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d57a74f950
Fixes langchain-ai/deepagents#3774 ## Summary `Pregel.update_state` / `aupdate_state` on a fresh thread silently dropped the first write to a `DeltaChannel`-backed channel (e.g. `DeepAgentState.messages`). This PR persists the first write under a lazily-created stub checkpoint so the read-path ancestor walk can replay it. ## Root cause `DeltaChannel` reads its value back by walking ancestor checkpoints and replaying writes attached to them — non-snapshot steps don't store the value in `channel_values`. In `bulk_update_state` the channel writes were only persisted via `checkpointer.put_writes(...)` when a previous checkpoint existed: ```python channel_writes = [w for w in task.writes if w[0] != PUSH] if saved and channel_writes: checkpointer.put_writes(checkpoint_config, channel_writes, task_id) ``` On a fresh thread `saved is None`, so the `if saved` guard skipped persistence entirely. `create_checkpoint` then bumped the channel version but stored neither a value nor replayable writes, so reads returned `[]`. ## Fix In both `bulk_update_state` (sync) and `abulk_update_state` (async), when the thread has no persisted parent **and** at least one write targets a `DeltaChannel`, lazily persist an empty stub checkpoint and use it as the parent for both the channel writes and the new update checkpoint. Mirrors the existing exit-mode pattern in `_loop._put_exit_delta_writes`. The behavior for non-delta writes on a fresh thread is preserved (skip `put_writes` — values are stored directly in the new checkpoint's `channel_values`), so non-delta `update_state` paths add no extra checkpoint rows. ## Test coverage New tests in `libs/langgraph/tests/test_delta_channel_update_state.py` (9 tests, sync + async): - **Fresh-thread regression** (the bug): single `update_state` writes a message and reads back via `get_state`. Without the fix, both sync and async fail with `assert [] == ['hello']`. - **`update_state` after `invoke`**: pins down the previously-working non-fresh-thread path so the lazy-stub change doesn't regress it. - **Consecutive `update_state`s**: second call sees a real parent (`saved is not None`) and takes the original write path; both messages round-trip in chronological order. - **Update-by-id end-to-end via `update_state`**: `_messages_delta_reducer`'s dedup-by-id semantics work through the `update_state` path, not just `invoke`. - **`bulk_update_state` with multiple per-superstep updates**: locks in the per-task `put_writes` loop so all task writes persist (not just the last task's). - **State-history chain shape**: validates the lazy stub via the public API — `get_state_history` returns `[update_checkpoint, stub]` where the stub has `source='update'`, `step=-1`, no parent, and the update checkpoint's `parent_config` points at the stub. ## Verification - All 9 tests in `tests/test_delta_channel_update_state.py` pass. - All 4 existing delta-channel suites pass (`test_delta_channel_exit_mode.py`, `test_delta_channel_migration.py`, `test_delta_channel_id_stability.py`, `test_delta_channel_supersteps_bound.py` — 30 tests, 39 total with the new file). - All `update_state`-related tests across `test_pregel`, `test_pregel_async`, `test_time_travel`, `test_time_travel_async` pass (10 tests). - `make format`, `make lint`, full `make test` pass locally in `libs/langgraph`. --------- Co-authored-by: Cursor <cursoragent@cursor.com>