fix(langgraph): keep _on_started backward-compatible with overrides predating cause#7987
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Nick Hollon (nick-hollon-lc) merged 1 commit intoJun 2, 2026
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… state #7928 added a keyword-only `cause` argument to the `_TasksLifecycleBase._on_started` hook and passed it unconditionally, breaking overrides that predate it — including downstream/third-party transformers like deepagents' `SubagentTransformer` (`TypeError: _on_started() got an unexpected keyword argument 'cause'`), which errored subagent runs. Deliver `cause` via instance state (`self._pending_cause`, set immediately before the call) instead of the call signature. `_on_started` reverts to its original `(ns, graph_name, trigger_call_id)` signature, so no override — old or new — ever breaks, with no introspection and no lockstep downstream release. `LifecycleTransformer` reads `self._pending_cause`; `SubgraphTransformer` ignores it as before. Adds a regression test.
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Sydney Runkle (sydney-runkle)
approved these changes
Jun 2, 2026
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Problem
#7928 added a keyword-only
causeargument to the_TasksLifecycleBase._on_startedhook and passes it unconditionally. Subclasses that override_on_startedwithout acauseparameter — including downstream/third-party transformers like deepagents'SubagentTransformer(which subclasses this base) — then raise:erroring the run whenever a subagent/subgraph is discovered. deepagents pulls langgraph transitively (
langchain<1.3.0,>=1.2.2, no direct pin), so shipping thecausechange in a 1.2.x release would break published deepagents.Fix
Deliver
causevia instance state rather than the call signature:_handle_task_startsetsself._pending_causeimmediately before calling_on_started, and overrides that surfacecauseread it from there._on_startedreverts to its original(self, ns, graph_name, trigger_call_id)signature.Why this shape:
_on_startedsignature is identical to what every existing override implements, so no override — old or new — breaks. No introspection/reflection.LifecycleTransformerreadsself._pending_cause(it surfacescauseon the wireLifecyclePayload);SubgraphTransformerignores it, as before.Tests
_on_started(self, ns, graph_name, trigger_call_id)override (nocause) processes a task-start event without raising. Existing transformer tests unchanged (27 pass); format/lint/mypy clean.deep_agentsubagent runs (which previously errored) complete cleanly, 6/6; full sdk-py integration suite green.(Switched from an earlier introspection-based approach to this instance-state delivery — cleaner and signature-stable.)