diff --git a/src/google/adk/flows/llm_flows/contents.py b/src/google/adk/flows/llm_flows/contents.py
index be471a44eb..0d45950196 100644
--- a/src/google/adk/flows/llm_flows/contents.py
+++ b/src/google/adk/flows/llm_flows/contents.py
@@ -686,6 +686,29 @@ async def _add_instructions_to_user_content(
     llm_request: The LLM request to modify
     instruction_contents: List of instruction-related contents to insert
   """
+
+  def is_valid_instruction_position(
+      llm_request: LlmRequest, index: int
+  ) -> bool:
+    """Checks if instructions can be inserted after a given index.
+
+    A valid insertion point is after a model response that is not a tool call.
+    This prevents injecting instructions in the middle of a user's turn or a
+    tool-use sequence.
+
+    Args:
+      llm_request: The LLM request containing the conversation contents.
+      index: The index of the content to check.
+
+    Returns:
+      True if the position after this index is a valid insertion point.
+    """
+    content_at_index = llm_request.contents[index]
+    is_user_message = content_at_index.role == 'user'
+    is_tool_request = any(part.function_call for part in content_at_index.parts)
+
+    return not is_user_message and not is_tool_request
+
   if not instruction_contents:
     return
 
@@ -695,7 +718,7 @@ async def _add_instructions_to_user_content(
 
   if llm_request.contents:
     for i in range(len(llm_request.contents) - 1, -1, -1):
-      if llm_request.contents[i].role != 'user':
+      if is_valid_instruction_position(llm_request, i):
         insert_index = i + 1
         break
       elif i == 0:
diff --git a/tests/unittests/flows/llm_flows/test_contents.py b/tests/unittests/flows/llm_flows/test_contents.py
index 9e77407b27..549c09440d 100644
--- a/tests/unittests/flows/llm_flows/test_contents.py
+++ b/tests/unittests/flows/llm_flows/test_contents.py
@@ -427,3 +427,89 @@ async def test_events_with_empty_content_are_skipped():
       types.UserContent("Hello"),
       types.UserContent("How are you?"),
   ]
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "initial_contents, expected_insertion_index",
+    [
+        (
+            [
+                types.UserContent("First user message"),
+                types.ModelContent("Model response"),
+                types.ModelContent([
+                    types.Part(
+                        function_call=types.FunctionCall(
+                            name="test_tool", args={}
+                        )
+                    )
+                ]),
+                types.Content(
+                    parts=[
+                        types.Part(
+                            function_response=types.FunctionResponse(
+                                name="test_tool", response={}
+                            )
+                        )
+                    ],
+                    role="user",
+                ),
+                types.UserContent("Final user message"),
+            ],
+            2,
+        ),
+        (
+            [
+                types.UserContent("First user message"),
+                types.UserContent("Second user message"),
+                types.ModelContent("Model response"),
+                types.UserContent("Third user message"),
+                types.UserContent("Fourth user message"),
+            ],
+            3,
+        ),
+        (
+            [
+                types.UserContent("First user message"),
+                types.UserContent("Second user message"),
+            ],
+            0,
+        ),
+        ([], 0),
+        (
+            [
+                types.UserContent("User message"),
+                types.ModelContent("Model response"),
+            ],
+            2,
+        ),
+    ],
+    ids=[
+        "skips_function_call_and_user_content",
+        "skips_trailing_user_content",
+        "inserts_at_start_when_all_user_content",
+        "inserts_at_start_for_empty_content",
+        "inserts_at_end_when_last_is_model_content",
+    ],
+)
+async def test_add_instructions_to_user_content(
+    initial_contents, expected_insertion_index
+):
+  """Tests that instructions are correctly inserted into the content list."""
+  agent = Agent(model="gemini-2.5-flash", name="test_agent")
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent
+  )
+  instruction_contents = [
+      types.Content(parts=[types.Part(text="System instruction")], role="user")
+  ]
+  llm_request = LlmRequest(model="gemini-2.5-flash", contents=initial_contents)
+
+  await contents._add_instructions_to_user_content(
+      invocation_context, llm_request, instruction_contents
+  )
+
+  assert len(llm_request.contents) == len(initial_contents) + 1
+  assert (
+      llm_request.contents[expected_insertion_index] == instruction_contents[0]
+  )