diff --git a/temporal/api/command/v1/message.proto b/temporal/api/command/v1/message.proto
index ee839115b..b8be64315 100644
--- a/temporal/api/command/v1/message.proto
+++ b/temporal/api/command/v1/message.proto
@@ -16,6 +16,7 @@ import "temporal/api/enums/v1/command_type.proto";
 import "temporal/api/common/v1/message.proto";
 import "temporal/api/failure/v1/message.proto";
 import "temporal/api/taskqueue/v1/message.proto";
+import "temporal/api/session/v1/message.proto";
 import "temporal/api/workflow/v1/message.proto";
 import "temporal/api/sdk/v1/user_metadata.proto";
 import "temporal/api/sdk/v1/event_group_marker.proto";
@@ -68,6 +69,8 @@ message ScheduleActivityTaskCommandAttributes {
     // Priority metadata. If this message is not present, or any fields are not
     // present, they inherit the values from the workflow.
     temporal.api.common.v1.Priority priority = 14;
+
+    temporal.api.session.v1.ActivitySessionOptions session_options = 15;
 }
 
 message RequestCancelActivityTaskCommandAttributes {
diff --git a/temporal/api/history/v1/message.proto b/temporal/api/history/v1/message.proto
index 1c6efd305..8f5a233bf 100644
--- a/temporal/api/history/v1/message.proto
+++ b/temporal/api/history/v1/message.proto
@@ -20,6 +20,7 @@ import "temporal/api/common/v1/message.proto";
 import "temporal/api/deployment/v1/message.proto";
 import "temporal/api/failure/v1/message.proto";
 import "temporal/api/nexus/v1/message.proto";
+import "temporal/api/session/v1/message.proto";
 import "temporal/api/taskqueue/v1/message.proto";
 import "temporal/api/update/v1/message.proto";
 import "temporal/api/workflow/v1/message.proto";
@@ -465,6 +466,9 @@ message ActivityTaskScheduledEventAttributes {
     // Priority metadata. If this message is not present, or any fields are not
     // present, they inherit the values from the workflow.
     temporal.api.common.v1.Priority priority = 14;
+    // Preserves the scheduling command's session routing options so activity state
+    // can be reconstructed from history.
+    temporal.api.session.v1.ActivitySessionOptions session_options = 15;
 }
 
 message ActivityTaskStartedEventAttributes {
diff --git a/temporal/api/session/v1/message.proto b/temporal/api/session/v1/message.proto
new file mode 100644
index 000000000..4a7cdb2e5
--- /dev/null
+++ b/temporal/api/session/v1/message.proto
@@ -0,0 +1,44 @@
+syntax = "proto3";
+
+package temporal.api.session.v1;
+
+option go_package = "go.temporal.io/api/session/v1;session";
+option java_package = "io.temporal.api.session.v1";
+option java_multiple_files = true;
+option java_outer_classname = "MessageProto";
+option ruby_package = "Temporalio::Api::Session::V1";
+option csharp_namespace = "Temporalio.Api.Session.V1";
+
+// EXPERIMENTAL. Subject to redesign or removal.
+// Configures a worker's polling for sessions that keep workflow and activity
+// tasks on a stable worker destination. Activities opt into session routing
+// through ActivitySessionOptions on their scheduling commands.
+message WorkerSessionOptions {
+    oneof isolation {
+        SingleSessionOptions single_session = 1;
+    }
+}
+
+// EXPERIMENTAL. Subject to redesign or removal.
+// Dedicates a sticky destination to the first workflow matched to it.
+// Workflow sticky polls may be forwarded to the normal queue until the first
+// workflow task is dispatched. The server persists that workflow ID and does
+// not forward subsequent polls, including after workflow completion.
+// Activity sticky polls are never forwarded to the normal queue.
+message SingleSessionOptions {
+    // Stable sticky task queue name, shared by workflow and activity polls.
+    // Reuse this value across worker restarts within the same session, but not
+    // across concurrently running workers. This is distinct from worker identity.
+    // The SDK must also set the polled sticky task queue's name to this value.
+    string stable_identity = 1;
+}
+
+message ActivitySessionOptions {
+    oneof session {
+        // Execute this activity as part of its workflow's session.
+        // Currently routes to the workflow's sticky task queue when true.
+        bool use_workflow_session = 1;
+
+        // later session_id, etc
+    }
+}
diff --git a/temporal/api/workflowservice/v1/request_response.proto b/temporal/api/workflowservice/v1/request_response.proto
index 2ad815a5e..e570a9466 100644
--- a/temporal/api/workflowservice/v1/request_response.proto
+++ b/temporal/api/workflowservice/v1/request_response.proto
@@ -38,6 +38,7 @@ import "temporal/api/replication/v1/message.proto";
 import "temporal/api/rules/v1/message.proto";
 import "temporal/api/sdk/v1/worker_config.proto";
 import "temporal/api/schedule/v1/message.proto";
+import "temporal/api/session/v1/message.proto";
 import "temporal/api/taskqueue/v1/message.proto";
 import "temporal/api/update/v1/message.proto";
 import "temporal/api/version/v1/message.proto";
@@ -323,6 +324,9 @@ message PollWorkflowTaskQueueRequest {
     // Removed in 1.55.0; was temporal.api.worker.v1.WorkerHeartbeat worker_heartbeat
     reserved 7;
     reserved "worker_heartbeat";
+
+    // EXPERIMENTAL. Subject to redesign or removal. Only applicable to sticky polls.
+    temporal.api.session.v1.WorkerSessionOptions session_options = 11;
 }
 
 message PollWorkflowTaskQueueResponse {
@@ -553,6 +557,9 @@ message PollActivityTaskQueueRequest {
     // Removed in 1.55.0; was temporal.api.worker.v1.WorkerHeartbeat worker_heartbeat
     reserved 7;
     reserved "worker_heartbeat";
+
+    // EXPERIMENTAL. Subject to redesign or removal. Only applicable to sticky polls.
+    temporal.api.session.v1.WorkerSessionOptions session_options = 11;
 }
 
 message PollActivityTaskQueueResponse {