fix: prevent duplicate synthetic user event on single-turn agent resumption

When resuming a single-turn agent node from an HITL tool confirmation pause,
prepare_llm_agent_input was unconditionally appending a synthetic user event,
shadowing the user's FunctionResponse and causing an infinite confirmation loop.
This change skips synthetic user input injection when resume_inputs are present.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 969178203

src/google/adk/workflow/_llm_agent_wrapper.py

@@ -291,7 +291,9 @@ def prepare_llm_agent_input(
   """Prepares the input for running LlmAgent as a node.
 
   For ``single_turn`` mode, append a user-role event with the input
-  directly to session.events (legacy behavior).
+  directly to session.events (legacy behavior). When resuming with
+  ``resume_inputs``, skip appending to avoid injecting duplicate synthetic
+  user events that shadow user function responses.
 
   For ``task`` mode, the input is the parent's task-delegation FC
   args.  Those are NOT appended here — the content-builder
@@ -305,7 +307,17 @@ def prepare_llm_agent_input(
   For workflow nodes running in a sub-branch, stamp the input event with that
   branch. A private node input should not look like the shared root user turn.
   """
-  if node_input is None or agent.mode != 'single_turn':
+  # Skip injection if:
+  # 1. No input was provided.
+  # 2. Agent is not single_turn (task mode handles its own leading turn).
+  # 3. Resuming from pause/interrupt (e.g. HITL confirmation): node is re-run
+  #    with resume_inputs; injecting synthetic user input would shadow the
+  #    user's FunctionResponse on the branch tail and cause infinite loops.
+  if (
+      node_input is None
+      or agent.mode != 'single_turn'
+      or bool(ctx.resume_inputs)
+  ):
     return
   agent_input = to_user_content(node_input)
   user_event = Event(author='user', message=agent_input)

tests/unittests/workflow/test_llm_agent_as_node.py

@@ -239,6 +239,39 @@ async def test_single_turn_input_event_inherits_branch_and_scope(
   assert event.isolation_scope == 'scope-1'
 
 
+@pytest.mark.asyncio
+async def test_single_turn_input_skipped_when_resuming(
+    request: pytest.FixtureRequest,
+):
+  """Resuming a single-turn agent node skips duplicate user input and retains initial input."""
+  from google.adk.workflow._llm_agent_wrapper import prepare_llm_agent_input
+
+  agent = _make_agent(mode='single_turn')
+  ic = await create_parent_invocation_context(request.function.__name__, agent)
+  ic.branch = 'parent.worker@1'
+  # Pre-populate session with turn 1's initial user input
+  original_event = Event(
+      author='user',
+      branch='parent.worker@1',
+      content=types.Content(
+          role='user', parts=[types.Part(text='turn 1 initial input')]
+      ),
+  )
+  ic.session.events.append(original_event)
+
+  ctx = Context(
+      invocation_context=ic, resume_inputs={'worker@1': {'confirmed': True}}
+  )
+
+  initial_len = len(ic.session.events)
+  prepare_llm_agent_input(agent, ctx, 'hello')
+
+  # Verify no duplicate user input was appended on resume
+  assert len(ic.session.events) == initial_len
+  # Verify the resumed node still sees the initial user input from turn 1
+  assert ic.session.events[-1].content.parts[0].text == 'turn 1 initial input'
+
+
 # --- build_node auto-wrapping ---