diff --git a/src/google/adk/models/lite_llm.py b/src/google/adk/models/lite_llm.py
index 79182d7b0a..9d7ddc74b7 100644
--- a/src/google/adk/models/lite_llm.py
+++ b/src/google/adk/models/lite_llm.py
@@ -98,6 +98,9 @@
 # 1. FinishReason.TOOL_CALL enum does not exist (as of google-genai 0.8.0)
 # 2. Tool calls represent normal completion (model stopped to invoke tools)
 # 3. Gemini native responses use STOP for tool calls (see lite_llm.py:910)
+# 4. Mapping tool-related finish reasons to STOP preserves backward
+#    compatibility with existing ADK consumers that assume STOP
+#    indicates a successful, non-error completion.
 _FINISH_REASON_MAPPING = {
     "length": types.FinishReason.MAX_TOKENS,
     "stop": types.FinishReason.STOP,
@@ -490,7 +493,7 @@ def _append_fallback_user_content_if_missing(
         content.parts = []
       content.parts.append(
           types.Part.from_text(
-              text="Handle the requests as specified in the System Instruction."
+              text="Handle the incoming request according to the provided requirements."
           )
       )
       return
@@ -500,8 +503,7 @@ def _append_fallback_user_content_if_missing(
           parts=[
               types.Part.from_text(
                   text=(
-                      "Handle the requests as specified in the System"
-                      " Instruction."
+                      "Handle the incoming request according to the provided requirements."
                   )
               ),
           ],
diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index 2ebbc5dfe8..598cb0356b 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -979,14 +979,14 @@ async def test_generate_content_async_adds_fallback_user_message(
   ]
   assert any(
       message.get("content")
-      == "Handle the requests as specified in the System Instruction."
+      == "Handle the incoming request according to the provided requirements."
       for message in user_messages
   )
   assert (
       sum(1 for content in llm_request.contents if content.role == "user") == 1
   )
   assert llm_request.contents[-1].parts[0].text == (
-      "Handle the requests as specified in the System Instruction."
+      "Handle the incoming request according to the provided requirements."
   )