fix: stabilize function call IDs across streaming 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_id_cache: Optional[dict[str, str]] = None,
 ) -> Event:
   """Finalize and build the model response event from LLM response.
 
@@ -91,6 +92,9 @@ 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_id_cache: Optional dict mapping function call names to
+      previously generated IDs. Used to keep IDs stable across partial
+      and final streaming events.
 
   Returns:
     The finalized Event with LLM response data merged in.
@@ -103,7 +107,9 @@ def _finalize_model_response_event(
   if finalized_event.content:
     function_calls = finalized_event.get_function_calls()
     if function_calls:
-      functions.populate_client_function_call_id(finalized_event)
+      functions.populate_client_function_call_id(
+          finalized_event, function_call_id_cache
+      )
       finalized_event.long_running_tool_ids = (
           functions.get_long_running_function_calls(
               function_calls, llm_request.tools_dict
@@ -827,6 +833,9 @@ async def _run_one_step_async(
         author=invocation_context.agent.name,
         branch=invocation_context.branch,
     )
+    # Cache maps function call names to generated IDs so that partial and
+    # final streaming events for the same call share a stable ID.
+    function_call_id_cache: dict[str, str] = {}
     async with Aclosing(
         self._call_llm_async(
             invocation_context, llm_request, model_response_event
@@ -840,6 +849,7 @@ async def _run_one_step_async(
                 llm_request,
                 llm_response,
                 model_response_event,
+                function_call_id_cache,
             )
         ) as agen:
           async for event in agen:
@@ -886,6 +896,7 @@ async def _postprocess_async(
       llm_request: LlmRequest,
       llm_response: LlmResponse,
       model_response_event: Event,
+      function_call_id_cache: Optional[dict[str, str]] = None,
   ) -> AsyncGenerator[Event, None]:
     """Postprocess after calling the LLM.
 
@@ -894,6 +905,9 @@ 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_id_cache: Optional dict mapping function call names to
+        previously generated IDs. Keeps IDs stable across partial and final
+        streaming events.
 
     Yields:
       A generator of events.
@@ -917,7 +931,8 @@ 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_id_cache,
     )
     yield model_response_event
 
@@ -1197,9 +1212,11 @@ def _finalize_model_response_event(
       llm_request: LlmRequest,
       llm_response: LlmResponse,
       model_response_event: Event,
+      function_call_id_cache: Optional[dict[str, str]] = None,
   ) -> Event:
     return _finalize_model_response_event(
-        llm_request, llm_response, model_response_event
+        llm_request, llm_response, model_response_event,
+        function_call_id_cache,
     )
 
   async def _resolve_toolset_auth(

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

@@ -181,12 +181,25 @@ def generate_client_function_call_id() -> str:
   return f'{AF_FUNCTION_CALL_ID_PREFIX}{uuid.uuid4()}'
 
 
-def populate_client_function_call_id(model_response_event: Event) -> None:
+def populate_client_function_call_id(
+    model_response_event: Event,
+    function_call_id_cache: Optional[dict[str, str]] = None,
+) -> None:
   if not model_response_event.get_function_calls():
     return
-  for function_call in model_response_event.get_function_calls():
+  for idx, function_call in enumerate(
+      model_response_event.get_function_calls()
+  ):
     if not function_call.id:
-      function_call.id = generate_client_function_call_id()
+      # Use (name, index) as cache key so that two calls to the same
+      # function in a single response keep separate stable IDs.
+      cache_key = f'{function_call.name}:{idx}'
+      if function_call_id_cache is not None and cache_key in function_call_id_cache:
+        function_call.id = function_call_id_cache[cache_key]
+      else:
+        function_call.id = generate_client_function_call_id()
+        if function_call_id_cache is not None:
+          function_call_id_cache[cache_key] = function_call.id
 
 
 def remove_client_function_call_id(content: Optional[types.Content]) -> None:

tests/unittests/flows/llm_flows/test_streaming_function_call_ids.py

@@ -0,0 +1,196 @@
+# 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 function call IDs stay stable across streaming events."""
+
+from google.adk.events.event import Event
+from google.adk.flows.llm_flows.base_llm_flow import _finalize_model_response_event
+from google.adk.flows.llm_flows.functions import populate_client_function_call_id
+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_fc_response(name: str, args: dict | None = None, partial: bool = False) -> LlmResponse:
+  """Create an LlmResponse containing a single function call."""
+  fc = types.FunctionCall(name=name, args=args or {})
+  return LlmResponse(
+      content=types.Content(role='model', parts=[types.Part(function_call=fc)]),
+      partial=partial,
+  )
+
+
+def _make_multi_fc_response(calls: list[tuple[str, dict]], partial: bool = False) -> LlmResponse:
+  """Create an LlmResponse containing multiple function calls."""
+  parts = [
+      types.Part(function_call=types.FunctionCall(name=name, args=args))
+      for name, args in calls
+  ]
+  return LlmResponse(
+      content=types.Content(role='model', parts=parts),
+      partial=partial,
+  )
+
+
+class TestPopulateClientFunctionCallIdWithCache:
+  """Tests for populate_client_function_call_id with ID caching."""
+
+  def test_generates_id_and_stores_in_cache(self):
+    event = Event(author='agent')
+    event.content = types.Content(
+        role='model',
+        parts=[types.Part(function_call=types.FunctionCall(name='get_weather', args={}))],
+    )
+    cache: dict[str, str] = {}
+    populate_client_function_call_id(event, cache)
+    fc = event.get_function_calls()[0]
+    assert fc.id.startswith('adk-')
+    assert 'get_weather:0' in cache
+    assert cache['get_weather:0'] == fc.id
+
+  def test_reuses_cached_id(self):
+    cache: dict[str, str] = {'get_weather:0': 'adk-cached-id-123'}
+
+    event = Event(author='agent')
+    event.content = types.Content(
+        role='model',
+        parts=[types.Part(function_call=types.FunctionCall(name='get_weather', args={}))],
+    )
+    populate_client_function_call_id(event, cache)
+    assert event.get_function_calls()[0].id == 'adk-cached-id-123'
+
+  def test_no_cache_generates_new_id_each_time(self):
+    event1 = Event(author='agent')
+    event1.content = types.Content(
+        role='model',
+        parts=[types.Part(function_call=types.FunctionCall(name='get_weather', args={}))],
+    )
+    event2 = Event(author='agent')
+    event2.content = types.Content(
+        role='model',
+        parts=[types.Part(function_call=types.FunctionCall(name='get_weather', args={}))],
+    )
+    populate_client_function_call_id(event1)
+    populate_client_function_call_id(event2)
+    assert event1.get_function_calls()[0].id != event2.get_function_calls()[0].id
+
+  def test_multiple_calls_same_name_get_separate_ids(self):
+    event = Event(author='agent')
+    event.content = types.Content(
+        role='model',
+        parts=[
+            types.Part(function_call=types.FunctionCall(name='search', args={'q': 'a'})),
+            types.Part(function_call=types.FunctionCall(name='search', args={'q': 'b'})),
+        ],
+    )
+    cache: dict[str, str] = {}
+    populate_client_function_call_id(event, cache)
+    fcs = event.get_function_calls()
+    assert fcs[0].id != fcs[1].id
+    assert cache['search:0'] == fcs[0].id
+    assert cache['search:1'] == fcs[1].id
+
+  def test_skips_function_calls_that_already_have_ids(self):
+    event = Event(author='agent')
+    event.content = types.Content(
+        role='model',
+        parts=[types.Part(function_call=types.FunctionCall(
+            name='get_weather', args={}, id='server-provided-id'))],
+    )
+    cache: dict[str, str] = {}
+    populate_client_function_call_id(event, cache)
+    assert event.get_function_calls()[0].id == 'server-provided-id'
+    assert len(cache) == 0
+
+
+class TestFinalizeModelResponseEventWithCache:
+  """Tests that _finalize_model_response_event preserves IDs via cache."""
+
+  def test_partial_and_final_share_same_function_call_id(self):
+    model_response_event = Event(
+        author='agent',
+        invocation_id='inv-1',
+    )
+    llm_request = LlmRequest(model='mock', contents=[])
+    cache: dict[str, str] = {}
+
+    # Partial event
+    partial_response = _make_fc_response('get_weather', partial=True)
+    partial_event = _finalize_model_response_event(
+        llm_request, partial_response, model_response_event, cache,
+    )
+    partial_id = partial_event.get_function_calls()[0].id
+    assert partial_id.startswith('adk-')
+
+    # Final event — same function call must get the same ID
+    final_response = _make_fc_response('get_weather', partial=False)
+    final_event = _finalize_model_response_event(
+        llm_request, final_response, model_response_event, cache,
+    )
+    final_id = final_event.get_function_calls()[0].id
+    assert final_id == partial_id
+
+  def test_without_cache_ids_differ(self):
+    model_response_event = Event(
+        author='agent',
+        invocation_id='inv-1',
+    )
+    llm_request = LlmRequest(model='mock', contents=[])
+
+    partial_response = _make_fc_response('get_weather', partial=True)
+    partial_event = _finalize_model_response_event(
+        llm_request, partial_response, model_response_event,
+    )
+    partial_id = partial_event.get_function_calls()[0].id
+
+    final_response = _make_fc_response('get_weather', partial=False)
+    final_event = _finalize_model_response_event(
+        llm_request, final_response, model_response_event,
+    )
+    final_id = final_event.get_function_calls()[0].id
+
+    # Without cache, IDs are different (this is the bug scenario)
+    assert final_id != partial_id
+
+  def test_multi_function_call_streaming_preserves_all_ids(self):
+    model_response_event = Event(
+        author='agent',
+        invocation_id='inv-1',
+    )
+    llm_request = LlmRequest(model='mock', contents=[])
+    cache: dict[str, str] = {}
+
+    # Partial with two function calls
+    partial_response = _make_multi_fc_response(
+        [('search', {'q': 'weather'}), ('lookup', {'id': '42'})],
+        partial=True,
+    )
+    partial_event = _finalize_model_response_event(
+        llm_request, partial_response, model_response_event, cache,
+    )
+    partial_ids = [fc.id for fc in partial_event.get_function_calls()]
+
+    # Final with same two function calls
+    final_response = _make_multi_fc_response(
+        [('search', {'q': 'weather'}), ('lookup', {'id': '42'})],
+        partial=False,
+    )
+    final_event = _finalize_model_response_event(
+        llm_request, final_response, model_response_event, cache,
+    )
+    final_ids = [fc.id for fc in final_event.get_function_calls()]
+
+    assert partial_ids == final_ids
+    assert partial_ids[0] != partial_ids[1]  # different calls have different IDs