@@ -163,6 +163,63 @@ message ActivityExecutionInfo {
     string canceled_reason = 32;
 }
 
+// NewActivityExecutionInfo is a shared message that encapsulates all the
+// required arguments to starting a standalone activity in different contexts.
+message NewActivityExecutionInfo {
+    // Identifier for this activity. Must be unique among activities in the same namespace.
+    string activity_id = 1;
+
+    // The type of the activity to execute.
+    temporal.api.common.v1.ActivityType activity_type = 2;
+
+    // Task queue to schedule the activity on.
+    temporal.api.taskqueue.v1.TaskQueue task_queue = 3;
+
+    // Serialized arguments to the activity.
+    temporal.api.common.v1.Payloads input = 4;
+
+    // Indicates how long the caller is willing to wait for an activity completion. Limits how long
+    // retries will be attempted. Either this or `start_to_close_timeout` must be specified.
+    //
+    // (-- api-linter: core::0140::prepositions=disabled
+    //     aip.dev/not-precedent: "to" is used to indicate interval. --)
+    google.protobuf.Duration schedule_to_close_timeout = 5;
+
+    // Limits time an activity task can stay in a task queue before a worker picks it up. This
+    // timeout is always non retryable, as all a retry would achieve is to put it back into the same
+    // queue. Defaults to `schedule_to_close_timeout` if not specified.
+    //
+    // (-- api-linter: core::0140::prepositions=disabled
+    //     aip.dev/not-precedent: "to" is used to indicate interval. --)
+    google.protobuf.Duration schedule_to_start_timeout = 6;
+
+    // Maximum time an activity is allowed to execute after being picked up by a worker. This
+    // timeout is always retryable. Either this or `schedule_to_close_timeout` must be
+    // specified.
+    //
+    // (-- api-linter: core::0140::prepositions=disabled
+    //     aip.dev/not-precedent: "to" is used to indicate interval. --)
+    google.protobuf.Duration start_to_close_timeout = 7;
+
+    // Maximum permitted time between successful worker heartbeats.
+    google.protobuf.Duration heartbeat_timeout = 8;
+
+    // The retry policy for the activity. Will never exceed `schedule_to_close_timeout`.
+    temporal.api.common.v1.RetryPolicy retry_policy = 9;
+
+    // Header for context propagation and tracing purposes.
+    temporal.api.common.v1.Header header = 10;
+
+    // Search attributes for indexing.
+    temporal.api.common.v1.SearchAttributes search_attributes = 11;
+
+    // Metadata for use by user interfaces to display the fixed as-of-start summary and details of the activity.
+    temporal.api.sdk.v1.UserMetadata user_metadata = 12;
+
+    // Priority metadata.
+    temporal.api.common.v1.Priority priority = 13;
+}
+
 // Limited activity information returned in the list response.
 // When adding fields here, ensure that it is also present in ActivityExecutionInfo (note that it
 // may already be present in ActivityExecutionInfo but not at the top-level).
 
@@ -17,11 +17,10 @@ option csharp_namespace = "Temporalio.Api.Schedule.V1";
 import "google/protobuf/duration.proto";
 import "google/protobuf/timestamp.proto";
 
+import "temporal/api/activity/v1/message.proto";
 import "temporal/api/common/v1/message.proto";
 import "temporal/api/enums/v1/schedule.proto";
 import "temporal/api/enums/v1/workflow.proto";
-import "temporal/api/sdk/v1/user_metadata.proto";
-import "temporal/api/taskqueue/v1/message.proto";
 import "temporal/api/workflow/v1/message.proto";
 
 // CalendarSpec describes an event specification relative to the calendar,
@@ -231,54 +230,6 @@ message SchedulePolicies {
     bool keep_original_workflow_id = 4;
 }
 
-// ScheduleActivityAction describes a standalone activity to start on each schedule trigger.
-message ScheduleActivityAction {
-    // The activity ID. May have a timestamp appended for uniqueness.
-    string activity_id = 1;
-
-    // The type of the activity to execute.
-    temporal.api.common.v1.ActivityType activity_type = 2;
-
-    // Task queue to schedule the activity on.
-    temporal.api.taskqueue.v1.TaskQueue task_queue = 3;
-
-    // Serialized arguments to the activity.
-    temporal.api.common.v1.Payloads input = 4;
-
-    // Timeout from schedule to close of the activity.
-    // (-- api-linter: core::0140::prepositions=disabled
-    //     aip.dev/not-precedent: standard name of field --)
-    google.protobuf.Duration schedule_to_close_timeout = 5;
-
-    // Timeout from schedule to start of the activity.
-    // (-- api-linter: core::0140::prepositions=disabled
-    //     aip.dev/not-precedent: standard name of field --)
-    google.protobuf.Duration schedule_to_start_timeout = 6;
-
-    // Timeout from start to close of the activity.
-    // (-- api-linter: core::0140::prepositions=disabled
-    //     aip.dev/not-precedent: standard name of field --)
-    google.protobuf.Duration start_to_close_timeout = 7;
-
-    // Heartbeat timeout.
-    google.protobuf.Duration heartbeat_timeout = 8;
-
-    // Retry policy for the activity.
-    temporal.api.common.v1.RetryPolicy retry_policy = 9;
-
-    // Request header for context propagation.
-    temporal.api.common.v1.Header header = 10;
-
-    // Search attributes for the activity.
-    temporal.api.common.v1.SearchAttributes search_attributes = 11;
-
-    // Metadata for display purposes.
-    temporal.api.sdk.v1.UserMetadata user_metadata = 12;
-
-    // Priority metadata.
-    temporal.api.common.v1.Priority priority = 13;
-}
-
 message ScheduleAction {
     oneof action {
         // All fields of NewWorkflowExecutionInfo are valid except for:
@@ -288,10 +239,11 @@ message ScheduleAction {
         // it may have a timestamp appended for uniqueness.
         temporal.api.workflow.v1.NewWorkflowExecutionInfo start_workflow = 1;
 
-        // Start a standalone activity execution.
+        // All fields of NewActivityExecutionInfo are valid except for:
+        // - activity_id (will be set by the server)
         // The activity id of the started activity may not match this exactly,
         // it may have a timestamp appended for uniqueness.
-        ScheduleActivityAction start_activity = 2;
+        temporal.api.activity.v1.NewActivityExecutionInfo start_activity = 2;
     }
 }
 
 
@@ -2931,10 +2931,6 @@ message StartActivityExecutionRequest {
     temporal.api.sdk.v1.UserMetadata user_metadata = 17;
     // Priority metadata.
     temporal.api.common.v1.Priority priority = 18;
-
-    // Callbacks to invoke on activity completion.
-    // Used internally by the scheduler to track activity lifecycle.
-    repeated temporal.api.common.v1.Callback completion_callbacks = 19;
 }
 
 message StartActivityExecutionResponse {