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docs/FIX_STALE_PENDING_WRITE_CHECKPOINT_20260901.md

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+# Fix: stale pending writes cannot roll back the latest LangGraph checkpoint
+
+## Problem
+`RepositoryCheckpointSaver.aput_writes()` used the latest full checkpoint payload, appended pending writes and persisted that payload as a new checkpoint row. Because the repository is append-only, an `aput_writes()` from an older super-step could finish after a newer `aput()` and become the newest database row. The next request would then restore an older transaction state.
+
+Observed symptom in a single session:
+- turn N opens a new transaction and asks for confirmation;
+- turn N+1 restores an older completed transaction;
+- a standalone confirmation such as `sim` is routed to no-match.
+
+## Fix
+`aput_writes()` now compares `config.configurable.checkpoint_id` with the durable latest checkpoint id. If the write belongs to an older checkpoint, it is ignored instead of re-persisting the stale full checkpoint as latest.
+
+Writes for the actual latest checkpoint continue to be persisted normally.
+
+## Invariant
+For a thread/session, checkpoint ordering is monotonic: a delayed pending write may enrich its own checkpoint, but it must never make a previous checkpoint become the session's latest state.