fix: Ensure user content is included in LLM requests when provided by…

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

@@ -223,12 +223,12 @@ def _get_contents(
   for event in events:
     if (
         not event.content
-        or not event.content.role
         or not event.content.parts
         or event.content.parts[0].text == ''
     ):
       # Skip events without content, or generated neither by user nor by model
       # or has empty text.
+      # Doesn't skip events with user content but without a role.
       # E.g. events purely for mutating session states.
 
       continue

src/google/adk/models/base_llm.py

@@ -97,6 +97,12 @@ def _maybe_append_user_content(self, llm_request: LlmRequest):
       )
       return
 
+    # Insert user role for the content where the user message exists
+    # but not the role
+    if (llm_request.contents[-1].parts):
+      llm_request.contents[-1].role = "user"
+      return
+
     # Insert a user content to preserve user intent and to avoid empty
     # model response.
     if llm_request.contents[-1].role != 'user':

tests/unittests/models/test_base_llm.py

@@ -0,0 +1,48 @@
+# 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.
+
+import pytest
+from google.genai import types
+from google.adk.models.llm_request import LlmRequest
+from google.adk.models.lite_llm import _get_completion_inputs
+
+
+@pytest.mark.parametrize("content_kwargs", [
+    # Case 1: Explicit role provided
+    {"role": "user", "parts": [types.Part(text="This is an input text from user.")]},
+    # Case 2: Role omitted, should still be treated as 'user'
+    {"parts": [types.Part(text="This is an input text from user.")]}
+])
+def test_user_content_role_defaults_to_user(content_kwargs):
+    """
+    Verifies that user-provided messages are always passed to the LLM as 'user' role,
+    regardless of whether the role is explicitly set in types.Content.
+
+    The helper `_get_completion_inputs` should give normalize messages so that
+    explicit 'user' and implicit (missing role) are equivalent.
+    """
+    llm_request = LlmRequest(
+        contents=[types.Content(**content_kwargs)],
+        config=types.GenerateContentConfig()
+    )
+
+    messages, _, _, _ = _get_completion_inputs(llm_request)
+
+    assert all(
+        msg.get("role") == "user" for msg in messages
+    ), f"Expected role 'user' but got {messages}"
+    assert any(
+        "This is an input text from user." == (msg.get("content") or "") 
+        for msg in messages
+    ), f"Expected the user text to be preserved, but got {messages}"