fix: preserve function call IDs across SSE streaming partial/final events

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

@@ -81,6 +81,7 @@ def _finalize_model_response_event(
     llm_request: LlmRequest,
     llm_response: LlmResponse,
     model_response_event: Event,
+    function_call_ids: Optional[dict[tuple[str, int], str]] = None,
 ) -> Event:
   """Finalize and build the model response event from LLM response.
 
@@ -91,6 +92,11 @@ def _finalize_model_response_event(
     llm_request: The original LLM request.
     llm_response: The LLM response from the model.
     model_response_event: The base event to populate.
+    function_call_ids: Optional mutable dict mapping (function_name, index) to
+      previously assigned client function call IDs. Used during SSE streaming
+      to ensure partial and final events for the same function call share the
+      same ID. When provided, newly generated IDs are stored back into this
+      dict for reuse by subsequent events in the same streaming sequence.
 
   Returns:
     The finalized Event with LLM response data merged in.
@@ -103,7 +109,23 @@ def _finalize_model_response_event(
   if finalized_event.content:
     function_calls = finalized_event.get_function_calls()
     if function_calls:
+      # Restore previously assigned IDs before populating new ones so that
+      # partial and final events in an SSE stream share the same IDs.
+      if function_call_ids is not None:
+        for i, fc in enumerate(function_calls):
+          key = (fc.name, i)
+          if key in function_call_ids:
+            fc.id = function_call_ids[key]
+
       functions.populate_client_function_call_id(finalized_event)
+
+      # Persist any newly generated IDs for subsequent events.
+      if function_call_ids is not None:
+        for i, fc in enumerate(function_calls):
+          key = (fc.name, i)
+          if fc.id:
+            function_call_ids.setdefault(key, fc.id)
+
       finalized_event.long_running_tool_ids = (
           functions.get_long_running_function_calls(
               function_calls, llm_request.tools_dict
@@ -821,6 +843,9 @@ async def _run_one_step_async(
         author=invocation_context.agent.name,
         branch=invocation_context.branch,
     )
+    # Track function call IDs across partial/final events in SSE streaming
+    # so that the same function call keeps the same client-generated ID.
+    function_call_ids: dict[tuple[str, int], str] = {}
     async with Aclosing(
         self._call_llm_async(
             invocation_context, llm_request, model_response_event
@@ -834,6 +859,7 @@ async def _run_one_step_async(
                 llm_request,
                 llm_response,
                 model_response_event,
+                function_call_ids,
             )
         ) as agen:
           async for event in agen:
@@ -880,6 +906,7 @@ async def _postprocess_async(
       llm_request: LlmRequest,
       llm_response: LlmResponse,
       model_response_event: Event,
+      function_call_ids: Optional[dict[tuple[str, int], str]] = None,
   ) -> AsyncGenerator[Event, None]:
     """Postprocess after calling the LLM.
 
@@ -888,6 +915,8 @@ async def _postprocess_async(
       llm_request: The original LLM request.
       llm_response: The LLM response from the LLM call.
       model_response_event: A mutable event for the LLM response.
+      function_call_ids: Optional mutable dict for preserving function call IDs
+        across partial and final events in an SSE streaming sequence.
 
     Yields:
       A generator of events.
@@ -911,7 +940,7 @@ async def _postprocess_async(
 
     # Builds the event.
     model_response_event = self._finalize_model_response_event(
-        llm_request, llm_response, model_response_event
+        llm_request, llm_response, model_response_event, function_call_ids
     )
     yield model_response_event
 
@@ -1191,9 +1220,10 @@ def _finalize_model_response_event(
       llm_request: LlmRequest,
       llm_response: LlmResponse,
       model_response_event: Event,
+      function_call_ids: Optional[dict[tuple[str, int], str]] = None,
   ) -> Event:
     return _finalize_model_response_event(
-        llm_request, llm_response, model_response_event
+        llm_request, llm_response, model_response_event, function_call_ids
     )
 
   async def _resolve_toolset_auth(

tests/unittests/flows/llm_flows/test_streaming_function_call_id.py

@@ -0,0 +1,186 @@
+# Copyright 2026 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.
+
+"""Tests that SSE streaming preserves function call IDs across partial/final events.
+
+Regression test for https://github.com/google/adk-python/issues/4609
+"""
+
+from google.adk.events.event import Event
+from google.adk.flows.llm_flows.base_llm_flow import _finalize_model_response_event
+from google.adk.models.llm_request import LlmRequest
+from google.adk.models.llm_response import LlmResponse
+from google.genai import types
+import pytest
+
+
+def _make_base_event() -> Event:
+    return Event(
+        id=Event.new_id(),
+        invocation_id="test-inv",
+        author="test-agent",
+    )
+
+
+def _make_llm_response(*, partial: bool, fc_name: str = "get_weather") -> LlmResponse:
+    return LlmResponse(
+        content=types.Content(
+            role="model",
+            parts=[
+                types.Part(
+                    function_call=types.FunctionCall(
+                        name=fc_name,
+                        args={"location": "NYC"},
+                    )
+                )
+            ],
+        ),
+        partial=partial,
+    )
+
+
+def _make_llm_request() -> LlmRequest:
+    req = LlmRequest()
+    req.tools_dict = {}
+    return req
+
+
+class TestStreamingFunctionCallIdConsistency:
+    """Ensure partial and final events share the same function call ID."""
+
+    def test_partial_and_final_share_same_id(self):
+        """The core regression: partial event ID must equal final event ID."""
+        llm_request = _make_llm_request()
+        function_call_ids: dict[tuple[str, int], str] = {}
+
+        # Simulate partial event
+        partial_event = _finalize_model_response_event(
+            llm_request,
+            _make_llm_response(partial=True),
+            _make_base_event(),
+            function_call_ids,
+        )
+        partial_fc_id = partial_event.get_function_calls()[0].id
+        assert partial_fc_id is not None
+        assert partial_fc_id.startswith("adk-")
+
+        # Simulate final event (new Event object, same streaming sequence)
+        final_event = _finalize_model_response_event(
+            llm_request,
+            _make_llm_response(partial=False),
+            _make_base_event(),
+            function_call_ids,
+        )
+        final_fc_id = final_event.get_function_calls()[0].id
+
+        assert final_fc_id == partial_fc_id
+
+    def test_without_function_call_ids_dict_generates_different_ids(self):
+        """Without the fix dict, each event gets a fresh ID (old behaviour)."""
+        llm_request = _make_llm_request()
+
+        partial_event = _finalize_model_response_event(
+            llm_request,
+            _make_llm_response(partial=True),
+            _make_base_event(),
+        )
+        final_event = _finalize_model_response_event(
+            llm_request,
+            _make_llm_response(partial=False),
+            _make_base_event(),
+        )
+
+        # Without the dict, IDs differ (demonstrating the old bug)
+        assert (
+            partial_event.get_function_calls()[0].id
+            != final_event.get_function_calls()[0].id
+        )
+
+    def test_multiple_function_calls_preserve_ids(self):
+        """Each function call in a multi-call response keeps its own stable ID."""
+        llm_request = _make_llm_request()
+        function_call_ids: dict[tuple[str, int], str] = {}
+
+        def make_multi_fc_response(partial: bool) -> LlmResponse:
+            return LlmResponse(
+                content=types.Content(
+                    role="model",
+                    parts=[
+                        types.Part(
+                            function_call=types.FunctionCall(
+                                name="get_weather",
+                                args={"location": "NYC"},
+                            )
+                        ),
+                        types.Part(
+                            function_call=types.FunctionCall(
+                                name="get_time",
+                                args={"timezone": "EST"},
+                            )
+                        ),
+                    ],
+                ),
+                partial=partial,
+            )
+
+        partial_event = _finalize_model_response_event(
+            llm_request,
+            make_multi_fc_response(partial=True),
+            _make_base_event(),
+            function_call_ids,
+        )
+        partial_ids = [fc.id for fc in partial_event.get_function_calls()]
+
+        final_event = _finalize_model_response_event(
+            llm_request,
+            make_multi_fc_response(partial=False),
+            _make_base_event(),
+            function_call_ids,
+        )
+        final_ids = [fc.id for fc in final_event.get_function_calls()]
+
+        assert partial_ids == final_ids
+        # The two function calls should have different IDs from each other
+        assert partial_ids[0] != partial_ids[1]
+
+    def test_server_provided_id_is_preserved(self):
+        """If the server already provides an ID, it should not be overwritten."""
+        llm_request = _make_llm_request()
+        function_call_ids: dict[tuple[str, int], str] = {}
+
+        server_id = "server-provided-id-123"
+        response = LlmResponse(
+            content=types.Content(
+                role="model",
+                parts=[
+                    types.Part(
+                        function_call=types.FunctionCall(
+                            id=server_id,
+                            name="get_weather",
+                            args={"location": "NYC"},
+                        )
+                    )
+                ],
+            ),
+            partial=False,
+        )
+
+        event = _finalize_model_response_event(
+            llm_request,
+            response,
+            _make_base_event(),
+            function_call_ids,
+        )
+
+        assert event.get_function_calls()[0].id == server_id