fix: keep existing header values while merging tracking headers for llm_request.config.http_options in Gemini.generate_content_async

PiperOrigin-RevId: 788135222

Author: xuanyang15

src/google/adk/models/google_llm.py

@@ -116,12 +116,15 @@ async def generate_content_async(
     )
     logger.debug(_build_request_log(llm_request))
 
-    # add tracking headers to custom headers given it will override the headers
-    # set in the api client constructor
-    if llm_request.config and llm_request.config.http_options:
-      if not llm_request.config.http_options.headers:
-        llm_request.config.http_options.headers = {}
-      llm_request.config.http_options.headers.update(self._tracking_headers)
+    # Always add tracking headers to custom headers given it will override
+    # the headers set in the api client constructor to avoid tracking headers
+    # being dropped if user provides custom headers or overrides the api client.
+    if llm_request.config:
+      if not llm_request.config.http_options:
+        llm_request.config.http_options = types.HttpOptions()
+      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(
@@ -333,6 +336,21 @@ 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():
+      current_value = headers.get(key, None)
+      if not current_value:
+        headers[key] = tracking_header_value
+      else:
+        # Merge tracking headers with existing headers and avoid duplicates.
+        value_parts = set(current_value.split(' ')) | set(
+            tracking_header_value.split(' ')
+        )
+        headers[key] = ' '.join(sorted(value_parts))
+    return headers
+
 
 def _build_function_declaration_log(
     func_decl: types.FunctionDeclaration,