fix(langgraph): replay task results on nested resume

[Serhiy Bzhezytskyy (serhiy-bzhezytskyy)]

Summary

Fixes #2667

A task() that completed before an interrupt() records its result as a pending write, so resuming should reuse it. It does when the graph runs standalone. Composed as a subgraph, the body runs again on every resume — one duplicated side effect per resume, and task() is the wrapper for the non-idempotent step.

Root cause

A subgraph's loop receives checkpoint_id as a key in its configurable, and skipDoneTasks tests for the key rather than a value:

// libs/langgraph-core/src/pregel/loop.ts
const skipDoneTasks = config.configurable
  ? !("checkpoint_id" in config.configurable)
  : true;

So it is false on every nested run and the two sites that hand a recorded result to a task never fire — _matchWrites() for a pushed Call task, and the pending-writes loop in tick(). task.writes stays empty, the short-circuit in runner.ts that returns a recorded RETURN write cannot run, and the body executes again.

The predicate itself is load-bearing. Python fixed the same bug in langchain-ai/langgraph#6161 by conditioning the derivation on resuming inside a nested graph; ported here, that shape fails the Direct-to-subgraph time travel group in time_travel.test.ts, and making the check test the value instead of the key fails seven tests in the same file.

Changes

• pregel/loop.ts: gate applying pending writes on isTimeTraveling — the predicate _first() already computes and already uses to drop stale RESUME writes — instead of skipDoneTasks, whose derivation is untouched. isReplaying still derives from it, so it still reads true for every nested run.
• tests/nested-resume-task-replay.test.ts: standalone resume (the control), nested resume, and an explicit checkpoint_id replay, which must still re-run the task.

Test plan

• the nested-resume case fails expected 2 to be 1 with the change stashed
• pnpm lint, pnpm format:check and the langgraph-core suite pass locally, including the time-travel suites
• Changeset for @langchain/langgraph

Reproduces on @langchain/langgraph 1.4.9, on main, and on Python langgraph 1.2.10 — langchain-ai/langgraph#6161 fixed it there, and the later rename of skip_done_tasks to is_replaying dropped the nested-resume condition.

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Latest commit: 6060c05

The changes in this PR will be included in the next version bump.

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Name	Type
@langchain/langgraph	Patch

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