diff --git a/src/google/adk/flows/llm_flows/base_llm_flow.py b/src/google/adk/flows/llm_flows/base_llm_flow.py
index 424bb580e1..d00980d084 100644
--- a/src/google/adk/flows/llm_flows/base_llm_flow.py
+++ b/src/google/adk/flows/llm_flows/base_llm_flow.py
@@ -90,11 +90,26 @@ def _finalize_model_response_event(
   Args:
     llm_request: The original LLM request.
     llm_response: The LLM response from the model.
-    model_response_event: The base event to populate.
+    model_response_event: The base event to populate.  When SSE streaming is
+      active, this same object is passed for every partial *and* the final
+      response.  After the first partial event is finalized, the assigned
+      ``adk-*`` function-call IDs are written back into
+      ``model_response_event.content`` so that subsequent calls (including the
+      final non-partial event) can reuse the same IDs instead of generating
+      new ones.
 
   Returns:
     The finalized Event with LLM response data merged in.
   """
+  # Collect any function-call IDs that were already assigned during a previous
+  # partial-event finalization for this same logical LLM turn.  We extract them
+  # *before* the merge so they are not lost when llm_response overwrites content.
+  prior_fc_ids: list[str | None] = []
+  if model_response_event.content:
+    prior_fc_ids = [
+        fc.id for fc in (model_response_event.get_function_calls() or [])
+    ]
+
   finalized_event = Event.model_validate({
       **model_response_event.model_dump(exclude_none=True),
       **llm_response.model_dump(exclude_none=True),
@@ -103,7 +118,19 @@ def _finalize_model_response_event(
   if finalized_event.content:
     function_calls = finalized_event.get_function_calls()
     if function_calls:
+      # Restore previously-assigned IDs (by position) so that partial and
+      # final SSE events for the same function call share the same ID.
+      if prior_fc_ids:
+        for idx, fc in enumerate(function_calls):
+          if idx < len(prior_fc_ids) and prior_fc_ids[idx]:
+            fc.id = prior_fc_ids[idx]
+
       functions.populate_client_function_call_id(finalized_event)
+
+      # Persist the now-assigned IDs back into model_response_event so that
+      # the next call (e.g. the final non-partial event) can reuse them.
+      model_response_event.content = finalized_event.content
+
       finalized_event.long_running_tool_ids = (
           functions.get_long_running_function_calls(
               function_calls, llm_request.tools_dict
diff --git a/src/google/adk/tools/transfer_to_agent_tool.py b/src/google/adk/tools/transfer_to_agent_tool.py
index 4db933999a..9f51e7fe36 100644
--- a/src/google/adk/tools/transfer_to_agent_tool.py
+++ b/src/google/adk/tools/transfer_to_agent_tool.py
@@ -23,17 +23,15 @@
 from .tool_context import ToolContext
 
 
+# Note: For most use cases, you should use TransferToAgentTool instead of this
+# function directly. TransferToAgentTool provides additional enum constraints
+# that prevent LLMs from hallucinating invalid agent names.
 def transfer_to_agent(agent_name: str, tool_context: ToolContext) -> None:
   """Transfer the question to another agent.
 
   This tool hands off control to another agent when it's more suitable to
   answer the user's question according to the agent's description.
 
-  Note:
-    For most use cases, you should use TransferToAgentTool instead of this
-    function directly. TransferToAgentTool provides additional enum constraints
-    that prevent LLMs from hallucinating invalid agent names.
-
   Args:
     agent_name: the agent name to transfer to.
   """
diff --git a/tests/unittests/flows/llm_flows/test_base_llm_flow.py b/tests/unittests/flows/llm_flows/test_base_llm_flow.py
index 3dfadbcabf..c9a7b384a0 100644
--- a/tests/unittests/flows/llm_flows/test_base_llm_flow.py
+++ b/tests/unittests/flows/llm_flows/test_base_llm_flow.py
@@ -487,3 +487,78 @@ async def call(self, **kwargs):
     assert result1.grounding_metadata == {'foo': 'bar'}
     assert result2.grounding_metadata == {'foo': 'bar'}
     assert result3.grounding_metadata == {'foo': 'bar'}
+
+
+# ---------------------------------------------------------------------------
+# Tests for _finalize_model_response_event function-call ID consistency
+# ---------------------------------------------------------------------------
+
+from google.adk.flows.llm_flows.base_llm_flow import _finalize_model_response_event
+
+
+def _make_fc_response(fc_name: str, fc_id: str | None = None, partial: bool = False) -> LlmResponse:
+    """Helper: build an LlmResponse with a single function call."""
+    return LlmResponse(
+        content=types.Content(
+            role='model',
+            parts=[
+                types.Part(
+                    function_call=types.FunctionCall(
+                        name=fc_name,
+                        args={'x': 1},
+                        id=fc_id,
+                    )
+                )
+            ],
+        ),
+        partial=partial,
+    )
+
+
+def test_finalize_model_response_event_consistent_fc_id_across_partial_and_final():
+    """Function call IDs must be identical in partial and final SSE events.
+
+    Regression test for https://github.com/google/adk-python/issues/4609.
+    When SSE streaming is active, _finalize_model_response_event is called
+    once for the partial event and once for the final event, both sharing the
+    same model_response_event object.  The assigned adk-* ID must be the same
+    in both calls.
+    """
+    llm_request = LlmRequest()
+    llm_request.tools_dict = {}
+    base_event = Event(
+        invocation_id='inv1',
+        author='agent',
+    )
+
+    # First call: partial streaming event (function call has no ID from LLM)
+    partial_response = _make_fc_response('my_tool', fc_id=None, partial=True)
+    partial_finalized = _finalize_model_response_event(
+        llm_request, partial_response, base_event
+    )
+    partial_fc_id = partial_finalized.get_function_calls()[0].id
+    assert partial_fc_id is not None
+    assert partial_fc_id.startswith('adk-')
+
+    # Second call: final (non-partial) event for the same function call
+    final_response = _make_fc_response('my_tool', fc_id=None, partial=False)
+    final_finalized = _finalize_model_response_event(
+        llm_request, final_response, base_event
+    )
+    final_fc_id = final_finalized.get_function_calls()[0].id
+
+    assert final_fc_id == partial_fc_id, (
+        f'Function call ID changed between partial ({partial_fc_id!r}) and '
+        f'final ({final_fc_id!r}) SSE events — HITL workflows will break.'
+    )
+
+
+def test_finalize_model_response_event_preserves_llm_assigned_id():
+    """If the LLM already assigned an ID, it must be preserved as-is."""
+    llm_request = LlmRequest()
+    llm_request.tools_dict = {}
+    base_event = Event(invocation_id='inv1', author='agent')
+
+    response = _make_fc_response('my_tool', fc_id='llm-assigned-id')
+    finalized = _finalize_model_response_event(llm_request, response, base_event)
+    assert finalized.get_function_calls()[0].id == 'llm-assigned-id'