diff --git a/openapi/openapiv2.json b/openapi/openapiv2.json
index ea37162a0..2f2ff73f7 100644
--- a/openapi/openapiv2.json
+++ b/openapi/openapiv2.json
@@ -16743,6 +16743,10 @@
         "pollerAutoscalingAutoEnroll": {
           "type": "boolean",
           "description": "When true, workers should use poller autoscaling by default unless explicitly configured otherwise."
+        },
+        "workflowTaskCompletionPagination": {
+          "type": "boolean",
+          "description": "True if the namespace supports paginated workflow task completion, i.e.\n`RespondWorkflowTaskCompleted` requests split across multiple pages. SDKs must check this\ncapability before sending a request with `intermediate_page` set to true."
         }
       },
       "description": "Namespace capability details. Should contain what features are enabled in a namespace."
diff --git a/openapi/openapiv3.yaml b/openapi/openapiv3.yaml
index a2ec27cdd..d0cdbf076 100644
--- a/openapi/openapiv3.yaml
+++ b/openapi/openapiv3.yaml
@@ -13144,6 +13144,12 @@ components:
         pollerAutoscalingAutoEnroll:
           type: boolean
           description: When true, workers should use poller autoscaling by default unless explicitly configured otherwise.
+        workflowTaskCompletionPagination:
+          type: boolean
+          description: |-
+            True if the namespace supports paginated workflow task completion, i.e.
+             `RespondWorkflowTaskCompleted` requests split across multiple pages. SDKs must check this
+             capability before sending a request with `intermediate_page` set to true.
       description: Namespace capability details. Should contain what features are enabled in a namespace.
     NamespaceInfo_Limits:
       type: object
diff --git a/temporal/api/errordetails/v1/message.proto b/temporal/api/errordetails/v1/message.proto
index 2abb60db6..d2ae7c32d 100644
--- a/temporal/api/errordetails/v1/message.proto
+++ b/temporal/api/errordetails/v1/message.proto
@@ -137,3 +137,9 @@ message NexusOperationExecutionAlreadyStartedFailure {
     string start_request_id = 1;
     string run_id = 2;
 }
+
+// An error indicating that the server lost the buffered pages of a paginated workflow task
+// completion. This is a transient error: the workflow task is still valid, and the client 
+// should resend all pages from page 0 using the same task token.
+message WorkflowTaskCompletionBufferLostFailure {
+}
diff --git a/temporal/api/namespace/v1/message.proto b/temporal/api/namespace/v1/message.proto
index a8b49dc8c..1c12dbaea 100644
--- a/temporal/api/namespace/v1/message.proto
+++ b/temporal/api/namespace/v1/message.proto
@@ -58,6 +58,10 @@ message NamespaceInfo {
         bool workflow_update_callbacks = 12;
         // When true, workers should use poller autoscaling by default unless explicitly configured otherwise.
         bool poller_autoscaling_auto_enroll = 13;
+        // True if the namespace supports paginated workflow task completion, i.e.
+        // `RespondWorkflowTaskCompleted` requests split across multiple pages. SDKs must check this
+        // capability before sending a request with `intermediate_page` set to true.
+        bool workflow_task_completion_pagination = 14;
     }
 
     // Namespace configured limits
diff --git a/temporal/api/workflowservice/v1/request_response.proto b/temporal/api/workflowservice/v1/request_response.proto
index 263c3c8a0..92472bd18 100644
--- a/temporal/api/workflowservice/v1/request_response.proto
+++ b/temporal/api/workflowservice/v1/request_response.proto
@@ -424,6 +424,16 @@ message RespondWorkflowTaskCompletedRequest {
     // tasks (e.g. activity cancellation) to this specific worker instance.
     string worker_control_task_queue = 20;
 
+    // 0-indexed page number when the workflow task completion is split across multiple
+    // requests ("pages"). 0 for single-page requests. May only be set to non-zero value
+    // when the namespace capability workflow_task_completion_pagination is true.
+    int32 page_number = 21;
+
+    // True for non-final pages of a paginated workflow task completion. The final page's
+    // `page_number` tells the server how many intermediate pages (0..page_number-1) preceded it.
+    // May only be used when the namespace capability workflow_task_completion_pagination is true.
+    bool intermediate_page = 22;
+
     // SDK capability details.
     message Capabilities {
         // True if the SDK can handle speculative workflow task with command events. If true, the