fix(langgraph): let error_handler recover on every path and stream failed task results

- A node's error_handler ran, but the original error was still raised when the node ran on the executor path: streaming messages or custom, with subgraphs=True, or next to a parallel node. The runner marks a handled future as such, but the executors re-raised every failed future on exit without checking. The marker set now lives in the executor module, and both executors skip those futures. - With only tasks or debug streaming, a failing node's result was queued but never streamed, because the single-task fast path raised before the caller drained the queue. It now yields once before re-raising. Ideas from #8611 by SnowingFox and #6984 by Br1an67.
feat(cli): add the priority section to langgraph.json (#9393 )
2026-10-24 17:01:45 +02:00 · 2026-10-23 07:26:51 -04:00 · 2026-10-22 17:26:01 -04:00 · 2026-10-22 17:25:38 -04:00 · 2026-10-22 16:27:58 -04:00 · 2026-10-21 21:22:40 +09:00
35 changed files with 924 additions and 72 deletions
@@ -36,11 +36,11 @@ wheels = [

 [[package]]
 name = "certifi"
-version = "2026.4.22"
+version = "2026.10.19"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/25/ee/6caf7a40c36a1220410afe15a1cc64993a1f864871f698c0f93acb72842a/certifi-2026.4.22.tar.gz", hash = "sha256:8d455352a37b71bf76a79caa83a3d6c25afee4a385d632127b6afb3963f1c580", size = 137077, upload-time = "2026-04-22T11:26:11.191Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a6/bd/1436c50cc46699de0230fbaacc1d1de7bfec413176099949e9d5112e88f2/certifi-2026.10.19.tar.gz", hash = "sha256:035c1e85572b141bf1c3a3eff0e33a8eb74917522f2559075ff8d2fea780a1ae", size = 137598, upload-time = "2026-10-20T02:24:16.604Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/22/30/7cd8fdcdfbc5b869528b079bfb76dcdf6056b1a2097a662e5e8c04f42965/certifi-2026.4.22-py3-none-any.whl", hash = "sha256:3cb2210c8f88ba2318d29b0388d1023c8492ff72ecdde4ebdaddbb13a31b1c4a", size = 135707, upload-time = "2026-04-22T11:26:09.372Z" },
+    { url = "https://files.pythonhosted.org/packages/c4/51/29046e48f693c54e5b7412216d447a1d8362c9653148d98c02cdf65fa733/certifi-2026.10.19-py3-none-any.whl", hash = "sha256:42d35340cc5b9ba78e818ddbff0c5b1c2b779de448a340fd1b989f5fa8670e1b", size = 136464, upload-time = "2026-10-20T02:24:15.072Z" },
 ]

 [[package]]
@@ -40,11 +40,11 @@ wheels = [

 [[package]]
 name = "certifi"
-version = "2026.1.4"
+version = "2026.10.19"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/e0/2d/a891ca51311197f6ad14a7ef42e2399f36cf2f9bd44752b3dc4eab60fdc5/certifi-2026.1.4.tar.gz", hash = "sha256:ac726dd470482006e014ad384921ed6438c457018f4b3d204aea4281258b2120", size = 154268, upload-time = "2026-01-04T02:42:41.825Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a6/bd/1436c50cc46699de0230fbaacc1d1de7bfec413176099949e9d5112e88f2/certifi-2026.10.19.tar.gz", hash = "sha256:035c1e85572b141bf1c3a3eff0e33a8eb74917522f2559075ff8d2fea780a1ae", size = 137598, upload-time = "2026-10-20T02:24:16.604Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/e6/ad/3cc14f097111b4de0040c83a525973216457bbeeb63739ef1ed275c1c021/certifi-2026.1.4-py3-none-any.whl", hash = "sha256:9943707519e4add1115f44c2bc244f782c0249876bf51b6599fee1ffbedd685c", size = 152900, upload-time = "2026-01-04T02:42:40.15Z" },
+    { url = "https://files.pythonhosted.org/packages/c4/51/29046e48f693c54e5b7412216d447a1d8362c9653148d98c02cdf65fa733/certifi-2026.10.19-py3-none-any.whl", hash = "sha256:42d35340cc5b9ba78e818ddbff0c5b1c2b779de448a340fd1b989f5fa8670e1b", size = 136464, upload-time = "2026-10-20T02:24:15.072Z" },
 ]

 [[package]]
@@ -1364,6 +1364,17 @@ def test_search_empty_prefix_is_unconstrained(store: SqliteStore) -> None:
    }


+@pytest.mark.parametrize("batched", [False, True])
+def test_search_rejects_nul_namespace_prefix(store: SqliteStore, batched: bool) -> None:
+    store.put(("a",), "key", {"v": 1})
+
+    with pytest.raises(InvalidNamespaceError, match="NUL"):
+        if batched:
+            store.batch([SearchOp(("a\x00",))])
+        else:
+            store.search(("a\x00",))
+
+
 def test_namespace_labels_with_trailing_newline(store: SqliteStore) -> None:
    """Labels may contain newlines, and must not match a differently-named label.

@@ -49,11 +49,11 @@ wheels = [

 [[package]]
 name = "certifi"
-version = "2026.1.4"
+version = "2026.10.19"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/e0/2d/a891ca51311197f6ad14a7ef42e2399f36cf2f9bd44752b3dc4eab60fdc5/certifi-2026.1.4.tar.gz", hash = "sha256:ac726dd470482006e014ad384921ed6438c457018f4b3d204aea4281258b2120", size = 154268, upload-time = "2026-01-04T02:42:41.825Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a6/bd/1436c50cc46699de0230fbaacc1d1de7bfec413176099949e9d5112e88f2/certifi-2026.10.19.tar.gz", hash = "sha256:035c1e85572b141bf1c3a3eff0e33a8eb74917522f2559075ff8d2fea780a1ae", size = 137598, upload-time = "2026-10-20T02:24:16.604Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/e6/ad/3cc14f097111b4de0040c83a525973216457bbeeb63739ef1ed275c1c021/certifi-2026.1.4-py3-none-any.whl", hash = "sha256:9943707519e4add1115f44c2bc244f782c0249876bf51b6599fee1ffbedd685c", size = 152900, upload-time = "2026-01-04T02:42:40.15Z" },
+    { url = "https://files.pythonhosted.org/packages/c4/51/29046e48f693c54e5b7412216d447a1d8362c9653148d98c02cdf65fa733/certifi-2026.10.19-py3-none-any.whl", hash = "sha256:42d35340cc5b9ba78e818ddbff0c5b1c2b779de448a340fd1b989f5fa8670e1b", size = 136464, upload-time = "2026-10-20T02:24:15.072Z" },
 ]

 [[package]]
@@ -1333,6 +1333,10 @@ def _validate_namespace_labels(namespace: tuple[str, ...]) -> None:
            raise InvalidNamespaceError(
                f"Invalid namespace label '{label}' found in {namespace}. Namespace labels cannot contain periods ('.')."
            )
+        elif "\x00" in label:
+            raise InvalidNamespaceError(
+                f"Invalid namespace label '{label}' found in {namespace}. Namespace labels cannot contain NUL bytes ('\\x00')."
+            )
        elif not label:
            raise InvalidNamespaceError(
                f"Namespace labels cannot be empty strings. Got {label} in {namespace}"
@@ -532,7 +532,7 @@ async def test_cannot_put_empty_namespace() -> None:
    assert (await async_store.aget(("valid", "namespace"), "key")) is None


-INVALID_NAMESPACES = [("foo.bar",), ("foo", ""), (123,)]
+INVALID_NAMESPACES = [("foo.bar",), ("foo\x00",), ("foo", ""), (123,)]
 NAMESPACE_METHODS = ["get", "delete", "search", "prefix", "suffix"]


@@ -51,11 +51,11 @@ wheels = [

 [[package]]
 name = "certifi"
-version = "2026.1.4"
+version = "2026.10.19"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/e0/2d/a891ca51311197f6ad14a7ef42e2399f36cf2f9bd44752b3dc4eab60fdc5/certifi-2026.1.4.tar.gz", hash = "sha256:ac726dd470482006e014ad384921ed6438c457018f4b3d204aea4281258b2120", size = 154268, upload-time = "2026-01-04T02:42:41.825Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a6/bd/1436c50cc46699de0230fbaacc1d1de7bfec413176099949e9d5112e88f2/certifi-2026.10.19.tar.gz", hash = "sha256:035c1e85572b141bf1c3a3eff0e33a8eb74917522f2559075ff8d2fea780a1ae", size = 137598, upload-time = "2026-10-20T02:24:16.604Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/e6/ad/3cc14f097111b4de0040c83a525973216457bbeeb63739ef1ed275c1c021/certifi-2026.1.4-py3-none-any.whl", hash = "sha256:9943707519e4add1115f44c2bc244f782c0249876bf51b6599fee1ffbedd685c", size = 152900, upload-time = "2026-01-04T02:42:40.15Z" },
+    { url = "https://files.pythonhosted.org/packages/c4/51/29046e48f693c54e5b7412216d447a1d8362c9653148d98c02cdf65fa733/certifi-2026.10.19-py3-none-any.whl", hash = "sha256:42d35340cc5b9ba78e818ddbff0c5b1c2b779de448a340fd1b989f5fa8670e1b", size = 136464, upload-time = "2026-10-20T02:24:15.072Z" },
 ]

 [[package]]
@@ -23,6 +23,7 @@ from langgraph_cli.schemas import (
    GraphDef,
    HttpConfig,
    IndexConfig,
+    PriorityConfig,
    SecurityConfig,
    SerdeConfig,
    StoreConfig,
@@ -113,6 +114,7 @@ def add_descriptions_to_schema(schema, cls):
                StoreConfig,
                IndexConfig,
                AuthConfig,
+                PriorityConfig,
                SecurityConfig,
                HttpConfig,
                CorsConfig,
@@ -1776,17 +1776,17 @@ baseline-browser-mapping@^2.11.12:
  integrity sha512-H0ulySigv6icDJ1F7SjtdCD6PrhTpdYCmP0CactWy1+ekh0AFd0o1Wn5T8b+hnTmdBx19u9yhL6wvCylXMY7zw==

 brace-expansion@^1.1.7:
-  version "1.1.11"
-  resolved "https://registry.yarnpkg.com/brace-expansion/-/brace-expansion-1.1.11.tgz#3c7fcbf529d87226f3d2f52b966ff5271eb441dd"
-  integrity sha512-iCuPHDFgrHX7H2vEI/5xpz07zSHB00TpugqhmYtVmMO6518mCuRMoOYFldEBl0g187ufozdaHgWKcYFb61qGiA==
+  version "1.1.21"
+  resolved "https://registry.yarnpkg.com/brace-expansion/-/brace-expansion-1.1.21.tgz#edf4fab5c64d051aea5a8def49aba1c7522279f3"
+  integrity sha512-9zeA+KLZNNzglF2TPKRQEDyx6Yby7daAkuy8MiPzpXPsYDWi/DRM8jmwUDxokQjYqBpv5DgPiwD4h4ZZSy1Ujw==
  dependencies:
    balanced-match "^1.0.0"
    concat-map "0.0.1"

 brace-expansion@^2.0.2:
-  version "2.0.2"
-  resolved "https://registry.yarnpkg.com/brace-expansion/-/brace-expansion-2.0.2.tgz#54fc53237a613d854c7bd37463aad17df87214e7"
-  integrity sha512-Jt0vHyM+jmUBqojB7E1NIYadt0vI0Qxjxd2TErW94wDz+E2LAm5vKMXXwg6ZZBTHPuUlDgQHKXvjGBdfcF1ZDQ==
+  version "2.1.7"
+  resolved "https://registry.yarnpkg.com/brace-expansion/-/brace-expansion-2.1.7.tgz#14ea3836731823bb24740225ea43b941b436f9d1"
+  integrity sha512-uZbew1NqdmPDTMJ8ah1y+b+9QEJrfkXFk3RcTQw3X0jW/xRUvFKsg1CfQdSYGdTbXZWExtU3J3ccxtnfw1Fi0g==
  dependencies:
    balanced-match "^1.0.0"

@@ -581,9 +581,9 @@ base64-js@^1.5.1:
  integrity sha512-AKpaYlHn8t4SVbOHCy+b5+KKgvR4vrsD8vbvrbiQJps7fKDTkjkDry6ji0rUJjC0kzbNePLwzxq8iypo41qeWA==

 brace-expansion@^1.1.7:
-  version "1.1.12"
-  resolved "https://registry.yarnpkg.com/brace-expansion/-/brace-expansion-1.1.12.tgz#ab9b454466e5a8cc3a187beaad580412a9c5b843"
-  integrity sha512-9T9UjW3r0UW5c1Q7GTwllptXwhvYmEzFhzMfZ9H7FQWt+uZePjZPjBP/W1ZEyZ1twGWom5/56TF4lPcqjnDHcg==
+  version "1.1.21"
+  resolved "https://registry.yarnpkg.com/brace-expansion/-/brace-expansion-1.1.21.tgz#edf4fab5c64d051aea5a8def49aba1c7522279f3"
+  integrity sha512-9zeA+KLZNNzglF2TPKRQEDyx6Yby7daAkuy8MiPzpXPsYDWi/DRM8jmwUDxokQjYqBpv5DgPiwD4h4ZZSy1Ujw==
  dependencies:
    balanced-match "^1.0.0"
    concat-map "0.0.1"
@@ -847,6 +847,7 @@ def dev(
        store=config_json.get("store"),
        wait_for_client=wait_for_client,
        auth=config_json.get("auth"),
+        priority=config_json.get("priority"),
        http=config_json.get("http"),
        ui=config_json.get("ui"),
        ui_config=config_json.get("ui_config"),
@@ -442,6 +442,7 @@ def validate_config(
        "store": config.get("store"),
        "auth": config.get("auth"),
        "encryption": config.get("encryption"),
+        "priority": config.get("priority"),
        "http": config.get("http"),
        # Pass through webhooks config so it can be injected into the image
        "webhooks": config.get("webhooks"),
@@ -606,6 +607,19 @@ def validate_config(
                    f"Invalid encryption.path format: '{encryption_conf['path']}'. "
                    "Must be in format './path/to/file.py:attribute_name'"
                )
+    # Validate priority config
+    if priority_conf := config.get("priority"):
+        if "path" in priority_conf:
+            if ":" not in priority_conf["path"]:
+                raise ValueError(
+                    f"Invalid priority.path format: '{priority_conf['path']}'. "
+                    "Must be in format './path/to/file.py:attribute_name'"
+                )
+            if _is_node_graph(priority_conf["path"]):
+                raise ValueError(
+                    "JavaScript priority callbacks are not supported: "
+                    f"'{priority_conf['path']}'. Use a Python file or module instead."
+                )
    if http_conf := config.get("http"):
        if "app" in http_conf:
            if ":" not in http_conf["app"]:
@@ -649,6 +663,7 @@ _KNOWN_CONFIG_KEYS = {
    "store",
    "auth",
    "encryption",
+    "priority",
    "http",
    "webhooks",
    "checkpointer",
@@ -1080,6 +1095,49 @@ def _update_encryption_path(
    )


+def _update_priority_path(
+    config_path: pathlib.Path, config: Config, local_deps: LocalDeps
+) -> None:
+    """Update priority.path to use Docker container paths."""
+    priority_conf = config.get("priority")
+    if not priority_conf or not (path_str := priority_conf.get("path")):
+        return
+
+    module_str, sep, attr_str = path_str.partition(":")
+    if not sep or not module_str.startswith("."):
+        return  # Already validated or absolute path
+
+    resolved = config_path.parent / module_str
+    if not resolved.exists():
+        raise FileNotFoundError(
+            f"Priority file not found: {resolved} (from {path_str})"
+        )
+    if not resolved.is_file():
+        raise IsADirectoryError(f"Priority path must be a file: {resolved}")
+
+    # Check faux packages first (they take precedence)
+    for faux_path, (_, destpath) in local_deps.faux_pkgs.items():
+        if resolved.is_relative_to(faux_path):
+            new_path = f"{destpath}/{resolved.relative_to(faux_path)}:{attr_str}"
+            priority_conf["path"] = new_path
+            return
+
+    # Check real packages
+    for real_path in local_deps.real_pkgs:
+        if resolved.is_relative_to(real_path):
+            new_path = (
+                f"/deps/{real_path.name}/{resolved.relative_to(real_path)}:{attr_str}"
+            )
+            priority_conf["path"] = new_path
+            return
+
+    raise ValueError(
+        f"Priority file '{resolved}' not covered by dependencies.\n"
+        "Add its parent directory to the 'dependencies' array in your config.\n"
+        f"Current dependencies: {config['dependencies']}"
+    )
+
+
 def _update_checkpointer_path(
    config_path: pathlib.Path, config: Config, local_deps: LocalDeps
 ) -> None:
@@ -1213,6 +1271,9 @@ def _build_runtime_env_vars(config: Config) -> list[str]:
    if (encryption_config := config.get("encryption")) is not None:
        env_vars.append(f"ENV LANGGRAPH_ENCRYPTION='{json.dumps(encryption_config)}'")

+    if (priority_config := config.get("priority")) is not None:
+        env_vars.append(f"ENV LANGGRAPH_PRIORITY='{json.dumps(priority_config)}'")
+
    if (http_config := config.get("http")) is not None:
        env_vars.append(f"ENV LANGGRAPH_HTTP='{json.dumps(http_config)}'")

@@ -1380,6 +1441,8 @@ def python_config_to_docker(
    _update_auth_path(config_path, config, local_deps)
    # Rewrite encryption path, so it points to the correct location in the Docker container
    _update_encryption_path(config_path, config, local_deps)
+    # Rewrite priority path, so it points to the correct location in the Docker container
+    _update_priority_path(config_path, config, local_deps)
    # Rewrite checkpointer path, so it points to the correct location in the Docker container
    _update_checkpointer_path(config_path, config, local_deps)
    # Rewrite HTTP app path, so it points to the correct location in the Docker container
@@ -373,6 +373,27 @@ class EncryptionConfig(TypedDict, total=False):
    """


+class PriorityConfig(TypedDict, total=False):
+    """Configuration for custom run priority logic.
+
+    Allows you to decide each thread's priority settings with a Python function
+    that the server calls at the first run on a thread with no settings, which
+    can come long after the thread was created."""
+
+    path: str
+    """Required. Path to an instance of the Priority() class that registers an assign function.
+
+    Format: "path/to/file.py:my_priority"
+
+    Example:
+        {
+            "priority": {
+                "path": "./priority.py:my_priority"
+            }
+        }
+    """
+
+
 class CorsConfig(TypedDict, total=False):
    """Specifies Cross-Origin Resource Sharing (CORS) rules for your server.

@@ -753,6 +774,13 @@ class Config(TypedDict, total=False):
    Allows you to implement custom encryption for sensitive data stored in the database.
    """

+    priority: PriorityConfig | None
+    """Optional. Custom run priority config, including the path to your Python priority logic.
+
+    Allows you to decide each thread's priority settings at the first run on a thread with no settings,
+    which can come long after the thread was created.
+    """
+
    http: HttpConfig | None
    """Optional. Configuration for the built-in HTTP server, controlling which custom routes are exposed
    and how cross-origin requests are handled.
@@ -784,6 +812,7 @@ __all__ = [
    "CheckpointerConfig",
    "AuthConfig",
    "EncryptionConfig",
+    "PriorityConfig",
    "HttpConfig",
    "MiddlewareOrders",
    "Distros",
@@ -910,6 +910,7 @@ def python_config_to_docker_uv_lock(
    for section, key in [
        ("auth", "path"),
        ("encryption", "path"),
+        ("priority", "path"),
        ("checkpointer", "path"),
        ("http", "app"),
    ]:
@@ -196,6 +196,17 @@
                }
              ]
            },
+            "priority": {
+              "anyOf": [
+                {
+                  "$ref": "#/$defs/PriorityConfig"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "Optional. Custom run priority config, including the path to your Python priority logic.\n\nAllows you to decide each thread's priority settings at the first run on a thread with no settings,\nwhich can come long after the thread was created.\n"
+            },
            "source": {
              "anyOf": [
                {
@@ -438,6 +449,17 @@
                }
              ]
            },
+            "priority": {
+              "anyOf": [
+                {
+                  "$ref": "#/$defs/PriorityConfig"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "Optional. Custom run priority config, including the path to your Python priority logic.\n\nAllows you to decide each thread's priority settings at the first run on a thread with no settings,\nwhich can come long after the thread was created.\n"
+            },
            "source": {
              "$ref": "#/$defs/UvSource"
            },
@@ -666,6 +688,17 @@
                }
              ]
            },
+            "priority": {
+              "anyOf": [
+                {
+                  "$ref": "#/$defs/PriorityConfig"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "Optional. Custom run priority config, including the path to your Python priority logic.\n\nAllows you to decide each thread's priority settings at the first run on a thread with no settings,\nwhich can come long after the thread was created.\n"
+            },
            "source": {
              "anyOf": [
                {
@@ -1170,6 +1203,18 @@
      },
      "required": []
    },
+    "PriorityConfig": {
+      "title": "PriorityConfig",
+      "description": "Configuration for custom run priority logic.\n\nAllows you to decide each thread's priority settings with a Python function\nthat the server calls at the first run on a thread with no settings, which\ncan come long after the thread was created.",
+      "type": "object",
+      "properties": {
+        "path": {
+          "type": "string",
+          "description": "Required. Path to an instance of the Priority() class that registers an assign function.\n"
+        }
+      },
+      "required": []
+    },
    "UvSource": {
      "title": "UvSource",
      "description": "Deployment source rooted at a uv project or workspace.",
@@ -196,6 +196,17 @@
                }
              ]
            },
+            "priority": {
+              "anyOf": [
+                {
+                  "$ref": "#/$defs/PriorityConfig"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "Optional. Custom run priority config, including the path to your Python priority logic.\n\nAllows you to decide each thread's priority settings at the first run on a thread with no settings,\nwhich can come long after the thread was created.\n"
+            },
            "source": {
              "anyOf": [
                {
@@ -438,6 +449,17 @@
                }
              ]
            },
+            "priority": {
+              "anyOf": [
+                {
+                  "$ref": "#/$defs/PriorityConfig"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "Optional. Custom run priority config, including the path to your Python priority logic.\n\nAllows you to decide each thread's priority settings at the first run on a thread with no settings,\nwhich can come long after the thread was created.\n"
+            },
            "source": {
              "$ref": "#/$defs/UvSource"
            },
@@ -666,6 +688,17 @@
                }
              ]
            },
+            "priority": {
+              "anyOf": [
+                {
+                  "$ref": "#/$defs/PriorityConfig"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "Optional. Custom run priority config, including the path to your Python priority logic.\n\nAllows you to decide each thread's priority settings at the first run on a thread with no settings,\nwhich can come long after the thread was created.\n"
+            },
            "source": {
              "anyOf": [
                {
@@ -1170,6 +1203,18 @@
      },
      "required": []
    },
+    "PriorityConfig": {
+      "title": "PriorityConfig",
+      "description": "Configuration for custom run priority logic.\n\nAllows you to decide each thread's priority settings with a Python function\nthat the server calls at the first run on a thread with no settings, which\ncan come long after the thread was created.",
+      "type": "object",
+      "properties": {
+        "path": {
+          "type": "string",
+          "description": "Required. Path to an instance of the Priority() class that registers an assign function.\n"
+        }
+      },
+      "required": []
+    },
    "UvSource": {
      "title": "UvSource",
      "description": "Deployment source rooted at a uv project or workspace.",
@@ -2,10 +2,12 @@ import json
 import pathlib
 import re
 import shutil
+import sys
 import tempfile
 import textwrap
 from contextlib import contextmanager
 from pathlib import Path
+from unittest.mock import MagicMock

 import click
 from click.testing import CliRunner
@@ -371,6 +373,32 @@ def test_dev_command_requires_ssl_certfile_and_keyfile_together(tmp_path) -> Non
    assert "Both --ssl-certfile and --ssl-keyfile must be provided" in result.output


+def test_dev_command_passes_priority_to_the_server(tmp_path, monkeypatch) -> None:
+    config_path = tmp_path / "langgraph.json"
+    config_path.write_text(
+        json.dumps(
+            {
+                "dependencies": ["."],
+                "graphs": {"agent": "./agent.py:graph"},
+                "priority": {"path": "./priority.py:priority"},
+            }
+        ),
+        encoding="utf-8",
+    )
+    # The test environment doesn't install langgraph-api, so stand in for its server.
+    server = MagicMock()
+    monkeypatch.setitem(sys.modules, "langgraph_api.cli", server)
+    # dev appends the working directory and its dependencies to sys.path.
+    monkeypatch.setattr(sys, "path", list(sys.path))
+
+    result = CliRunner().invoke(cli, ["dev", "--config", str(config_path)])
+
+    assert result.exit_code == 0, result.output
+    assert server.run_server.call_args.kwargs["priority"] == {
+        "path": "./priority.py:priority"
+    }
+
+
 def test_deploy_list_command(monkeypatch) -> None:
    runner = CliRunner()
    captured: dict[str, str] = {}
@@ -16,6 +16,7 @@ from langgraph_cli.config import (
    config_to_docker,
    default_base_image,
    docker_tag,
+    get_unknown_keys,
    has_disallowed_build_command_content,
    validate_config,
    validate_config_file,
@@ -155,6 +156,7 @@ def test_validate_config():
        "store": None,
        "auth": None,
        "encryption": None,
+        "priority": None,
        "webhooks": None,
        "checkpointer": None,
        "http": None,
@@ -184,6 +186,7 @@ def test_validate_config():
        "store": None,
        "auth": None,
        "encryption": None,
+        "priority": None,
        "webhooks": None,
        "checkpointer": None,
        "http": None,
@@ -1289,6 +1292,109 @@ def test_config_to_docker_python_encryption_formatted():
    )


+@pytest.mark.parametrize(
+    "path", ["./priority.py:priority", "my_package.priority:priority"]
+)
+def test_config_to_docker_python_priority(path: str):
+    # Test that priority config is kept by validation and isn't an unknown key
+    raw_config = {
+        "python_version": "3.11",
+        "graphs": {"agent": "./agent.py:graph"},
+        "dependencies": ["."],
+        "priority": {"path": path},
+    }
+    validated = validate_config(raw_config)
+
+    assert validated["priority"] == {"path": path}
+    assert get_unknown_keys(raw_config) == []
+
+
+def test_config_to_docker_python_priority_bad_path():
+    # Test that invalid priority path format raises ValueError
+    with pytest.raises(ValueError, match="Invalid priority.path format"):
+        validate_config(
+            {
+                "python_version": "3.11",
+                "graphs": {"agent": "./agent.py:graph"},
+                "dependencies": ["."],
+                "priority": {"path": "./priority.py"},  # Missing :attribute
+            }
+        )
+
+
+@pytest.mark.parametrize("ext", [".ts", ".mts", ".cts", ".js", ".mjs", ".cjs"])
+def test_config_to_docker_python_priority_js_path(ext: str):
+    # Test that a JavaScript priority path is rejected
+    with pytest.raises(ValueError, match="JavaScript priority callbacks"):
+        validate_config(
+            {
+                "python_version": "3.11",
+                "graphs": {"agent": "./agent.py:graph"},
+                "dependencies": ["."],
+                "priority": {"path": f"./priority{ext}:priority"},
+            }
+        )
+
+
+def test_config_to_docker_nodejs_priority():
+    # Test that a Node.js-only project can name a Python priority file
+    actual_docker_stdin, _ = config_to_docker(
+        PATH_TO_CONFIG,
+        validate_config(
+            {
+                "node_version": "20",
+                "graphs": {"agent": "./graphs/agent.js:graph"},
+                "priority": {"path": "./priority.py:priority"},
+            }
+        ),
+        base_image="langchain/langgraphjs-api",
+    )
+    assert (
+        """ENV LANGGRAPH_PRIORITY='{"path": "./priority.py:priority"}'"""
+        in actual_docker_stdin
+    )
+
+
+def test_config_to_docker_python_priority_formatted():
+    # Test that priority config is written to the Docker output with its container path
+    actual_docker_stdin, _ = config_to_docker(
+        PATH_TO_CONFIG,
+        validate_config(
+            {
+                "python_version": "3.11",
+                "dependencies": ["."],
+                "graphs": {"agent": "./graphs/agent.py:graph"},
+                "priority": {"path": "./agent.py:my_priority"},
+            }
+        ),
+        base_image="langchain/langgraph-api",
+    )
+    assert (
+        "ENV LANGGRAPH_PRIORITY="
+        """'{"path": "/deps/outer-unit_tests/unit_tests/agent.py:my_priority"}'"""
+        in actual_docker_stdin
+    )
+
+
+def test_config_to_docker_python_priority_not_in_dependencies():
+    # Test that a priority file outside the dependencies raises ValueError
+    with pytest.raises(
+        ValueError, match="Priority file .* not covered by dependencies"
+    ):
+        config_to_docker(
+            PATH_TO_CONFIG,
+            validate_config(
+                {
+                    "python_version": "3.11",
+                    "dependencies": ["./graphs"],
+                    "graphs": {"agent": "./graphs/agent.py:graph"},
+                    "priority": {"path": "./agent.py:my_priority"},
+                }
+            ),
+            base_image="langchain/langgraph-api",
+        )
+
+
 def test_config_to_docker_nodejs_internal_docker_tag():
    graphs = {"agent": "./graphs/agent.js:graph"}
    actual_docker_stdin, additional_contexts = config_to_docker(
@@ -1731,6 +1837,38 @@ def test_config_to_docker_uv_lock_honors_root_workspace_sources():
        )


+def test_config_to_docker_uv_lock_rewrites_priority_path():
+    with tempfile.TemporaryDirectory() as tmpdir:
+        tmpdir_path = pathlib.Path(tmpdir)
+        project_root, config_path = _write_uv_lock_workspace(
+            tmpdir_path,
+            agent_sources="[tool.uv.sources]\nshared = { workspace = true }",
+        )
+        (project_root / "libs" / "shared" / "src" / "shared" / "priority.py").touch()
+
+        config = validate_config(
+            {
+                "python_version": "3.11",
+                "graphs": {
+                    "agent": "../../apps/agent/src/agent/graph.py:graph",
+                },
+                "source": {"kind": "uv", "root": "../..", "package": "agent"},
+                "priority": {
+                    "path": "../../libs/shared/src/shared/priority.py:priority"
+                },
+            }
+        )
+        docker, _ = config_to_docker(
+            config_path, config, base_image="langchain/langgraph-api:0.2.47"
+        )
+
+        assert (
+            "ENV LANGGRAPH_PRIORITY="
+            """'{"path": "/deps/workspace/libs/shared/src/shared/priority.py:priority"}'"""
+            in docker
+        )
+
+
 def test_config_to_docker_uv_lock_ignores_unrelated_workspace_package_sources():
    with tempfile.TemporaryDirectory() as tmpdir:
        tmpdir_path = pathlib.Path(tmpdir)
@@ -52,11 +52,11 @@ wheels = [

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+version = "2026.10.19"
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+    { url = "https://files.pythonhosted.org/packages/c4/51/29046e48f693c54e5b7412216d447a1d8362c9653148d98c02cdf65fa733/certifi-2026.10.19-py3-none-any.whl", hash = "sha256:42d35340cc5b9ba78e818ddbff0c5b1c2b779de448a340fd1b989f5fa8670e1b", size = 136464, upload-time = "2026-10-20T02:24:15.072Z" },
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 [[package]]
@@ -3,6 +3,7 @@ from __future__ import annotations
 import asyncio
 import concurrent.futures
 import time
+import weakref
 from collections.abc import Awaitable, Callable, Coroutine
 from contextlib import AbstractAsyncContextManager, AbstractContextManager, ExitStack
 from contextvars import copy_context
@@ -37,6 +38,13 @@ class Submit(Protocol[P, T]):
    ) -> concurrent.futures.Future[T]: ...


+# Futures whose exception a node's error handler took over; the executors
+# don't re-raise them on exit.
+SKIP_RERAISE_SET: weakref.WeakSet[concurrent.futures.Future | asyncio.Future] = (
+    weakref.WeakSet()
+)
+
+
 class BackgroundExecutor(AbstractContextManager):
    """A context manager that runs sync tasks in the background.
    Uses a thread pool executor to delegate tasks to separate threads.
@@ -111,7 +119,7 @@ class BackgroundExecutor(AbstractContextManager):
        if exc_type is None:
            # re-raise the first exception that occurred in a task
            for task, (_, reraise) in tasks.items():
-                if not reraise:
+                if not reraise or task in SKIP_RERAISE_SET:
                    continue
                try:
                    task.result()
@@ -202,7 +210,7 @@ class AsyncBackgroundExecutor(AbstractAsyncContextManager):
        if exc_type is None:
            # re-raise the first exception that occurred in a task
            for task, (_, reraise) in tasks.items():
-                if not reraise:
+                if not reraise or task in SKIP_RERAISE_SET:
                    continue
                try:
                    if exc := task.exception():
@@ -43,7 +43,7 @@ from langgraph._internal._typing import MISSING
 from langgraph.constants import TAG_HIDDEN
 from langgraph.errors import GraphBubbleUp, GraphInterrupt, is_invalid_resume
 from langgraph.pregel._algo import Call
-from langgraph.pregel._executor import Submit
+from langgraph.pregel._executor import SKIP_RERAISE_SET, Submit
 from langgraph.pregel._retry import arun_with_retry, run_with_retry
 from langgraph.pregel._task_status import CONTROL_WRITES
 from langgraph.types import (
@@ -68,10 +68,6 @@ EXCLUDED_FRAME_FNAMES = (
    "concurrent/futures/_base.py",
 )

-SKIP_RERAISE_SET: weakref.WeakSet[concurrent.futures.Future | asyncio.Future] = (
-    weakref.WeakSet()
-)
-

 class FuturesDict(Generic[F, E], dict[F, PregelExecutableTask | None]):
    event: E
@@ -239,6 +235,8 @@ class PregelRunner:
                        fut.set_exception(exc)
                        futures.done.add(fut)
                elif reraise and id(exc) not in self._handled_exception_ids:
+                    # let the caller stream this task's result before it raises
+                    yield
                    if tb := exc.__traceback__:
                        while tb.tb_next is not None and any(
                            tb.tb_frame.f_code.co_filename.endswith(name)
@@ -430,6 +428,8 @@ class PregelRunner:
                        fut.set_exception(exc)
                        futures.done.add(fut)
                elif reraise and id(exc) not in self._handled_exception_ids:
+                    # let the caller stream this task's result before it raises
+                    yield
                    if tb := exc.__traceback__:
                        while tb.tb_next is not None and any(
                            tb.tb_frame.f_code.co_filename.endswith(name)
@@ -4346,6 +4346,25 @@ def test_enum_node_names():
    assert graph.invoke({"foo": "hello"}) == {"foo": "hello", "bar": "hello!"}


+@pytest.mark.parametrize("stream_mode", ["tasks", "debug"])
+def test_failing_task_result_is_streamed_before_the_error(stream_mode: str) -> None:
+    class State(TypedDict):
+        x: str
+
+    def failing(state: State) -> State:
+        raise ValueError("boom")
+
+    graph = StateGraph(State).add_node("failing", failing).add_edge(START, "failing")
+    errors = []
+    with pytest.raises(ValueError, match="boom"):
+        for chunk in graph.compile().stream({"x": ""}, stream_mode=stream_mode):
+            payload = chunk["payload"] if stream_mode == "debug" else chunk
+            if "error" in payload:
+                errors.append(payload["error"])
+
+    assert [type(error) for error in errors] == [ValueError]
+
+
 def test_debug_retry(sync_checkpointer: BaseCheckpointSaver):
    class State(TypedDict):
        messages: Annotated[list[str], operator.add]
@@ -5809,6 +5809,60 @@ async def test_store_injected_async(
    )  # still overwriting the same one


+@pytest.mark.parametrize("stream_mode", ["tasks", "debug"])
+async def test_failing_task_result_is_streamed_before_the_error(
+    stream_mode: str,
+) -> None:
+    class State(TypedDict):
+        x: str
+
+    async def failing(state: State) -> State:
+        raise ValueError("boom")
+
+    graph = StateGraph(State).add_node("failing", failing).add_edge(START, "failing")
+    errors = []
+    with pytest.raises(ValueError, match="boom"):
+        async for chunk in graph.compile().astream({"x": ""}, stream_mode=stream_mode):
+            payload = chunk["payload"] if stream_mode == "debug" else chunk
+            if "error" in payload:
+                errors.append(payload["error"])
+
+    assert [type(error) for error in errors] == [ValueError]
+
+
+@pytest.mark.parametrize(
+    ("parallel", "stream_mode"), [(False, "custom"), (True, "values")]
+)
+async def test_graph_error_handler_recovers_on_the_executor_path(
+    parallel: bool, stream_mode: str
+) -> None:
+    class State(TypedDict):
+        x: str
+
+    async def failing(state: State) -> State:
+        raise ValueError("boom")
+
+    async def handler(state: State) -> Command:
+        return Command(update={"x": "handled"}, goto=END)
+
+    builder = (
+        StateGraph(State)
+        .add_node("failing", failing, error_handler=handler)
+        .add_edge(START, "failing")
+    )
+    if parallel:
+        builder.add_node("sibling", lambda state: {}).add_edge(START, "sibling")
+
+    chunks = [
+        chunk
+        async for chunk in builder.compile().astream(
+            {"x": ""}, stream_mode=[stream_mode, "values"]
+        )
+    ]
+
+    assert [c[-1] for c in chunks if c[0] == "values"][-1] == {"x": "handled"}
+
+
 async def test_debug_retry(async_checkpointer: BaseCheckpointSaver):
    class State(TypedDict):
        messages: Annotated[list[str], operator.add]
@@ -2097,6 +2097,47 @@ def test_graph_error_handler_can_route_with_command():
    assert result["foo"] == "handled_next"


+# Each case runs the failing node on the executor path instead of the
+# single-task fast path.
+@pytest.mark.parametrize(
+    ("parallel", "stream_mode", "subgraphs"),
+    [
+        (False, "custom", False),
+        (False, "messages", False),
+        (False, "values", True),
+        (True, "values", False),
+    ],
+)
+def test_graph_error_handler_recovers_on_the_executor_path(
+    parallel: bool, stream_mode: str, subgraphs: bool
+) -> None:
+    class State(TypedDict):
+        x: str
+
+    def failing(state: State) -> State:
+        raise ValueError("boom")
+
+    def handler(state: State) -> Command:
+        return Command(update={"x": "handled"}, goto=END)
+
+    builder = (
+        StateGraph(State)
+        .add_node("failing", failing, error_handler=handler)
+        .add_edge(START, "failing")
+    )
+    if parallel:
+        builder.add_node("sibling", lambda state: {}).add_edge(START, "sibling")
+    graph = builder.compile()
+
+    chunks = list(
+        graph.stream(
+            {"x": ""}, stream_mode=[stream_mode, "values"], subgraphs=subgraphs
+        )
+    )
+
+    assert [c[-1] for c in chunks if c[-2] == "values"][-1] == {"x": "handled"}
+
+
 def test_graph_error_handler_failure_fails_run():
    class State(TypedDict):
        foo: str
@@ -202,11 +202,11 @@ wheels = [

 [[package]]
 name = "certifi"
-version = "2026.1.4"
+version = "2026.10.19"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/e0/2d/a891ca51311197f6ad14a7ef42e2399f36cf2f9bd44752b3dc4eab60fdc5/certifi-2026.1.4.tar.gz", hash = "sha256:ac726dd470482006e014ad384921ed6438c457018f4b3d204aea4281258b2120", size = 154268, upload-time = "2026-01-04T02:42:41.825Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a6/bd/1436c50cc46699de0230fbaacc1d1de7bfec413176099949e9d5112e88f2/certifi-2026.10.19.tar.gz", hash = "sha256:035c1e85572b141bf1c3a3eff0e33a8eb74917522f2559075ff8d2fea780a1ae", size = 137598, upload-time = "2026-10-20T02:24:16.604Z" }
 wheels = [
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+    { url = "https://files.pythonhosted.org/packages/c4/51/29046e48f693c54e5b7412216d447a1d8362c9653148d98c02cdf65fa733/certifi-2026.10.19-py3-none-any.whl", hash = "sha256:42d35340cc5b9ba78e818ddbff0c5b1c2b779de448a340fd1b989f5fa8670e1b", size = 136464, upload-time = "2026-10-20T02:24:15.072Z" },
 ]

 [[package]]
@@ -1919,7 +1919,7 @@ requires-dist = [
    { name = "langchain-core", specifier = ">=1.4.0,<2" },
    { name = "langchain-protocol", specifier = ">=0.0.15" },
    { name = "orjson", specifier = ">=3.11.5" },
-    { name = "websockets", specifier = ">=14,<17" },
+    { name = "websockets", specifier = ">=14" },
 ]

 [package.metadata.requires-dev]
@@ -49,11 +49,11 @@ wheels = [

 [[package]]
 name = "certifi"
-version = "2026.1.4"
+version = "2026.10.19"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/e0/2d/a891ca51311197f6ad14a7ef42e2399f36cf2f9bd44752b3dc4eab60fdc5/certifi-2026.1.4.tar.gz", hash = "sha256:ac726dd470482006e014ad384921ed6438c457018f4b3d204aea4281258b2120", size = 154268, upload-time = "2026-01-04T02:42:41.825Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a6/bd/1436c50cc46699de0230fbaacc1d1de7bfec413176099949e9d5112e88f2/certifi-2026.10.19.tar.gz", hash = "sha256:035c1e85572b141bf1c3a3eff0e33a8eb74917522f2559075ff8d2fea780a1ae", size = 137598, upload-time = "2026-10-20T02:24:16.604Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/e6/ad/3cc14f097111b4de0040c83a525973216457bbeeb63739ef1ed275c1c021/certifi-2026.1.4-py3-none-any.whl", hash = "sha256:9943707519e4add1115f44c2bc244f782c0249876bf51b6599fee1ffbedd685c", size = 152900, upload-time = "2026-01-04T02:42:40.15Z" },
+    { url = "https://files.pythonhosted.org/packages/c4/51/29046e48f693c54e5b7412216d447a1d8362c9653148d98c02cdf65fa733/certifi-2026.10.19-py3-none-any.whl", hash = "sha256:42d35340cc5b9ba78e818ddbff0c5b1c2b779de448a340fd1b989f5fa8670e1b", size = 136464, upload-time = "2026-10-20T02:24:15.072Z" },
 ]

 [[package]]
@@ -614,7 +614,7 @@ requires-dist = [
    { name = "langchain-core", specifier = ">=1.4.0,<2" },
    { name = "langchain-protocol", specifier = ">=0.0.15" },
    { name = "orjson", specifier = ">=3.11.5" },
-    { name = "websockets", specifier = ">=14,<17" },
+    { name = "websockets", specifier = ">=14" },
 ]

 [package.metadata.requires-dev]
@@ -34,6 +34,14 @@
 - **`thread.output`** — awaitable that resolves to the terminal thread state
  `values` dict after the run lifecycle completes.

+- **`Priority` (beta)** — register an async function with `@priority.assign`
+  and name the object in the `priority` section of `langgraph.json`. The
+  server calls the function with a `PriorityContext` at the first run on a
+  thread with no settings and stores the `PrioritySettings` it returns
+  (`priority`, `fairness_group`, `fairness_weight`) on the thread. Requires
+  `langgraph-api` 0.18.0 or later and a `langgraph-cli` that supports the
+  section; older versions of either ignore it.
+
 ### Changed

 - `client.threads.stream()` now accepts `transport="sse"` (default) or
@@ -2,6 +2,8 @@ from langgraph_sdk.auth import Auth
 from langgraph_sdk.client import get_client, get_sync_client
 from langgraph_sdk.encryption import Encryption
 from langgraph_sdk.encryption.types import DecryptResult, EncryptionContext
+from langgraph_sdk.priority import Priority
+from langgraph_sdk.priority.types import PriorityContext, PrioritySettings

 __version__ = "0.4.8"

@@ -10,6 +12,9 @@ __all__ = [
    "DecryptResult",
    "Encryption",
    "EncryptionContext",
+    "Priority",
+    "PriorityContext",
+    "PrioritySettings",
    "get_client",
    "get_sync_client",
 ]
@@ -0,0 +1,155 @@
+"""Custom run priority for LangGraph.
+
+.. warning::
+    This API is in beta and may change in future versions.
+
+This module lets developers decide the priority settings of each thread with
+an async function that the server calls at the first run on a thread with no
+settings. The function is registered on an object that `langgraph.json` names,
+and runs server-side.
+"""
+
+from __future__ import annotations
+
+import functools
+import inspect
+import typing
+import warnings
+from collections.abc import Awaitable, Callable
+
+from langgraph_sdk.encryption import DuplicateHandlerError, LangGraphBetaWarning
+from langgraph_sdk.priority import types
+
+_AssignHandler = Callable[
+    [types.PriorityContext], Awaitable[types.PrioritySettings | None]
+]
+_AssignHandlerT = typing.TypeVar("_AssignHandlerT", bound=_AssignHandler)
+
+
+@functools.lru_cache(maxsize=1)
+def _warn_priority_beta() -> None:
+    warnings.warn(
+        "The Priority API is in beta and may change in future versions.",
+        LangGraphBetaWarning,
+        stacklevel=4,
+    )
+
+
+def _validate_handler(fn: typing.Callable) -> None:
+    """Validate that the assign handler is async and takes one parameter.
+
+    Raises:
+        TypeError: If the handler is not an async function or has wrong parameter count
+    """
+    if not inspect.iscoroutinefunction(fn):
+        raise TypeError(
+            f"Priority assign handler must be an async function, got {type(fn)}"
+        )
+
+    sig = inspect.signature(fn)
+    params = [
+        p
+        for p in sig.parameters.values()
+        if p.kind in (p.POSITIONAL_ONLY, p.POSITIONAL_OR_KEYWORD)
+    ]
+    if len(params) != 1:
+        raise TypeError(
+            "Priority assign handler must accept exactly 1 parameter (ctx), "
+            f"got {len(params)}"
+        )
+
+
+class Priority:
+    """Decide the priority settings of your LangGraph application's threads.
+
+    .. warning::
+        This API is in beta and may change in future versions.
+
+    To use, create a separate Python file and add the path to the file to your
+    LangGraph API configuration file (`langgraph.json`). Within that file, create
+    an instance of the Priority class and register an assign function with
+    `@priority.assign`.
+
+    Example `langgraph.json` file:
+
+    ```json
+    {
+      "dependencies": ["."],
+      "graphs": {
+        "agent": "./my_agent/agent.py:graph"
+      },
+      "env": ".env",
+      "priority": {
+        "path": "./priority.py:priority"
+      }
+    }
+    ```
+
+    The server calls the assign function at the first run on a thread with no
+    settings, which can come long after the thread was created, and stores the
+    settings it returns on the thread. Later runs on the thread use the stored
+    settings. The function can be called more than once for one thread, for
+    example after it raises, and only one result is stored, so keep it free of
+    side effects.
+
+    ???+ example "Basic Usage"
+
+        ```python
+        from langgraph_sdk import Priority, PriorityContext, PrioritySettings
+
+        priority = Priority()
+
+        @priority.assign
+        async def assign(ctx: PriorityContext) -> PrioritySettings | None:
+            return {
+                "priority": 3 if "premium" in ctx.permissions else 0,
+                # Stored unencrypted, so an opaque ID rather than personal data.
+                "fairness_group": ctx.user["org_id"],
+            }
+        ```
+    """
+
+    __slots__ = ("_assign_handler",)
+
+    types = types
+    """Reference to priority type definitions.
+
+    Provides access to the type definitions used by the priority system:
+    PriorityContext and PrioritySettings.
+    """
+
+    def __init__(self) -> None:
+        """Initialize the Priority instance."""
+        _warn_priority_beta()
+        self._assign_handler: _AssignHandler | None = None
+
+    def assign(self, fn: _AssignHandlerT) -> _AssignHandlerT:
+        """Register the function that assigns a thread's priority settings.
+
+        The function receives a `PriorityContext` and returns `PrioritySettings`,
+        or `None` for the defaults.
+
+        Example:
+            ```python
+            @priority.assign
+            async def assign(ctx: PriorityContext) -> PrioritySettings | None:
+                if ctx.cron_id is not None:
+                    return {"priority": -2}
+                return None
+            ```
+
+        Args:
+            fn: The assign handler function
+
+        Returns:
+            The registered handler function
+
+        Raises:
+            DuplicateHandlerError: If an assign handler is already registered
+            TypeError: If the handler has an invalid signature
+        """
+        if self._assign_handler is not None:
+            raise DuplicateHandlerError("Priority assign handler already registered")
+        _validate_handler(fn)
+        self._assign_handler = fn
+        return fn
@@ -0,0 +1,67 @@
+"""Priority types for LangGraph.
+
+This module defines the context the server passes to the function registered
+with `@priority.assign`, and the settings that function returns.
+"""
+
+from __future__ import annotations
+
+import typing
+from collections.abc import Sequence
+from dataclasses import dataclass
+
+from langgraph_sdk.auth.types import BaseUser
+
+
+@dataclass(frozen=True, slots=True)
+class PriorityContext:
+    """Context passed to the `@priority.assign` function at the first run on a thread with no settings."""
+
+    user: BaseUser
+    """The user from the deployment's auth, custom auth or LangSmith auth. Without auth, `user.identity` is empty."""
+
+    permissions: Sequence[str]
+    """The user's permissions from the same auth. Without auth, it's empty."""
+
+    thread_id: str | None
+    """The thread's ID, or `None` when the run creates its thread without being given an ID."""
+
+    thread_metadata: dict[str, typing.Any]
+    """The thread's metadata."""
+
+    assistant_id: str
+    """The ID of the assistant the run uses."""
+
+    graph_id: str
+    """The ID of the graph the assistant runs."""
+
+    assistant_metadata: dict[str, typing.Any]
+    """The assistant's metadata."""
+
+    run_metadata: dict[str, typing.Any]
+    """The run's metadata."""
+
+    cron_id: str | None
+    """The ID of the cron that started the run, or `None` when no cron started it."""
+
+
+class PrioritySettings(typing.TypedDict, total=False):
+    """Priority settings returned by the `@priority.assign` function.
+
+    Every key is optional: a missing key or a key set to `None` takes its
+    default, and returning `None` instead of a dict gives every key its default.
+    A value outside its range, or an empty `fairness_group`, is an error.
+    """
+
+    priority: int | None
+    """An integer from -5 to 5, where 5 runs first and -5 runs last. Defaults to 0."""
+
+    fairness_group: str | None
+    """The thread's fairness group, a non-empty string. Defaults to `"default"`.
+
+    The server stores it unencrypted, even when the deployment uses custom
+    encryption, so use an opaque ID rather than personal data.
+    """
+
+    fairness_weight: int | None
+    """The weight of the thread's fairness group, an integer from 1 to 100. Defaults to 1."""
@@ -16,7 +16,7 @@ dependencies = [
    "orjson>=3.11.5",
    "langchain-protocol>=0.0.15",
    "langchain-core>=1.4.0,<2",
-    "websockets>=14,<17",
+    "websockets>=14",
 ]

 [tool.hatch.version]
@@ -0,0 +1,128 @@
+import dataclasses
+
+import pytest
+
+import langgraph_sdk
+from langgraph_sdk import Priority, PriorityContext, PrioritySettings
+from langgraph_sdk.encryption import DuplicateHandlerError, LangGraphBetaWarning
+from langgraph_sdk.priority import _warn_priority_beta
+
+
+class _User:
+    is_authenticated = True
+    display_name = "user-1"
+    identity = "user-1"
+    permissions: tuple[str, ...] = ()
+
+    def __getitem__(self, key):
+        return getattr(self, key)
+
+    def __contains__(self, key):
+        return hasattr(self, key)
+
+    def __iter__(self):
+        return iter(("identity", "display_name", "is_authenticated", "permissions"))
+
+
+def _context(thread_id: str | None) -> PriorityContext:
+    return PriorityContext(
+        user=_User(),
+        permissions=[],
+        thread_id=thread_id,
+        thread_metadata={},
+        assistant_id="assistant-1",
+        graph_id="agent",
+        assistant_metadata={"tier": "gold"},
+        run_metadata={},
+        cron_id=None,
+    )
+
+
+async def _no_params() -> PrioritySettings | None:
+    return None
+
+
+async def _two_params(_ctx: PriorityContext, _extra: object) -> PrioritySettings | None:
+    return None
+
+
+def test_priority_and_its_types_are_exported():
+    assert {"Priority", "PriorityContext", "PrioritySettings"} <= set(
+        langgraph_sdk.__all__
+    )
+    assert Priority.types.PriorityContext is PriorityContext
+    assert Priority.types.PrioritySettings is PrioritySettings
+
+
+def test_construction_warns_that_priority_is_beta():
+    # The warning fires once per process, so an earlier test may have used it up.
+    _warn_priority_beta.cache_clear()
+    with pytest.warns(LangGraphBetaWarning, match="Priority API is in beta"):
+        Priority()
+
+
+def test_assign_registers_and_returns_the_function():
+    priority = Priority()
+
+    async def assign(_ctx: PriorityContext) -> PrioritySettings | None:
+        return {"priority": 3, "fairness_group": "org-42"}
+
+    assert priority.assign(assign) is assign
+    assert priority._assign_handler is assign
+
+
+def test_assign_rejects_a_sync_function():
+    priority = Priority()
+
+    def assign(_ctx: PriorityContext) -> PrioritySettings | None:
+        return None
+
+    with pytest.raises(TypeError, match="must be an async function"):
+        priority.assign(assign)  # ty: ignore[invalid-argument-type]
+    assert priority._assign_handler is None
+
+
+@pytest.mark.parametrize("fn", [_no_params, _two_params])
+def test_assign_rejects_the_wrong_number_of_parameters(fn):
+    priority = Priority()
+
+    with pytest.raises(TypeError, match="must accept exactly 1 parameter"):
+        priority.assign(fn)
+    assert priority._assign_handler is None
+
+
+def test_assign_rejects_a_second_function():
+    priority = Priority()
+
+    @priority.assign
+    async def first(_ctx: PriorityContext) -> PrioritySettings | None:
+        return None
+
+    with pytest.raises(DuplicateHandlerError):
+
+        @priority.assign
+        async def second(_ctx: PriorityContext) -> PrioritySettings | None:
+            return None
+
+    assert priority._assign_handler is first
+
+
+def test_context_accepts_no_thread_id():
+    # `make lint` type-checks this call too, so `thread_id` must accept None there.
+    assert _context(thread_id=None).thread_id is None
+
+
+def test_context_is_frozen():
+    ctx = _context(thread_id="thread-1")
+
+    with pytest.raises(dataclasses.FrozenInstanceError):
+        ctx.thread_id = "thread-2"  # ty: ignore[invalid-assignment]
+
+
+def test_every_settings_key_is_optional():
+    assert PrioritySettings.__optional_keys__ == {
+        "priority",
+        "fairness_group",
+        "fairness_weight",
+    }
+    assert PrioritySettings.__required_keys__ == frozenset()
@@ -1,5 +1,5 @@
 version = 1
-revision = 5
+revision = 3
 requires-python = ">=3.10"
 resolution-markers = [
    "python_full_version >= '3.15'",
@@ -40,11 +40,11 @@ wheels = [

 [[package]]
 name = "certifi"
-version = "2026.2.25"
+version = "2026.10.19"
 source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/af/2d/7bf41579a8986e348fa033a31cdd0e4121114f6bce2457e8876010b092dd/certifi-2026.2.25.tar.gz", hash = "sha256:e887ab5cee78ea814d3472169153c2d12cd43b14bd03329a39a9c6e2e80bfba7", size = 155029, upload-time = "2026-02-25T02:54:17.342Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a6/bd/1436c50cc46699de0230fbaacc1d1de7bfec413176099949e9d5112e88f2/certifi-2026.10.19.tar.gz", hash = "sha256:035c1e85572b141bf1c3a3eff0e33a8eb74917522f2559075ff8d2fea780a1ae", size = 137598, upload-time = "2026-10-20T02:24:16.604Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/9a/3c/c17fb3ca2d9c3acff52e30b309f538586f9f5b9c9cf454f3845fc9af4881/certifi-2026.2.25-py3-none-any.whl", hash = "sha256:027692e4402ad994f1c42e52a4997a9763c646b73e4096e4d5d6db8af1d6f0fa", size = 153684, upload-time = "2026-02-25T02:54:15.766Z" },
+    { url = "https://files.pythonhosted.org/packages/c4/51/29046e48f693c54e5b7412216d447a1d8362c9653148d98c02cdf65fa733/certifi-2026.10.19-py3-none-any.whl", hash = "sha256:42d35340cc5b9ba78e818ddbff0c5b1c2b779de448a340fd1b989f5fa8670e1b", size = 136464, upload-time = "2026-10-20T02:24:15.072Z" },
 ]

 [[package]]
@@ -525,7 +525,7 @@ requires-dist = [
    { name = "langchain-core", specifier = ">=1.4.0,<2" },
    { name = "langchain-protocol", specifier = ">=0.0.15" },
    { name = "orjson", specifier = ">=3.11.5" },
-    { name = "websockets", specifier = ">=14,<17" },
+    { name = "websockets", specifier = ">=14" },
 ]

 [package.metadata.requires-dev]