diff --git a/src/google/adk/models/google_llm.py b/src/google/adk/models/google_llm.py
index c7b10aa61a..50c820c143 100644
--- a/src/google/adk/models/google_llm.py
+++ b/src/google/adk/models/google_llm.py
@@ -122,9 +122,9 @@ async def generate_content_async(
     if llm_request.config:
       if not llm_request.config.http_options:
         llm_request.config.http_options = types.HttpOptions()
-      if not llm_request.config.http_options.headers:
-        llm_request.config.http_options.headers = {}
-      llm_request.config.http_options.headers.update(self._tracking_headers)
+      llm_request.config.http_options.headers = self._merge_tracking_headers(
+          llm_request.config.http_options.headers
+      )
 
     if stream:
       responses = await self.api_client.aio.models.generate_content_stream(
@@ -336,6 +336,23 @@ async def _preprocess_request(self, llm_request: LlmRequest) -> None:
           llm_request.config.system_instruction = None
           await self._adapt_computer_use_tool(llm_request)
 
+  def _merge_tracking_headers(self, headers: dict[str, str]) -> dict[str, str]:
+    """Merge tracking headers to the given headers."""
+    headers = headers or {}
+    for key, tracking_header_value in self._tracking_headers.items():
+      custom_value = headers.get(key, None)
+      if not custom_value:
+        headers[key] = tracking_header_value
+        continue
+
+      # Merge tracking headers with existing headers and avoid duplicates.
+      value_parts = tracking_header_value.split(' ')
+      for custom_value_part in custom_value.split(' '):
+        if custom_value_part not in value_parts:
+          value_parts.append(custom_value_part)
+      headers[key] = ' '.join(value_parts)
+    return headers
+
 
 def _build_function_declaration_log(
     func_decl: types.FunctionDeclaration,
diff --git a/tests/unittests/models/test_google_llm.py b/tests/unittests/models/test_google_llm.py
index 4e99c5a567..03d18ec6d7 100644
--- a/tests/unittests/models/test_google_llm.py
+++ b/tests/unittests/models/test_google_llm.py
@@ -403,7 +403,7 @@ async def mock_coro():
 
     for key, value in config_arg.http_options.headers.items():
       if key in gemini_llm._tracking_headers:
-        assert value == gemini_llm._tracking_headers[key]
+        assert value == gemini_llm._tracking_headers[key] + " custom"
       else:
         assert value == custom_headers[key]