvocationContext, llm_response: LlmResponse
   ) -> AsyncGenerator[Event, None]:
+    from ...planners.built_in_planner import BuiltInPlanner
+
     if (
         not llm_response
         or not llm_response.content
@@ -83,7 +82,7 @@ class _NlPlanningResponse(BaseLlmResponseProcessor):
       return
 
     planner = _get_planner(invocation_context)
-    if not planner:
+    if not planner or isinstance(planner, BuiltInPlanner):
       return
 
     # Postprocess the LLM response.

tests/unittests/flows/llm_flows/test_nl_planning.py

@@ -0,0 +1,128 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for NL planning logic."""
+
+from unittest.mock import MagicMock
+
+from google.adk.agents.llm_agent import Agent
+from google.adk.flows.llm_flows._nl_planning import request_processor
+from google.adk.models.llm_request import LlmRequest
+from google.adk.planners.built_in_planner import BuiltInPlanner
+from google.adk.planners.plan_re_act_planner import PlanReActPlanner
+from google.genai import types
+import pytest
+
+from ... import testing_utils
+
+
+@pytest.mark.asyncio
+async def test_built_in_planner_content_list_unchanged():
+  """Test that BuiltInPlanner doesn't modify LlmRequest content list."""
+  planner = BuiltInPlanner(thinking_config=types.ThinkingConfig())
+  agent = Agent(name='test_agent', planner=planner)
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent, user_content='test message'
+  )
+  # Create user/model/user conversation with thought in model response
+  llm_request = LlmRequest(
+      contents=[
+          types.UserContent(parts=[types.Part(text='Hello')]),
+          types.ModelContent(
+              parts=[
+                  types.Part(text='thinking...', thought=True),
+                  types.Part(text='Here is my response'),
+              ]
+          ),
+          types.UserContent(parts=[types.Part(text='Follow up')]),
+      ]
+  )
+  original_contents = llm_request.contents.copy()
+
+  async for _ in request_processor.run_async(invocation_context, llm_request):
+    pass
+
+  assert llm_request.contents == original_contents
+
+
+@pytest.mark.asyncio
+async def test_built_in_planner_apply_thinking_config_called():
+  """Test that BuiltInPlanner.apply_thinking_config is called."""
+  planner = BuiltInPlanner(thinking_config=types.ThinkingConfig())
+  planner.apply_thinking_config = MagicMock()
+  agent = Agent(name='test_agent', planner=planner)
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent, user_content='test message'
+  )
+  llm_request = LlmRequest()
+
+  async for _ in request_processor.run_async(invocation_context, llm_request):
+    pass
+
+  planner.apply_thinking_config.assert_called_once_with(llm_request)
+
+
+@pytest.mark.asyncio
+async def test_plan_react_planner_instruction_appended():
+  """Test that PlanReActPlanner appends planning instruction."""
+  planner = PlanReActPlanner()
+  planner.build_planning_instruction = MagicMock(
+      return_value='Test instruction'
+  )
+  agent = Agent(name='test_agent', planner=planner)
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent, user_content='test message'
+  )
+
+  llm_request = LlmRequest()
+  llm_request.config.system_instruction = 'Original instruction'
+
+  async for _ in request_processor.run_async(invocation_context, llm_request):
+    pass
+
+  assert llm_request.config.system_instruction == ("""\
+Original instruction
+
+Test instruction""")
+
+
+@pytest.mark.asyncio
+async def test_remove_thought_from_request_with_thoughts():
+  """Test that PlanReActPlanner removes thought flags from content parts."""
+  planner = PlanReActPlanner()
+  agent = Agent(name='test_agent', planner=planner)
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent, user_content='test message'
+  )
+  llm_request = LlmRequest(
+      contents=[
+          types.UserContent(parts=[types.Part(text='initial query')]),
+          types.ModelContent(
+              parts=[
+                  types.Part(text='Text with thought', thought=True),
+                  types.Part(text='Regular text'),
+              ]
+          ),
+          types.UserContent(parts=[types.Part(text='follow up')]),
+      ]
+  )
+
+  async for _ in request_processor.run_async(invocation_context, llm_request):
+    pass
+
+  assert all(
+      part.thought is None
+      for content in llm_request.contents
+      for part in content.parts or []
+  )