update loop

## Description
The `_build_server_info` function was reading `assistant_id` and
`graph_id` from `config["metadata"]`, but the server puts these values
in `config["configurable"]`. This updates the source to read from
`configurable` consistently.

## Test Plan
- [ ] Verify `server_info.assistant_id` and `server_info.graph_id` are
correctly populated from `config["configurable"]`

_Opened collaboratively by Sydney Runkle and open-swe._

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Sydney Runkle <54324534+sydney-runkle@users.noreply.github.com>

libs/cli/js-monorepo-example/yarn.lock

@@ -217,11 +217,6 @@
   resolved "https://registry.yarnpkg.com/@types/json5/-/json5-0.0.29.tgz#ee28707ae94e11d2b827bcbe5270bcea7f3e71ee"
   integrity sha512-dRLjCWHYg4oaA77cxO64oO+7JwCwnIzkZPdrrC71jQmQtlhM556pwKo5bUzqvZndkVbeFLIIi+9TC40JNF5hNQ==
 
-"@types/uuid@^10.0.0":
-  version "10.0.0"
-  resolved "https://registry.yarnpkg.com/@types/uuid/-/uuid-10.0.0.tgz#e9c07fe50da0f53dc24970cca94d619ff03f6f6d"
-  integrity sha512-7gqG38EyHgyP1S+7+xomFtL+ZNHcKv6DwNaCZmJmo1vgMugyF3TCnXVg4t1uk89mLNwnLtnY3TpOpCOyp1/xHQ==
-
 "@typescript-eslint/eslint-plugin@^8.58.0":
   version "8.58.0"
   resolved "https://registry.yarnpkg.com/@typescript-eslint/eslint-plugin/-/eslint-plugin-8.58.0.tgz#ad40e492f1931f46da1bd888e52b9e56df9063aa"
@@ -343,13 +338,6 @@ ajv@^6.14.0:
     json-schema-traverse "^0.4.1"
     uri-js "^4.2.2"
 
-ansi-styles@^4.1.0:
-  version "4.3.0"
-  resolved "https://registry.yarnpkg.com/ansi-styles/-/ansi-styles-4.3.0.tgz#edd803628ae71c04c85ae7a0906edad34b648937"
-  integrity sha512-zbB9rCJAT1rbjiVDb2hqKFHNYLxgtk8NURxZ3IZwD3F6NtxbXZQCnnSi1Lkx+IDohdPlFp222wVALIheZJQSEg==
-  dependencies:
-    color-convert "^2.0.1"
-
 ansi-styles@^5.0.0:
   version "5.2.0"
   resolved "https://registry.yarnpkg.com/ansi-styles/-/ansi-styles-5.2.0.tgz#07449690ad45777d1924ac2abb2fc8895dba836b"
@@ -508,38 +496,11 @@ camelcase@6:
   resolved "https://registry.yarnpkg.com/camelcase/-/camelcase-6.3.0.tgz#5685b95eb209ac9c0c177467778c9c84df58ba9a"
   integrity sha512-Gmy6FhYlCY7uOElZUSbxo2UCDH8owEk996gkbrpsgGtrJLM3J7jGxl9Ic7Qwwj4ivOE5AWZWRMecDdF7hqGjFA==
 
-chalk@^4.1.2:
-  version "4.1.2"
-  resolved "https://registry.yarnpkg.com/chalk/-/chalk-4.1.2.tgz#aac4e2b7734a740867aeb16bf02aad556a1e7a01"
-  integrity sha512-oKnbhFyRIXpUuez8iBMmyEa4nbj4IOQyuhc/wy9kY7/WVPcwIO9VA668Pu8RkO7+0G76SLROeyw9CpQ061i4mA==
-  dependencies:
-    ansi-styles "^4.1.0"
-    supports-color "^7.1.0"
-
-color-convert@^2.0.1:
-  version "2.0.1"
-  resolved "https://registry.yarnpkg.com/color-convert/-/color-convert-2.0.1.tgz#72d3a68d598c9bdb3af2ad1e84f21d896abd4de3"
-  integrity sha512-RRECPsj7iu/xb5oKYcsFHSppFNnsj/52OVTRKb4zP5onXwVF3zVmmToNcOfGC+CRDpfK/U584fMg38ZHCaElKQ==
-  dependencies:
-    color-name "~1.1.4"
-
-color-name@~1.1.4:
-  version "1.1.4"
-  resolved "https://registry.yarnpkg.com/color-name/-/color-name-1.1.4.tgz#c2a09a87acbde69543de6f63fa3995c826c536a2"
-  integrity sha512-dOy+3AuW3a2wNbZHIuMZpTcgjGuLU/uBL/ubcZF9OXbDo8ff4O8yVp5Bf0efS8uEoYo5q4Fx7dY9OgQGXgAsQA==
-
 concat-map@0.0.1:
   version "0.0.1"
   resolved "https://registry.yarnpkg.com/concat-map/-/concat-map-0.0.1.tgz#d8a96bd77fd68df7793a73036a3ba0d5405d477b"
   integrity sha512-/Srv4dswyQNBfohGpz9o6Yb3Gz3SrUDqBH5rTuhGR7ahtlbYKnVxw2bCFMRljaA7EXHaXZ8wsHdodFvbkhKmqg==
 
-console-table-printer@^2.12.1:
-  version "2.14.6"
-  resolved "https://registry.yarnpkg.com/console-table-printer/-/console-table-printer-2.14.6.tgz#edfe0bf311fa2701922ed509443145ab51e06436"
-  integrity sha512-MCBl5HNVaFuuHW6FGbL/4fB7N/ormCy+tQ+sxTrF6QtSbSNETvPuOVbkJBhzDgYhvjWGrTma4eYJa37ZuoQsPw==
-  dependencies:
-    simple-wcswidth "^1.0.1"
-
 cross-spawn@^7.0.6:
   version "7.0.6"
   resolved "https://registry.yarnpkg.com/cross-spawn/-/cross-spawn-7.0.6.tgz#8a58fe78f00dcd70c370451759dfbfaf03e8ee9f"
@@ -1059,11 +1020,6 @@ has-bigints@^1.0.2:
   resolved "https://registry.yarnpkg.com/has-bigints/-/has-bigints-1.1.0.tgz#28607e965ac967e03cd2a2c70a2636a1edad49fe"
   integrity sha512-R3pbpkcIqv2Pm3dUwgjclDRVmWpTJW2DcMzcIhEXEx1oh/CEMObMm3KLmRJOdvhM7o4uQBnwr8pzRK2sJWIqfg==
 
-has-flag@^4.0.0:
-  version "4.0.0"
-  resolved "https://registry.yarnpkg.com/has-flag/-/has-flag-4.0.0.tgz#944771fd9c81c81265c4d6941860da06bb59479b"
-  integrity sha512-EykJT/Q1KjTWctppgIAgfSO0tKVuZUjhgMr17kqTumMl6Afv3EISleU7qZUzoXDFTAHTDC4NOoG/ZxU3EvlMPQ==
-
 has-property-descriptors@^1.0.0, has-property-descriptors@^1.0.2:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/has-property-descriptors/-/has-property-descriptors-1.0.2.tgz#963ed7d071dc7bf5f084c5bfbe0d1b6222586854"
@@ -1372,16 +1328,12 @@ keyv@^4.5.4:
     json-buffer "3.0.1"
 
 "langsmith@>=0.5.0 <1.0.0":
-  version "0.5.4"
-  resolved "https://registry.yarnpkg.com/langsmith/-/langsmith-0.5.4.tgz#f75b82b08e30db72a7d1d595b341e9666bd525e5"
-  integrity sha512-qYkNIoKpf0ZYt+cYzrDV+XI3FCexApmZmp8EMs3eDTMv0OvrHMLoxJ9IpkeoXJSX24+GPk0/jXjKx2hWerpy9w==
+  version "0.5.18"
+  resolved "https://registry.yarnpkg.com/langsmith/-/langsmith-0.5.18.tgz#c691ad23614f0b46eaf07d982e0ac988e1f43880"
+  integrity sha512-3zuZUWffTHQ+73EAwnodADtf534VNEZUpXr9jC12qyG8/IQuJET7PRsCpTb9wX2lmBspakwLUpqpj3tNm/0bVA==
   dependencies:
-    "@types/uuid" "^10.0.0"
-    chalk "^4.1.2"
-    console-table-printer "^2.12.1"
-    p-queue "^6.6.2"
-    semver "^7.6.3"
-    uuid "^10.0.0"
+    p-queue "6.6.2"
+    uuid "10.0.0"
 
 levn@^0.4.1:
   version "0.4.1"
@@ -1528,7 +1480,7 @@ p-locate@^5.0.0:
   dependencies:
     p-limit "^3.0.2"
 
-p-queue@^6.6.2:
+p-queue@6.6.2, p-queue@^6.6.2:
   version "6.6.2"
   resolved "https://registry.yarnpkg.com/p-queue/-/p-queue-6.6.2.tgz#2068a9dcf8e67dd0ec3e7a2bcb76810faa85e426"
   integrity sha512-RwFpb72c/BhQLEXIZ5K2e+AhgNVmIejGlTgiB9MzZ0e93GRvqZ7uSi0dvRF7/XIXDeNkra2fNHBxTyPDGySpjQ==
@@ -1690,11 +1642,6 @@ semver@^6.3.1:
   resolved "https://registry.yarnpkg.com/semver/-/semver-6.3.1.tgz#556d2ef8689146e46dcea4bfdd095f3434dffcb4"
   integrity sha512-BR7VvDCVHO+q2xBEWskxS6DJE1qRnb7DxzUrogb71CWoSficBxYsiAGd+Kl0mmq/MprG9yArRkyrQxTO6XjMzA==
 
-semver@^7.6.3:
-  version "7.7.2"
-  resolved "https://registry.yarnpkg.com/semver/-/semver-7.7.2.tgz#67d99fdcd35cec21e6f8b87a7fd515a33f982b58"
-  integrity sha512-RF0Fw+rO5AMf9MAyaRXI4AV0Ulj5lMHqVxxdSgiVbixSCXoEmmX/jk0CuJw4+3SqroYO9VoUh+HcuJivvtJemA==
-
 semver@^7.7.3:
   version "7.7.4"
   resolved "https://registry.yarnpkg.com/semver/-/semver-7.7.4.tgz#28464e36060e991fa7a11d0279d2d3f3b57a7e8a"
@@ -1783,11 +1730,6 @@ side-channel@^1.1.0:
     side-channel-map "^1.0.1"
     side-channel-weakmap "^1.0.2"
 
-simple-wcswidth@^1.0.1:
-  version "1.1.2"
-  resolved "https://registry.yarnpkg.com/simple-wcswidth/-/simple-wcswidth-1.1.2.tgz#66722f37629d5203f9b47c5477b1225b85d6525b"
-  integrity sha512-j7piyCjAeTDSjzTSQ7DokZtMNwNlEAyxqSZeCS+CXH7fJ4jx3FuJ/mTW3mE+6JLs4VJBbcll0Kjn+KXI5t21Iw==
-
 stop-iteration-iterator@^1.1.0:
   version "1.1.0"
   resolved "https://registry.yarnpkg.com/stop-iteration-iterator/-/stop-iteration-iterator-1.1.0.tgz#f481ff70a548f6124d0312c3aa14cbfa7aa542ad"
@@ -1838,13 +1780,6 @@ strip-json-comments@^3.1.1:
   resolved "https://registry.yarnpkg.com/strip-json-comments/-/strip-json-comments-3.1.1.tgz#31f1281b3832630434831c310c01cccda8cbe006"
   integrity sha512-6fPc+R4ihwqP6N/aIv2f1gMH8lOVtWQHoqC4yK6oSDVVocumAsfCqjkXnqiYMhmMwS/mEHLp7Vehlt3ql6lEig==
 
-supports-color@^7.1.0:
-  version "7.2.0"
-  resolved "https://registry.yarnpkg.com/supports-color/-/supports-color-7.2.0.tgz#1b7dcdcb32b8138801b3e478ba6a51caa89648da"
-  integrity sha512-qpCAvRl9stuOHveKsn7HncJRvv501qIacKzQlO/+Lwxc9+0q2wLyv4Dfvt80/DPn2pqOBsJdDiogXGR9+OvwRw==
-  dependencies:
-    has-flag "^4.0.0"
-
 supports-preserve-symlinks-flag@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/supports-preserve-symlinks-flag/-/supports-preserve-symlinks-flag-1.0.0.tgz#6eda4bd344a3c94aea376d4cc31bc77311039e09"
@@ -1966,7 +1901,7 @@ uri-js@^4.2.2:
   dependencies:
     punycode "^2.1.0"
 
-uuid@^10.0.0:
+uuid@10.0.0, uuid@^10.0.0:
   version "10.0.0"
   resolved "https://registry.yarnpkg.com/uuid/-/uuid-10.0.0.tgz#5a95aa454e6e002725c79055fd42aaba30ca6294"
   integrity sha512-8XkAphELsDnEGrDxUOHB3RGvXz6TeuYSGEZBOjtTtPm2lwhGBjLgOzLHB63IUWfBpNucQjND6d3AOudO+H3RWQ==

libs/cli/uv.lock

@@ -2318,28 +2318,28 @@ wheels = [
 
 [[package]]
 name = "uv"
-version = "0.11.3"
+version = "0.11.6"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/88/ed/f11c558e8d2e02fba6057dacd9e92a71557359a80bd5355452310b89f40f/uv-0.11.3.tar.gz", hash = "sha256:6a6fcaf1fec28bbbdf0dfc5a0a6e34be4cea08c6287334b08c24cf187300f20d", size = 4027684, upload-time = "2026-04-01T21:47:22.096Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/dd/f3/8aceeab67ea69805293ab290e7ca8cc1b61a064d28b8a35c76d8eba063dd/uv-0.11.6.tar.gz", hash = "sha256:e3b21b7e80024c95ff339fcd147ac6fc3dd98d3613c9d45d3a1f4fd1057f127b", size = 4073298, upload-time = "2026-04-09T12:09:01.738Z" }
 wheels = [
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 ]
 
 [[package]]

libs/langgraph/langgraph/pregel/_checkpoint_writer.py

@@ -0,0 +1,218 @@
+from __future__ import annotations
+
+import asyncio
+import os
+import queue
+import threading
+from collections.abc import Callable
+from contextlib import AbstractAsyncContextManager, AbstractContextManager
+from dataclasses import dataclass
+from types import TracebackType
+from typing import Any
+
+from langchain_core.runnables import RunnableConfig
+from langgraph.checkpoint.base import (
+    ChannelVersions,
+    Checkpoint,
+    CheckpointMetadata,
+)
+
+QUEUE_PUT_TIMEOUT = 0.05
+CHECKPOINT_BACKLOG_ENV_VAR = "LANGGRAPH_CHECKPOINT_BACKLOG"
+DEFAULT_CHECKPOINT_BACKLOG = 10
+
+
+def _resolve_checkpoint_backlog() -> int:
+    if raw := os.getenv(CHECKPOINT_BACKLOG_ENV_VAR):
+        try:
+            backlog = int(raw)
+        except ValueError:
+            return DEFAULT_CHECKPOINT_BACKLOG
+        if backlog > 0:
+            return backlog
+    return DEFAULT_CHECKPOINT_BACKLOG
+
+
+CHECKPOINT_BACKLOG = _resolve_checkpoint_backlog()
+
+
+@dataclass(frozen=True, slots=True)
+class CheckpointRequest:
+    config: RunnableConfig
+    checkpoint: Checkpoint
+    metadata: CheckpointMetadata
+    new_versions: ChannelVersions
+
+
+def _raise(error: BaseException) -> None:
+    raise error
+
+
+class SyncCheckpointWriter(AbstractContextManager):
+    __slots__ = ("put", "queue", "error", "closed", "thread")
+
+    def __init__(
+        self,
+        put: Callable[
+            [RunnableConfig, Checkpoint, CheckpointMetadata, ChannelVersions], Any
+        ],
+        *,
+        max_pending: int | None = None,
+    ) -> None:
+        self.put = put
+        max_pending = CHECKPOINT_BACKLOG if max_pending is None else max_pending
+        self.queue: queue.Queue[CheckpointRequest | None] = queue.Queue(max_pending)
+        self.error: BaseException | None = None
+        self.closed = False
+        self.thread = threading.Thread(
+            target=self._run,
+            name="langgraph-checkpoint-writer",
+            daemon=True,
+        )
+
+    def __enter__(self) -> SyncCheckpointWriter:
+        self.thread.start()
+        return self
+
+    def submit(self, request: CheckpointRequest) -> None:
+        self._ensure_open()
+        while True:
+            self._raise_if_broken()
+            try:
+                self.queue.put(request, timeout=QUEUE_PUT_TIMEOUT)
+            except queue.Full:
+                continue
+            else:
+                self._raise_if_broken()
+                return
+
+    def _run(self) -> None:
+        while True:
+            item = self.queue.get()
+            if item is None:
+                return
+            try:
+                self.put(
+                    item.config,
+                    item.checkpoint,
+                    item.metadata,
+                    item.new_versions,
+                )
+            except BaseException as exc:
+                self.error = exc
+                return
+
+    def _ensure_open(self) -> None:
+        if self.closed:
+            raise RuntimeError("Checkpoint writer is closed")
+
+    def _raise_if_broken(self) -> None:
+        if self.error is not None:
+            _raise(self.error)
+
+    def __exit__(
+        self,
+        exc_type: type[BaseException] | None,
+        exc_value: BaseException | None,
+        traceback: TracebackType | None,
+    ) -> bool | None:
+        self.closed = True
+        while self.thread.is_alive():
+            if self.error is not None:
+                break
+            try:
+                self.queue.put(None, timeout=QUEUE_PUT_TIMEOUT)
+            except queue.Full:
+                continue
+            else:
+                break
+        self.thread.join()
+        if exc_type is None and self.error is not None:
+            _raise(self.error)
+        return None
+
+
+class AsyncCheckpointWriter(AbstractAsyncContextManager):
+    __slots__ = ("put", "queue", "error", "closed", "task")
+
+    def __init__(
+        self,
+        put: Callable[
+            [RunnableConfig, Checkpoint, CheckpointMetadata, ChannelVersions], Any
+        ],
+        *,
+        max_pending: int | None = None,
+    ) -> None:
+        self.put = put
+        max_pending = CHECKPOINT_BACKLOG if max_pending is None else max_pending
+        self.queue: asyncio.Queue[CheckpointRequest | None] = asyncio.Queue(max_pending)
+        self.error: BaseException | None = None
+        self.closed = False
+        self.task: asyncio.Task[None] | None = None
+
+    async def __aenter__(self) -> AsyncCheckpointWriter:
+        self.task = asyncio.create_task(self._run(), name="langgraph-checkpoint-writer")
+        return self
+
+    async def submit(self, request: CheckpointRequest) -> None:
+        self._ensure_open()
+        while True:
+            self._raise_if_broken()
+            try:
+                await asyncio.wait_for(
+                    self.queue.put(request),
+                    timeout=QUEUE_PUT_TIMEOUT,
+                )
+            except asyncio.TimeoutError:
+                continue
+            else:
+                self._raise_if_broken()
+                return
+
+    async def _run(self) -> None:
+        while True:
+            item = await self.queue.get()
+            if item is None:
+                return
+            try:
+                await self.put(
+                    item.config,
+                    item.checkpoint,
+                    item.metadata,
+                    item.new_versions,
+                )
+            except BaseException as exc:
+                self.error = exc
+                return
+
+    def _ensure_open(self) -> None:
+        if self.closed:
+            raise RuntimeError("Checkpoint writer is closed")
+
+    def _raise_if_broken(self) -> None:
+        if self.error is not None:
+            _raise(self.error)
+
+    async def __aexit__(
+        self,
+        exc_type: type[BaseException] | None,
+        exc_value: BaseException | None,
+        traceback: TracebackType | None,
+    ) -> None:
+        self.closed = True
+        while self.task is not None and not self.task.done():
+            if self.error is not None:
+                break
+            try:
+                await asyncio.wait_for(
+                    self.queue.put(None),
+                    timeout=QUEUE_PUT_TIMEOUT,
+                )
+            except asyncio.TimeoutError:
+                continue
+            else:
+                break
+        if self.task is not None:
+            await self.task
+        if exc_type is None and self.error is not None:
+            _raise(self.error)

libs/langgraph/langgraph/pregel/_loop.py

@@ -2,7 +2,6 @@ from __future__ import annotations
 
 import asyncio
 import binascii
-import concurrent.futures
 from collections import defaultdict, deque
 from collections.abc import Callable, Iterator, Mapping, Sequence
 from contextlib import (
@@ -27,7 +26,6 @@ from langgraph.cache.base import BaseCache
 from langgraph.checkpoint.base import (
     WRITES_IDX_MAP,
     BaseCheckpointSaver,
-    ChannelVersions,
     Checkpoint,
     CheckpointMetadata,
     CheckpointTuple,
@@ -97,6 +95,11 @@ from langgraph.pregel._checkpoint import (
     create_checkpoint,
     empty_checkpoint,
 )
+from langgraph.pregel._checkpoint_writer import (
+    AsyncCheckpointWriter,
+    CheckpointRequest,
+    SyncCheckpointWriter,
+)
 from langgraph.pregel._executor import (
     AsyncBackgroundExecutor,
     BackgroundExecutor,
@@ -172,19 +175,6 @@ class PregelLoop:
     checkpointer_get_next_version: GetNextVersion
     checkpointer_put_writes: Callable[[RunnableConfig, WritesT, str], Any] | None
     checkpointer_put_writes_accepts_task_path: bool
-    _checkpointer_put_after_previous: (
-        Callable[
-            [
-                concurrent.futures.Future | None,
-                RunnableConfig,
-                Checkpoint,
-                str,
-                ChannelVersions,
-            ],
-            Any,
-        ]
-        | None
-    )
     _migrate_checkpoint: Callable[[Checkpoint], None] | None
     submit: Submit
     channels: Mapping[str, BaseChannel]
@@ -536,7 +526,7 @@ class PregelLoop:
         )
 
         # produce debug output
-        if self._checkpointer_put_after_previous is not None:
+        if self.checkpointer is not None:
             self._emit(
                 "checkpoints",
                 map_debug_checkpoint,
@@ -582,7 +572,7 @@ class PregelLoop:
 
         return True
 
-    def after_tick(self) -> None:
+    def _after_tick(self) -> CheckpointRequest | None:
         # finish superstep
         writes = [w for t in self.tasks.values() for w in t.writes]
         # all tasks have finished
@@ -607,15 +597,7 @@ class PregelLoop:
         # only replay (re-execute) done tasks on the first tick
         self.is_replaying = False
         # save checkpoint
-        self._put_checkpoint({"source": "loop"})
-        # after execution, check if we should interrupt
-        if self.interrupt_after and should_interrupt(
-            self.checkpoint, self.interrupt_after, self.tasks.values()
-        ):
-            self.status = "interrupt_after"
-            raise GraphInterrupt()
-        # unset resuming flag
-        self.config[CONF].pop(CONFIG_KEY_RESUMING, None)
+        return self._prepare_checkpoint({"source": "loop"})
 
     def match_cached_writes(self) -> Sequence[PregelExecutableTask]:
         raise NotImplementedError
@@ -664,7 +646,7 @@ class PregelLoop:
 
     def _first(
         self, *, input_keys: str | Sequence[str], updated_channels: set[str] | None
-    ) -> set[str] | None:
+    ) -> tuple[set[str] | None, CheckpointRequest | None]:
         # Resuming from a previous checkpoint requires two things:
         # 1. A prior checkpoint exists (channel_versions is non-empty)
         # 2. The input signals continuation (not a fresh run with new input)
@@ -758,6 +740,7 @@ class PregelLoop:
             )
             if updated_channels is not None:
                 updated_channels.update(null_updated_channels)
+        checkpoint_request = None
         # proceed past previous checkpoint
         if is_resuming:
             self.checkpoint["versions_seen"].setdefault(INTERRUPT, {})
@@ -800,7 +783,7 @@ class PregelLoop:
             )
             # save input checkpoint
             self.updated_channels = updated_channels
-            self._put_checkpoint({"source": "input"})
+            checkpoint_request = self._prepare_checkpoint({"source": "input"})
         elif CONFIG_KEY_RESUMING not in configurable:
             raise EmptyInputError(f"Received no input for {input_keys}")
         # Propagate resuming and replaying flags to subgraphs.
@@ -832,9 +815,11 @@ class PregelLoop:
         self.status = "pending"
         if is_resuming:
             self._push_graph_lifecycle_event("resume")
-        return updated_channels
+        return updated_channels, checkpoint_request
 
-    def _put_checkpoint(self, metadata: CheckpointMetadata) -> None:
+    def _prepare_checkpoint(
+        self, metadata: CheckpointMetadata
+    ) -> CheckpointRequest | None:
         # assign step and parents
         exiting = metadata is self.checkpoint_metadata
         if exiting and self.checkpoint["id"] == self.checkpoint_id_saved:
@@ -844,8 +829,7 @@ class PregelLoop:
             metadata["step"] = self.step
             metadata["parents"] = self.config[CONF].get(CONFIG_KEY_CHECKPOINT_MAP, {})
             self.checkpoint_metadata = metadata
-        # do checkpoint?
-        do_checkpoint = self._checkpointer_put_after_previous is not None and (
+        do_checkpoint = self.checkpointer is not None and (
             exiting or self.durability != "exit"
         )
         # create new checkpoint
@@ -867,9 +851,8 @@ class PregelLoop:
                 for value in self.checkpoint["channel_values"][TASKS]
             ]
             self.checkpoint["channel_values"][TASKS] = sanitized_tasks
-        # bail if no checkpointer
-
-        if do_checkpoint and self._checkpointer_put_after_previous is not None:
+        request = None
+        if do_checkpoint:
             self.prev_checkpoint_config = (
                 self.checkpoint_config
                 if CONFIG_KEY_CHECKPOINT_ID in self.checkpoint_config[CONF]
@@ -891,17 +874,11 @@ class PregelLoop:
                 self.checkpoint_previous_versions, channel_versions
             )
             self.checkpoint_previous_versions = channel_versions
-
-            # save it, without blocking
-            # if there's a previous checkpoint save in progress, wait for it
-            # ensuring checkpointers receive checkpoints in order
-            self._put_checkpoint_fut = self.submit(
-                self._checkpointer_put_after_previous,
-                getattr(self, "_put_checkpoint_fut", None),
-                self.checkpoint_config,
-                copy_checkpoint(self.checkpoint),
-                self.checkpoint_metadata,
-                new_versions,
+            request = CheckpointRequest(
+                config=self.checkpoint_config,
+                checkpoint=copy_checkpoint(self.checkpoint),
+                metadata=self.checkpoint_metadata,
+                new_versions=new_versions,
             )
             self.checkpoint_config = {
                 **self.checkpoint_config,
@@ -913,24 +890,13 @@ class PregelLoop:
         if not exiting:
             # increment step
             self.step += 1
+        return request
 
-    def _suppress_interrupt(
+    def _finalize_suppress(
         self,
         exc_type: type[BaseException] | None,
         exc_value: BaseException | None,
-        traceback: TracebackType | None,
     ) -> bool | None:
-        # persist current checkpoint and writes
-        if self.durability == "exit" and (
-            # if it's a top graph
-            not self.is_nested
-            # or a nested graph with error or interrupt
-            or exc_value is not None
-            # or a nested graph with checkpointer=True
-            or all(NS_END not in part for part in self.checkpoint_ns)
-        ):
-            self._put_checkpoint(self.checkpoint_metadata)
-            self._put_pending_writes()
         # suppress interrupt
         if isinstance(exc_value, GraphInterrupt) and not self.is_nested:
             interrupt = exc_value
@@ -975,6 +941,7 @@ class PregelLoop:
         elif exc_type is None:
             # save final output
             self.output = read_channels(self.channels, self.output_keys)
+        return None
 
     def _emit(
         self,
@@ -1122,26 +1089,57 @@ class SyncPregelLoop(PregelLoop, AbstractContextManager):
             )
         else:
             self.checkpointer_get_next_version = increment
-            self._checkpointer_put_after_previous = None  # type: ignore[assignment]
             self.checkpointer_put_writes = None
             self.checkpointer_put_writes_accepts_task_path = False
+        self._checkpoint_writer: SyncCheckpointWriter | None = None
 
-    def _checkpointer_put_after_previous(
-        self,
-        prev: concurrent.futures.Future | None,
-        config: RunnableConfig,
-        checkpoint: Checkpoint,
-        metadata: CheckpointMetadata,
-        new_versions: ChannelVersions,
-    ) -> RunnableConfig:
-        try:
-            if prev is not None:
-                prev.result()
-        finally:
+    def _dispatch_checkpoint_request(self, request: CheckpointRequest) -> None:
+        if self.durability == "async" and self._checkpoint_writer is not None:
+            self._checkpoint_writer.submit(request)
+        else:
             cast(BaseCheckpointSaver, self.checkpointer).put(
-                config, checkpoint, metadata, new_versions
+                request.config,
+                request.checkpoint,
+                request.metadata,
+                request.new_versions,
             )
 
+    def _put_checkpoint(self, metadata: CheckpointMetadata) -> None:
+        if request := self._prepare_checkpoint(metadata):
+            self._dispatch_checkpoint_request(request)
+
+    def _suppress_interrupt(
+        self,
+        exc_type: type[BaseException] | None,
+        exc_value: BaseException | None,
+        traceback: TracebackType | None,
+    ) -> bool | None:
+        # persist current checkpoint and writes
+        if self.durability == "exit" and (
+            # if it's a top graph
+            not self.is_nested
+            # or a nested graph with error or interrupt
+            or exc_value is not None
+            # or a nested graph with checkpointer=True
+            or all(NS_END not in part for part in self.checkpoint_ns)
+        ):
+            self._put_checkpoint(self.checkpoint_metadata)
+            self._put_pending_writes()
+        return self._finalize_suppress(exc_type, exc_value)
+
+    def after_tick(self) -> None:
+        # compute tick state and save checkpoint
+        if request := self._after_tick():
+            self._dispatch_checkpoint_request(request)
+        # after execution, check if we should interrupt
+        if self.interrupt_after and should_interrupt(
+            self.checkpoint, self.interrupt_after, self.tasks.values()
+        ):
+            self.status = "interrupt_after"
+            raise GraphInterrupt()
+        # unset resuming flag
+        self.config[CONF].pop(CONFIG_KEY_RESUMING, None)
+
     def match_cached_writes(self) -> Sequence[PregelExecutableTask]:
         if self.cache is None:
             return ()
@@ -1237,6 +1235,10 @@ class SyncPregelLoop(PregelLoop, AbstractContextManager):
             else []
         )
         self.submit = self.stack.enter_context(BackgroundExecutor(self.config))
+        if self.checkpointer is not None and self.durability == "async":
+            self._checkpoint_writer = self.stack.enter_context(
+                SyncCheckpointWriter(self.checkpointer.put)
+            )
         self.channels, self.managed = channels_from_checkpoint(
             self.specs, self.checkpoint
         )
@@ -1245,12 +1247,14 @@ class SyncPregelLoop(PregelLoop, AbstractContextManager):
         self.step = self.checkpoint_metadata["step"] + 1
         self.stop = self.step + self.config["recursion_limit"] + 1
         self.checkpoint_previous_versions = self.checkpoint["channel_versions"].copy()
-        self.updated_channels = self._first(
+        self.updated_channels, checkpoint_request = self._first(
             input_keys=self.input_keys,
             updated_channels=set(self.checkpoint.get("updated_channels"))  # type: ignore[arg-type]
             if self.checkpoint.get("updated_channels")
             else None,
         )
+        if checkpoint_request is not None:
+            self._dispatch_checkpoint_request(checkpoint_request)
 
         return self
 
@@ -1321,26 +1325,34 @@ class AsyncPregelLoop(PregelLoop, AbstractAsyncContextManager):
             )
         else:
             self.checkpointer_get_next_version = increment
-            self._checkpointer_put_after_previous = None  # type: ignore[assignment]
             self.checkpointer_put_writes = None
             self.checkpointer_put_writes_accepts_task_path = False
+        self._checkpoint_writer: AsyncCheckpointWriter | None = None
 
-    async def _checkpointer_put_after_previous(
-        self,
-        prev: asyncio.Task | None,
-        config: RunnableConfig,
-        checkpoint: Checkpoint,
-        metadata: CheckpointMetadata,
-        new_versions: ChannelVersions,
-    ) -> RunnableConfig:
-        try:
-            if prev is not None:
-                await prev
-        finally:
+    async def _dispatch_checkpoint_request(self, request: CheckpointRequest) -> None:
+        if self.durability == "async" and self._checkpoint_writer is not None:
+            await self._checkpoint_writer.submit(request)
+        else:
             await cast(BaseCheckpointSaver, self.checkpointer).aput(
-                config, checkpoint, metadata, new_versions
+                request.config,
+                request.checkpoint,
+                request.metadata,
+                request.new_versions,
             )
 
+    async def aafter_tick(self) -> None:
+        # compute tick state and save checkpoint
+        if request := self._after_tick():
+            await self._dispatch_checkpoint_request(request)
+        # after execution, check if we should interrupt
+        if self.interrupt_after and should_interrupt(
+            self.checkpoint, self.interrupt_after, self.tasks.values()
+        ):
+            self.status = "interrupt_after"
+            raise GraphInterrupt()
+        # unset resuming flag
+        self.config[CONF].pop(CONFIG_KEY_RESUMING, None)
+
     async def amatch_cached_writes(self) -> Sequence[PregelExecutableTask]:
         if self.cache is None:
             return []
@@ -1385,6 +1397,32 @@ class AsyncPregelLoop(PregelLoop, AbstractAsyncContextManager):
             },
         )
 
+    async def _asuppress_interrupt(
+        self,
+        exc_type: type[BaseException] | None,
+        exc_value: BaseException | None,
+        traceback: TracebackType | None,
+    ) -> bool | None:
+        """Async counterpart of SyncPregelLoop._suppress_interrupt.
+
+        Uses async checkpoint dispatch so we don't block the event loop
+        (the sync version was previously used here via stack.push, which
+        would call the blocking checkpointer.put instead of aput).
+        """
+        # persist current checkpoint and writes
+        if self.durability == "exit" and (
+            # if it's a top graph
+            not self.is_nested
+            # or a nested graph with error or interrupt
+            or exc_value is not None
+            # or a nested graph with checkpointer=True
+            or all(NS_END not in part for part in self.checkpoint_ns)
+        ):
+            if request := self._prepare_checkpoint(self.checkpoint_metadata):
+                await self._dispatch_checkpoint_request(request)
+            self._put_pending_writes()
+        return self._finalize_suppress(exc_type, exc_value)
+
     # context manager
 
     async def __aenter__(self) -> Self:
@@ -1441,20 +1479,26 @@ class AsyncPregelLoop(PregelLoop, AbstractAsyncContextManager):
         self.submit = await self.stack.enter_async_context(
             AsyncBackgroundExecutor(self.config)
         )
+        if self.checkpointer is not None and self.durability == "async":
+            self._checkpoint_writer = await self.stack.enter_async_context(
+                AsyncCheckpointWriter(self.checkpointer.aput)
+            )
         self.channels, self.managed = channels_from_checkpoint(
             self.specs, self.checkpoint
         )
-        self.stack.push(self._suppress_interrupt)
+        self.stack.push_async_exit(self._asuppress_interrupt)
         self.status = "input"
         self.step = self.checkpoint_metadata["step"] + 1
         self.stop = self.step + self.config["recursion_limit"] + 1
         self.checkpoint_previous_versions = self.checkpoint["channel_versions"].copy()
-        self.updated_channels = self._first(
+        self.updated_channels, checkpoint_request = self._first(
             input_keys=self.input_keys,
             updated_channels=set(self.checkpoint.get("updated_channels"))  # type: ignore[arg-type]
             if self.checkpoint.get("updated_channels")
             else None,
         )
+        if checkpoint_request is not None:
+            await self._dispatch_checkpoint_request(checkpoint_request)
 
         return self
 

libs/langgraph/langgraph/pregel/main.py

@@ -2775,9 +2775,6 @@ class Pregel(
                         )
                     loop.after_tick()
                     emit_graph_lifecycle_events(loop)
-                    # wait for checkpoint
-                    if durability_ == "sync":
-                        loop._put_checkpoint_fut.result()
             emit_graph_lifecycle_events(loop)
             # emit output
             yield from _output(
@@ -3196,11 +3193,8 @@ class Pregel(
                                 _state_mapper,
                             ):
                                 yield o
-                        loop.after_tick()
+                        await loop.aafter_tick()
                         await aemit_graph_lifecycle_events(loop)
-                        # wait for checkpoint
-                        if durability_ == "sync":
-                            await cast(asyncio.Future, loop._put_checkpoint_fut)
                 finally:
                     # ensure waiter doesn't remain pending on cancel/shutdown
                     if _cleanup_waiter is not None:
@@ -3715,15 +3709,14 @@ def _coerce_checkpoint_values(payload: Any, mapper: Callable[[Any], Any]) -> Non
 def _build_server_info(
     config: RunnableConfig, parent_runtime: Runtime[Any]
 ) -> ServerInfo | None:
-    """Build ServerInfo from config metadata and configurable.
+    """Build ServerInfo from config configurable.
 
-    The server puts assistant_id/graph_id in config metadata and the
+    The server puts assistant_id/graph_id in config configurable and the
     authenticated user dict in configurable["langgraph_auth_user"].
     """
-    metadata = config.get("metadata") or {}
     configurable = config.get(CONF) or {}
-    assistant_id = metadata.get("assistant_id")
-    graph_id = metadata.get("graph_id")
+    assistant_id = configurable.get("assistant_id")
+    graph_id = configurable.get("graph_id")
 
     # Read authenticated user from configurable (set by LangGraph Server).
     # We prefer isinstance(BaseUser) but fall back to hasattr("identity")

libs/langgraph/tests/test_pregel.py

@@ -54,6 +54,11 @@ from langgraph.pregel import (
     NodeBuilder,
     Pregel,
 )
+from langgraph.pregel._checkpoint_writer import (
+    CHECKPOINT_BACKLOG_ENV_VAR,
+    DEFAULT_CHECKPOINT_BACKLOG,
+    _resolve_checkpoint_backlog,
+)
 from langgraph.pregel._loop import SyncPregelLoop
 from langgraph.pregel._runner import PregelRunner
 from langgraph.types import (
@@ -3726,6 +3731,7 @@ def test_repeat_condition(snapshot: SnapshotAssertion) -> None:
             "end": END,
         },
     )
+
     workflow.add_conditional_edges(
         "Chart Generator",
         router,
@@ -3749,6 +3755,90 @@ def test_repeat_condition(snapshot: SnapshotAssertion) -> None:
     assert app.get_graph().draw_mermaid(with_styles=False) == snapshot
 
 
+def test_sync_durability_applies_checkpoint_backpressure() -> None:
+    first_put_started = threading.Event()
+    release_first_put = threading.Event()
+    put_calls = 0
+    visited: list[int] = []
+    result: dict[str, Any] = {}
+    error: dict[str, BaseException] = {}
+
+    class SlowFirstPutCheckpointer(InMemorySaver):
+        def put(
+            self,
+            config: RunnableConfig,
+            checkpoint: Checkpoint,
+            metadata: CheckpointMetadata,
+            new_versions: Any,
+        ) -> RunnableConfig:
+            nonlocal put_calls
+            put_calls += 1
+            if put_calls == 1:
+                first_put_started.set()
+                release_first_put.wait()
+            return super().put(config, checkpoint, metadata, new_versions)
+
+    class State(TypedDict):
+        counter: int
+
+    def increment(state: State) -> State:
+        visited.append(state["counter"])
+        return {"counter": state["counter"] + 1}
+
+    def should_continue(state: State) -> str:
+        return "loop" if state["counter"] < 4 else "done"
+
+    builder = StateGraph(State)
+    builder.add_node("increment", increment)
+    builder.add_edge(START, "increment")
+    builder.add_conditional_edges(
+        "increment", should_continue, {"loop": "increment", "done": END}
+    )
+
+    graph = builder.compile(checkpointer=SlowFirstPutCheckpointer())
+
+    def invoke() -> None:
+        try:
+            result["value"] = graph.invoke(
+                {"counter": 0},
+                {"configurable": {"thread_id": "1"}},
+                durability="async",
+            )
+        except BaseException as exc:
+            error["value"] = exc
+
+    thread = threading.Thread(target=invoke)
+    thread.start()
+
+    assert first_put_started.wait(timeout=1)
+    time.sleep(0.05)
+
+    assert thread.is_alive()
+    assert len(visited) <= DEFAULT_CHECKPOINT_BACKLOG + 1
+
+    release_first_put.set()
+    thread.join(timeout=1)
+
+    assert not thread.is_alive()
+    assert "value" not in error
+    assert result["value"] == {"counter": 4}
+
+
+def test_checkpoint_backlog_uses_env_override(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv(CHECKPOINT_BACKLOG_ENV_VAR, "7")
+    assert _resolve_checkpoint_backlog() == 7
+
+
+def test_checkpoint_backlog_invalid_env_uses_default(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setenv(CHECKPOINT_BACKLOG_ENV_VAR, "not-an-int")
+    assert _resolve_checkpoint_backlog() == DEFAULT_CHECKPOINT_BACKLOG
+
+    monkeypatch.setenv(CHECKPOINT_BACKLOG_ENV_VAR, "0")
+    assert _resolve_checkpoint_backlog() == DEFAULT_CHECKPOINT_BACKLOG
+
+
 def test_checkpoint_metadata(sync_checkpointer: BaseCheckpointSaver) -> None:
     """This test verifies that a run's configurable fields are merged with the
     previous checkpoint config for each step in the run.

libs/langgraph/tests/test_pregel_async.py

@@ -53,6 +53,7 @@ from langgraph.func import entrypoint, task
 from langgraph.graph import END, START, StateGraph
 from langgraph.graph.message import MessagesState, add_messages
 from langgraph.pregel import NodeBuilder, Pregel
+from langgraph.pregel._checkpoint_writer import DEFAULT_CHECKPOINT_BACKLOG
 from langgraph.pregel._loop import AsyncPregelLoop
 from langgraph.pregel._runner import PregelRunner
 from langgraph.types import (
@@ -484,6 +485,61 @@ async def test_checkpoint_put_after_cancellation_stream_events_anext() -> None:
         assert False, "Task should be cancelled"
 
 
+async def test_async_durability_applies_checkpoint_backpressure() -> None:
+    first_put_started = asyncio.Event()
+    release_first_put = asyncio.Event()
+    put_calls = 0
+    visited: list[int] = []
+
+    class SlowFirstPutCheckpointer(InMemorySaver):
+        async def aput(
+            self,
+            config: RunnableConfig,
+            checkpoint: Checkpoint,
+            metadata: CheckpointMetadata,
+            new_versions: ChannelVersions,
+        ) -> RunnableConfig:
+            nonlocal put_calls
+            put_calls += 1
+            if put_calls == 1:
+                first_put_started.set()
+                await release_first_put.wait()
+            return await super().aput(config, checkpoint, metadata, new_versions)
+
+    class State(TypedDict):
+        counter: int
+
+    def increment(state: State) -> State:
+        visited.append(state["counter"])
+        return {"counter": state["counter"] + 1}
+
+    def should_continue(state: State) -> str:
+        return "loop" if state["counter"] < 4 else "done"
+
+    builder = StateGraph(State)
+    builder.add_node("increment", increment)
+    builder.add_edge(START, "increment")
+    builder.add_conditional_edges(
+        "increment", should_continue, {"loop": "increment", "done": END}
+    )
+
+    graph = builder.compile(checkpointer=SlowFirstPutCheckpointer())
+    task = asyncio.create_task(
+        graph.ainvoke(
+            {"counter": 0}, {"configurable": {"thread_id": "1"}}, durability="async"
+        )
+    )
+
+    await first_put_started.wait()
+    await asyncio.sleep(0.05)
+
+    assert not task.done()
+    assert len(visited) <= DEFAULT_CHECKPOINT_BACKLOG + 1
+
+    release_first_put.set()
+    assert await task == {"counter": 4}
+
+
 async def test_node_cancellation_on_external_cancel() -> None:
     inner_task_cancelled = False
 

libs/langgraph/tests/test_runtime.py

@@ -501,13 +501,13 @@ async def test_execution_info_populated_in_graph_async() -> None:
     assert isinstance(info.node_first_attempt_time, float)
 
 
-def test_server_info_from_metadata() -> None:
-    """server_info is built from assistant_id/graph_id in config metadata."""
+def test_server_info_from_configurable() -> None:
+    """server_info is built from assistant_id/graph_id in config configurable."""
     captured: dict[str, Any] = {}
     compiled = _make_capture_graph(captured)
     compiled.invoke(
         {"message": "hi"},
-        config={"metadata": {"assistant_id": "asst-abc", "graph_id": "my-graph"}},
+        config={"configurable": {"assistant_id": "asst-abc", "graph_id": "my-graph"}},
     )
     si = captured["server_info"]
     assert si is not None
@@ -516,8 +516,8 @@ def test_server_info_from_metadata() -> None:
     assert si.user is None
 
 
-def test_server_info_none_without_metadata() -> None:
-    """server_info is None when no assistant_id/graph_id in metadata."""
+def test_server_info_none_without_configurable() -> None:
+    """server_info is None when no assistant_id/graph_id in configurable."""
     captured: dict[str, Any] = {}
     compiled = _make_capture_graph(captured)
     compiled.invoke({"message": "hi"})
@@ -579,8 +579,11 @@ def test_server_info_user_from_auth_user() -> None:
     compiled.invoke(
         {"message": "hi"},
         config={
-            "configurable": {"langgraph_auth_user": proxy},
-            "metadata": {"assistant_id": "asst-proxy", "graph_id": "graph-proxy"},
+            "configurable": {
+                "langgraph_auth_user": proxy,
+                "assistant_id": "asst-proxy",
+                "graph_id": "graph-proxy",
+            },
         },
     )
     si = captured["server_info"]