fix(langgraph): emit valid UUIDs for exit-mode delta task_ids for langgraph-api (#8165)

Fix exit-mode DeltaChannel `PutWrites` task IDs so they remain valid RFC
UUIDs while preserving superstep ordering for `ORDER BY task_id, idx`.

Follow-up to #7730: `_put_exit_delta_writes` used `f"{step:08d}-{tid}"`,
which produces a 6-segment string Postgres rejects for
`checkpoint_writes.task_id uuid` (e.g. `invalid input syntax for type
uuid`) in LangGraph-API.

This new `exit_delta_task_id()` helper embeds the superstep in the first
UUID group and keeps the original task UUID in the remaining segments
(e.g. `00000001-0270-bf16-1ef8-fb321bef9f3d`).

**Breaking changes:** None.

**Verification:**
- `cd libs/langgraph && make format && make lint && make test
TEST="tests/test_delta_channel_exit_mode.py"` (12 passed)
- New unit test asserts synthetic IDs parse as UUID, sort by superstep,
and that the old format is invalid

---------

Signed-off-by: Quanzheng Long <prclqz@gmail.com>
Co-authored-by: open-swe[bot] <215916821+open-swe[bot]@users.noreply.github.com>

libs/langgraph/langgraph/pregel/_checkpoint.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import uuid
 from collections.abc import Callable, Mapping
 from datetime import datetime, timezone
 from typing import Any, cast
@@ -34,6 +35,17 @@ def empty_checkpoint() -> Checkpoint:
     )
 
 
+def exit_delta_task_id(step: int, task_id: str) -> str:
+    """Synthetic task id for exit-mode DeltaChannel writes.
+
+    Embeds the superstep in the first UUID group so `ORDER BY task_id, idx`
+    preserves chronological order while remaining a valid RFC UUID (required by
+    Postgres `checkpoint_writes.task_id uuid` columns).
+    """
+    parts = str(uuid.UUID(task_id)).split("-")
+    return f"{step:08d}-{parts[1]}-{parts[2]}-{parts[3]}-{parts[4]}"
+
+
 def delta_channels_to_snapshot(
     channels: Mapping[str, BaseChannel],
     counters_since_delta_snapshot: Mapping[str, tuple[int, int]],

libs/langgraph/langgraph/pregel/_loop.py

@@ -102,6 +102,7 @@ from langgraph.pregel._checkpoint import (
     create_checkpoint,
     delta_channels_to_snapshot,
     empty_checkpoint,
+    exit_delta_task_id,
 )
 from langgraph.pregel._executor import (
     AsyncBackgroundExecutor,
@@ -1272,7 +1273,7 @@ class PregelLoop:
             },
         )
         for (step, tid), entries in grouped.items():
-            synth_tid = f"{step:08d}-{tid}"
+            synth_tid = exit_delta_task_id(step, tid)
             if self.checkpointer_put_writes_accepts_task_path:
                 fut = self.submit(
                     self.checkpointer_put_writes,

libs/langgraph/tests/test_delta_channel_exit_mode.py

@@ -6,6 +6,7 @@ channel), lazy stub creation when no parent exists, and proper read-path
 reconstruction via ancestor walks.
 """
 
+import uuid
 from typing import Annotated, Any
 
 import pytest
@@ -17,10 +18,28 @@ from typing_extensions import TypedDict
 from langgraph.channels.delta import DeltaChannel
 from langgraph.graph import START, StateGraph
 from langgraph.graph.message import _messages_delta_reducer
+from langgraph.pregel._checkpoint import exit_delta_task_id
 
 pytestmark = pytest.mark.anyio
 
 
+def test_exit_delta_task_id_is_valid_uuid_and_ordered() -> None:
+    """Exit-mode synthetic task ids must parse as UUID and sort by superstep."""
+    tid = "4f7226e4-0270-bf16-1ef8-fb321bef9f3d"
+    id1 = exit_delta_task_id(1, tid)
+    id7 = exit_delta_task_id(7, tid)
+
+    uuid.UUID(id1)
+    uuid.UUID(id7)
+    assert id1 < id7
+    assert id1.split("-")[0] == "00000001"
+    assert id7.split("-")[0] == "00000007"
+    assert id1.endswith("-0270-bf16-1ef8-fb321bef9f3d")
+
+    with pytest.raises(ValueError):
+        uuid.UUID(f"00000001-{tid}")
+
+
 def _build_graph(
     checkpointer: InMemorySaver,
     *,