fix(postgres): handle missing checkpoint_id in _get_channel_writes_history; update test signatures

Two fixes exposed by running the postgres test suite against a local
postgres instance:

1. `PostgresSaver._get_channel_writes_history` /
   `AsyncPostgresSaver._aget_channel_writes_history` required
   `checkpoint_id` in the passed config, raising `KeyError` when called
   with just `thread_id` (e.g. `graph.aget_state({"thread_id": "..."})`).
   Now resolves to the latest checkpoint via `get_tuple`/`aget_tuple`
   when the id is missing.

2. `test_get_checkpoint_no_channel_values` (sync + async) monkeypatched
   `_load_checkpoint_tuple` with the old `(value, cur)` signature. Method
   now takes `(value)` only since delta reconstruction moved out of the
   tuple-load path — updated both tests.

Local postgres (`brew install pgvector postgresql@16`, running on port
5441) now exercises all 40 non-vector postgres tests green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

libs/checkpoint-postgres/langgraph/checkpoint/postgres/__init__.py

@@ -8,6 +8,7 @@ from typing import Any
 
 from langchain_core.runnables import RunnableConfig
 from langgraph.checkpoint.base import (
+    DELTA_SENTINEL,
     WRITES_IDX_MAP,
     ChannelVersions,
     Checkpoint,
@@ -448,7 +449,15 @@ class PostgresSaver(BasePostgresSaver):
         """
         thread_id = config["configurable"]["thread_id"]
         checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
-        checkpoint_id = config["configurable"]["checkpoint_id"]
+        checkpoint_id = get_checkpoint_id(config)
+        if checkpoint_id is None:
+            # Caller didn't specify a target — resolve to the latest
+            # checkpoint on the thread. `get_tuple` without `checkpoint_id`
+            # returns the newest; its config carries the resolved id.
+            target = self.get_tuple(config)
+            if target is None:
+                return _ChannelWritesHistory(seed=DELTA_SENTINEL, writes=[])
+            checkpoint_id = target.config["configurable"]["checkpoint_id"]
         with self._cursor() as cur:
             cur.execute(SELECT_DELTA_PARENTS_SQL, (channel, thread_id, checkpoint_ns))
             parents_rows = cur.fetchall()

libs/checkpoint-postgres/langgraph/checkpoint/postgres/aio.py

@@ -8,6 +8,7 @@ from typing import Any
 
 from langchain_core.runnables import RunnableConfig
 from langgraph.checkpoint.base import (
+    DELTA_SENTINEL,
     WRITES_IDX_MAP,
     ChannelVersions,
     Checkpoint,
@@ -409,7 +410,12 @@ class AsyncPostgresSaver(BasePostgresSaver):
         """
         thread_id = config["configurable"]["thread_id"]
         checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
-        checkpoint_id = config["configurable"]["checkpoint_id"]
+        checkpoint_id = get_checkpoint_id(config)
+        if checkpoint_id is None:
+            target = await self.aget_tuple(config)
+            if target is None:
+                return _ChannelWritesHistory(seed=DELTA_SENTINEL, writes=[])
+            checkpoint_id = target.config["configurable"]["checkpoint_id"]
         async with self._cursor() as cur:
             await cur.execute(
                 SELECT_DELTA_PARENTS_SQL, (channel, thread_id, checkpoint_ns)

libs/checkpoint-postgres/tests/test_async.py

@@ -361,9 +361,9 @@ async def test_get_checkpoint_no_channel_values(
 
         load_checkpoint_tuple = saver._load_checkpoint_tuple
 
-        async def patched_load_checkpoint_tuple(value, cur):
+        async def patched_load_checkpoint_tuple(value):
             value["checkpoint"].pop("channel_values", None)
-            return await load_checkpoint_tuple(value, cur)
+            return await load_checkpoint_tuple(value)
 
         monkeypatch.setattr(
             saver, "_load_checkpoint_tuple", patched_load_checkpoint_tuple

libs/checkpoint-postgres/tests/test_sync.py

@@ -348,9 +348,9 @@ def test_get_checkpoint_no_channel_values(
 
         load_checkpoint_tuple = saver._load_checkpoint_tuple
 
-        def patched_load_checkpoint_tuple(value, cur):
+        def patched_load_checkpoint_tuple(value):
             value["checkpoint"].pop("channel_values", None)
-            return load_checkpoint_tuple(value, cur)
+            return load_checkpoint_tuple(value)
 
         monkeypatch.setattr(
             saver, "_load_checkpoint_tuple", patched_load_checkpoint_tuple