diff --git a/openapi/openapiv2.json b/openapi/openapiv2.json
index 1437120e5..1ea891029 100644
--- a/openapi/openapiv2.json
+++ b/openapi/openapiv2.json
@@ -12225,6 +12225,10 @@
         "deploymentOptions": {
           "$ref": "#/definitions/v1WorkerDeploymentOptions",
           "description": "Worker deployment options that user has set in the worker."
+        },
+        "cause": {
+          "$ref": "#/definitions/v1ActivityTaskFailedCause",
+          "description": "Why did the task fail? When unset, the failure is treated as an unspecified activity failure."
         }
       }
     },
@@ -12250,6 +12254,10 @@
         "resourceId": {
           "type": "string",
           "description": "Resource ID for routing. Contains \"workflow:workflow_id\" or \"activity:activity_id\" for standalone activities."
+        },
+        "cause": {
+          "$ref": "#/definitions/v1ActivityTaskFailedCause",
+          "description": "Why did the activity task fail? Optional; when unset the failure is treated as a normal\nactivity failure. See the type's doc for more."
         }
       }
     },
@@ -14046,6 +14054,17 @@
         }
       }
     },
+    "v1ActivityTaskFailedCause": {
+      "type": "string",
+      "enum": [
+        "ACTIVITY_TASK_FAILED_CAUSE_UNSPECIFIED",
+        "ACTIVITY_TASK_FAILED_CAUSE_PAYLOADS_TOO_LARGE",
+        "ACTIVITY_TASK_FAILED_CAUSE_EXTERNAL_STORAGE_FAILURE",
+        "ACTIVITY_TASK_FAILED_CAUSE_ACTIVITY_WORKER_UNHANDLED_FAILURE"
+      ],
+      "default": "ACTIVITY_TASK_FAILED_CAUSE_UNSPECIFIED",
+      "description": "Activity tasks can fail for various reasons. Note that some of these reasons can only originate\nfrom the server, and some of them can only originate from the SDK/worker.\n\n - ACTIVITY_TASK_FAILED_CAUSE_PAYLOADS_TOO_LARGE: A payload-bearing field on a request the worker sent for this activity task exceeded the\nper-field size limit configured on the server for the namespace.\nCheck the activity task failure message for more information.\n - ACTIVITY_TASK_FAILED_CAUSE_EXTERNAL_STORAGE_FAILURE: The worker failed to offload a payload to, or retrieve one from, external storage while\nprocessing this activity task.\nCheck the activity task failure message for more information.\n - ACTIVITY_TASK_FAILED_CAUSE_ACTIVITY_WORKER_UNHANDLED_FAILURE: The default cause for an activity task failure reported by a worker; a more specific cause\ntakes precedence whenever the condition is recognized.\nCheck the activity task failure message for more information."
+    },
     "v1ActivityTaskFailedEventAttributes": {
       "type": "object",
       "properties": {
@@ -14073,6 +14092,10 @@
         "workerVersion": {
           "$ref": "#/definitions/v1WorkerVersionStamp",
           "title": "Version info of the worker who processed this workflow task.\nDeprecated. This field should be cleaned up when versioning-2 API is removed. [cleanup-experimental-wv]"
+        },
+        "cause": {
+          "$ref": "#/definitions/v1ActivityTaskFailedCause",
+          "description": "Why did the task fail? When unset, the failure is treated as an unspecified activity failure."
         }
       }
     },
diff --git a/openapi/openapiv3.yaml b/openapi/openapiv3.yaml
index 007d77168..ba2bf2efc 100644
--- a/openapi/openapiv3.yaml
+++ b/openapi/openapiv3.yaml
@@ -10227,6 +10227,15 @@ components:
           description: |-
             Version info of the worker who processed this workflow task.
              Deprecated. This field should be cleaned up when versioning-2 API is removed. [cleanup-experimental-wv]
+        cause:
+          enum:
+            - ACTIVITY_TASK_FAILED_CAUSE_UNSPECIFIED
+            - ACTIVITY_TASK_FAILED_CAUSE_PAYLOADS_TOO_LARGE
+            - ACTIVITY_TASK_FAILED_CAUSE_EXTERNAL_STORAGE_FAILURE
+            - ACTIVITY_TASK_FAILED_CAUSE_ACTIVITY_WORKER_UNHANDLED_FAILURE
+          type: string
+          description: Why did the task fail? When unset, the failure is treated as an unspecified activity failure.
+          format: enum
     ActivityTaskScheduledEventAttributes:
       type: object
       properties:
@@ -15917,6 +15926,17 @@ components:
         resourceId:
           type: string
           description: Resource ID for routing. Contains "workflow:workflow_id" or "activity:activity_id" for standalone activities.
+        cause:
+          enum:
+            - ACTIVITY_TASK_FAILED_CAUSE_UNSPECIFIED
+            - ACTIVITY_TASK_FAILED_CAUSE_PAYLOADS_TOO_LARGE
+            - ACTIVITY_TASK_FAILED_CAUSE_EXTERNAL_STORAGE_FAILURE
+            - ACTIVITY_TASK_FAILED_CAUSE_ACTIVITY_WORKER_UNHANDLED_FAILURE
+          type: string
+          description: |-
+            Why did the activity task fail? Optional; when unset the failure is treated as a normal
+             activity failure. See the type's doc for more.
+          format: enum
     RespondActivityTaskFailedByIdResponse:
       type: object
       properties:
@@ -15969,6 +15989,15 @@ components:
           allOf:
             - $ref: '#/components/schemas/WorkerDeploymentOptions'
           description: Worker deployment options that user has set in the worker.
+        cause:
+          enum:
+            - ACTIVITY_TASK_FAILED_CAUSE_UNSPECIFIED
+            - ACTIVITY_TASK_FAILED_CAUSE_PAYLOADS_TOO_LARGE
+            - ACTIVITY_TASK_FAILED_CAUSE_EXTERNAL_STORAGE_FAILURE
+            - ACTIVITY_TASK_FAILED_CAUSE_ACTIVITY_WORKER_UNHANDLED_FAILURE
+          type: string
+          description: Why did the task fail? When unset, the failure is treated as an unspecified activity failure.
+          format: enum
     RespondActivityTaskFailedResponse:
       type: object
       properties:
diff --git a/temporal/api/enums/v1/failed_cause.proto b/temporal/api/enums/v1/failed_cause.proto
index f8809fd4e..c96de00db 100644
--- a/temporal/api/enums/v1/failed_cause.proto
+++ b/temporal/api/enums/v1/failed_cause.proto
@@ -92,6 +92,24 @@ enum WorkflowTaskFailedCause {
     WORKFLOW_TASK_FAILED_CAUSE_REQUEST_TOO_LARGE = 40;
 }
 
+// Activity tasks can fail for various reasons. Note that some of these reasons can only originate
+// from the server, and some of them can only originate from the SDK/worker.
+enum ActivityTaskFailedCause {
+    ACTIVITY_TASK_FAILED_CAUSE_UNSPECIFIED = 0;
+    // A payload-bearing field on a request the worker sent for this activity task exceeded the
+    // per-field size limit configured on the server for the namespace.
+    // Check the activity task failure message for more information.
+    ACTIVITY_TASK_FAILED_CAUSE_PAYLOADS_TOO_LARGE = 1;
+    // The worker failed to offload a payload to, or retrieve one from, external storage while
+    // processing this activity task.
+    // Check the activity task failure message for more information.
+    ACTIVITY_TASK_FAILED_CAUSE_EXTERNAL_STORAGE_FAILURE = 2;
+    // The default cause for an activity task failure reported by a worker; a more specific cause
+    // takes precedence whenever the condition is recognized.
+    // Check the activity task failure message for more information.
+    ACTIVITY_TASK_FAILED_CAUSE_ACTIVITY_WORKER_UNHANDLED_FAILURE = 3;
+}
+
 enum StartChildWorkflowExecutionFailedCause {
     START_CHILD_WORKFLOW_EXECUTION_FAILED_CAUSE_UNSPECIFIED = 0;
     START_CHILD_WORKFLOW_EXECUTION_FAILED_CAUSE_WORKFLOW_ALREADY_EXISTS = 1;
diff --git a/temporal/api/history/v1/message.proto b/temporal/api/history/v1/message.proto
index 0211c6f55..b40324d68 100644
--- a/temporal/api/history/v1/message.proto
+++ b/temporal/api/history/v1/message.proto
@@ -517,6 +517,8 @@ message ActivityTaskFailedEventAttributes {
     // Version info of the worker who processed this workflow task.
     // Deprecated. This field should be cleaned up when versioning-2 API is removed. [cleanup-experimental-wv]
     temporal.api.common.v1.WorkerVersionStamp worker_version = 6 [deprecated = true];
+    // Why did the task fail? When unset, the failure is treated as an unspecified activity failure.
+    temporal.api.enums.v1.ActivityTaskFailedCause cause = 7;
 }
 
 message ActivityTaskTimedOutEventAttributes {
diff --git a/temporal/api/workflowservice/v1/request_response.proto b/temporal/api/workflowservice/v1/request_response.proto
index c3dd95769..de1c271cd 100644
--- a/temporal/api/workflowservice/v1/request_response.proto
+++ b/temporal/api/workflowservice/v1/request_response.proto
@@ -739,6 +739,8 @@ message RespondActivityTaskFailedRequest {
     temporal.api.deployment.v1.Deployment deployment = 7 [deprecated = true];
     // Worker deployment options that user has set in the worker.
     temporal.api.deployment.v1.WorkerDeploymentOptions deployment_options = 8;
+    // Why did the task fail? When unset, the failure is treated as an unspecified activity failure.
+    temporal.api.enums.v1.ActivityTaskFailedCause cause = 10;
 }
 
 message RespondActivityTaskFailedResponse {
@@ -765,6 +767,9 @@ message RespondActivityTaskFailedByIdRequest {
     temporal.api.common.v1.Payloads last_heartbeat_details = 7;
     // Resource ID for routing. Contains "workflow:workflow_id" or "activity:activity_id" for standalone activities.
     string resource_id = 8;
+    // Why did the activity task fail? Optional; when unset the failure is treated as a normal
+    // activity failure. See the type's doc for more.
+    temporal.api.enums.v1.ActivityTaskFailedCause cause = 9;
 }
 
 message RespondActivityTaskFailedByIdResponse {