THardy98 · Oct 20, 2026 · Oct 21, 2026 · Oct 27, 2026 · Oct 27, 2026 · Oct 28, 2026
@@ -39,6 +39,7 @@ const SERDE_DERIVE_PREFIXES: &[&str] = &[
     ".temporal.api.replication",
     ".temporal.api.rules",
     ".temporal.api.schedule",
+    ".temporal.api.session",
     ".temporal.api.taskqueue",
     ".temporal.api.testservice",
     ".temporal.api.update",

@@ -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 {

@@ -155,6 +155,8 @@ enum ResourceExhaustedCause {
     RESOURCE_EXHAUSTED_CAUSE_WORKER_DEPLOYMENT_LIMITS = 10;
     // Namespace exceeds bandwidth limit.
     RESOURCE_EXHAUSTED_CAUSE_BANDWIDTH_LIMIT = 11;
+    // Caller exceeds Nexus requests per second limit.
+    RESOURCE_EXHAUSTED_CAUSE_CALLER_RPS_LIMIT = 12;
 }
 
 enum ResourceExhaustedScope {

@@ -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 {

@@ -0,0 +1,56 @@
+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";
+
+import "google/protobuf/empty.proto";
+
+// EXPERIMENTAL. Subject to redesign or removal.
+// Sent on workflow and activity polls. Presence means this worker can serve
+// sessions. Currently, the only supported Session type is Workflow Session
+// (aka Workflow-as-Session). A Workflow Session can receive both workflow
+// tasks and activity tasks of an execution.
+message WorkerSessionOptions {
+    // Admission control, if unset the worker accepts an arbitrary number of
+    // sessions (bounded by session slots in the future)
+    oneof admission {
+        // If specified, the worker only polls for work belonging to its session.
+        SingleSessionOptions single_session = 1;
+    }
+}
+
+// EXPERIMENTAL. Subject to redesign or removal.
+// Serve exactly one session for the worker's lifetime, including across
+// process restarts with the same stable_identity. The server binds the worker
+// to the first session that needs a worker, and dispatches no other work here,
+// even after that session completes.
+message SingleSessionOptions {
+    // Identity the binding is stored under. Reuse it across restarts of the
+    // same session, not across concurrently running workers. If multiple
+    // workers poll with the same stable_identity concurrently, the server
+    // treats them as pollers of a single worker and process affinity is lost.
+    //
+    // Note: this effectively acts as a configurable sticky queue.
+    // Note: we are currently unsure how this will evolve. May be deprecated as
+    // the session model develops.
+    string stable_identity = 1;
+}
+
+// EXPERIMENTAL. Subject to redesign or removal.
+// Resolved activity placement. Unset = normal task-queue routing.
+message ActivitySessionOptions {
+    oneof placement {
+        // Run in the workflow's current session (on its sticky queue).
+        // Currently requires the activity's task queue to be the workflow's,
+        // and the workflow's worker to serve sessions.
+        google.protobuf.Empty workflow_session = 1;
+
+        // Future: references to other types of sessions.
+    }
+}
@@ -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 {

@@ -16,6 +16,7 @@ import "temporal/api/enums/v1/workflow.proto";
 import "temporal/api/failure/v1/message.proto";
 import "temporal/api/sdk/v1/user_metadata.proto";
 import "temporal/api/sdk/v1/event_group_marker.proto";
+import "temporal/api/session/v1/message.proto";
 import "temporal/api/workflow/v1/message.proto";
 import "temporal/sdk/core/child_workflow/child_workflow.proto";
 import "temporal/sdk/core/nexus/nexus.proto";
@@ -106,6 +107,7 @@ message ScheduleActivity {
     coresdk.common.VersioningIntent versioning_intent = 15;
     // The Priority to use for this activity
     temporal.api.common.v1.Priority priority = 16;
+    temporal.api.session.v1.ActivitySessionOptions session_options = 17;
 }
 
 message ScheduleLocalActivity {

@@ -1915,6 +1915,7 @@ pub mod temporal {
                                 request_eager_execution: !s.do_not_eagerly_execute,
                                 use_workflow_build_id,
                                 priority: s.priority,
+                                session_options: s.session_options,
                             },
                         )
                     }
@@ -2619,6 +2620,11 @@ pub mod temporal {
                 include_proto_with_serde!("temporal.api.sdk.v1");
             }
         }
+        pub mod session {
+            pub mod v1 {
+                tonic::include_proto!("temporal.api.session.v1");
+            }
+        }
         pub mod taskqueue {
             pub mod v1 {
                 tonic::include_proto!("temporal.api.taskqueue.v1");

@@ -40,6 +40,14 @@ relevant information.
   name the run's last completed workflow task, and `failed_tasks`, the started event ids of the
   run's last 100 failed or timed out workflow tasks. Retries that Core runs locally keep the
   value. Core leaves it unset for other activities.
+* Experimental session workers can poll workflow-affine activity queues; individual activities opt
+  into their workflow's session, while normal activity routing remains the default. This increment
+  supports fixed-size activity slots with capacity of at least two; resource-based and custom
+  activity suppliers are not supported yet. Activity session options without a placement use normal
+  task-queue routing. Session workers advertise session support on their sticky polls. A workflow
+  task that schedules a session activity fails on a worker without session support, including
+  during replay, so replay workers must be configured with session options to replay such
+  workflows.
 
 ### Security
 * Ephemeral server downloads now use `zip` 9.0 for ZIP extraction, replacing the 8.x dependency

@@ -35,8 +35,7 @@ pub(crate) struct MeteredPermitDealer<SK: SlotKind> {
     max_permits: Option<usize>,
     metrics_ctx: MetricsContext,
     meter: Option<TemporalMeter>,
-    /// Only applies to permit dealers for workflow tasks. True if this permit dealer is associated
-    /// with a sticky queue poller.
+    /// True if this permit dealer is associated with a sticky queue poller.
     is_sticky_poller: bool,
     context_data: Arc<PermitDealerContextData>,
 }
@@ -198,7 +197,7 @@ where
     }
 }
 
-impl MeteredPermitDealer<WorkflowSlotKind> {
+impl<SK: SlotKind> MeteredPermitDealer<SK> {
     pub(crate) fn into_sticky(mut self) -> Self {
         self.is_sticky_poller = true;
         self

@@ -44,11 +44,11 @@ pub use worker::{
     FixedSizeSlotSupplier, LocalActivitySlotKind, NamespaceCapabilities, NexusSlotKind, PollError,
     PollerBehavior, ResourceBasedSlotsOptions, ResourceBasedSlotsOptionsBuilder,
     ResourceBasedTuner, ResourceBasedTunerConfig, ResourceController, ResourceSlotOptions,
-    SlotInfo, SlotInfoTrait, SlotKind, SlotKindType, SlotMarkUsedContext, SlotReleaseContext,
-    SlotReservationContext, SlotSupplier, SlotSupplierOptions, SlotSupplierPermit, TunerBuilder,
-    TunerHolder, TunerHolderOptions, TunerHolderOptionsBuilder, Worker, WorkerConfig,
-    WorkerConfigBuilder, WorkerTuner, WorkerValidationError, WorkerVersioningStrategy,
-    WorkflowErrorType, WorkflowSlotKind,
+    SessionWorkerOptions, SessionWorkerOptionsBuilder, SlotInfo, SlotInfoTrait, SlotKind,
+    SlotKindType, SlotMarkUsedContext, SlotReleaseContext, SlotReservationContext, SlotSupplier,
+    SlotSupplierOptions, SlotSupplierPermit, TunerBuilder, TunerHolder, TunerHolderOptions,
+    TunerHolderOptionsBuilder, Worker, WorkerConfig, WorkerConfigBuilder, WorkerTuner,
+    WorkerValidationError, WorkerVersioningStrategy, WorkflowErrorType, WorkflowSlotKind,
 };
 
 use crate::{

@@ -1,7 +1,9 @@
 mod poll_buffer;
 mod sticky;
 
-pub(crate) use poll_buffer::{ActivityTaskOptions, LongPollBuffer, WorkflowTaskOptions};
+pub(crate) use poll_buffer::{
+    ActivityTaskOptions, LongPollBuffer, PollRateLimiter, WorkflowTaskOptions,
+};
 pub(crate) use sticky::{StickyPollerShared, StickyTaskPoller, sticky_poller_behavior};
 pub use temporalio_client::{Client, ClientOptions, ClientTlsOptions, RetryOptions, TlsOptions};