diff --git a/protos/durable-task-scheduler/sandbox_service.proto b/protos/durable-task-scheduler/sandbox_service.proto
new file mode 100644
index 0000000..1d52ed7
--- /dev/null
+++ b/protos/durable-task-scheduler/sandbox_service.proto
@@ -0,0 +1,101 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+syntax = "proto3";
+
+package microsoft.durabletask.sandboxes;
+
+option csharp_namespace = "Microsoft.DurableTask.Protobuf.Sandboxes";
+
+service SandboxActivities {
+  // Opens a live sandbox activity worker session. The first message
+  // must be a start message with static worker metadata. Heartbeats carry
+  // dynamic state only. Closing the stream deregisters the worker.
+  rpc ConnectSandboxActivityWorker(stream SandboxActivityWorkerMessage) returns (SandboxActivityWorkerSessionResult);
+
+  // Creates or updates a sandbox worker profile before any live worker stream exists.
+  // This is a configuration contract and does not advertise active worker
+  // capacity.
+  rpc DeclareSandboxWorkerProfile(SandboxWorkerProfile) returns (DeclareSandboxWorkerProfileResult);
+
+  // Removes a sandbox worker profile so the backend stops
+  // waking new sandbox workers for the specified worker profile. Existing
+  // workers are not terminated by this RPC.
+  rpc RemoveSandboxWorkerProfile(RemoveSandboxWorkerProfileRequest) returns (RemoveSandboxWorkerProfileResult);
+}
+
+message SandboxActivityWorkerMessage {
+  oneof message {
+    SandboxActivityWorkerStart start = 1;
+    SandboxActivityWorkerHeartbeat heartbeat = 2;
+  }
+}
+
+message SandboxActivityWorkerStart {
+  string task_hub = 1;
+  int32 max_activities_count = 2;
+  // Sandbox provider the worker is running in. UNSPECIFIED = legacy
+  // (pre-provider-aware) workers.
+  SandboxProviderKind sandbox_provider = 3;
+  // DTS-generated sandbox identifier injected as DTS_SANDBOX_ID. This is not
+  // the ADC provider sandbox resource id.
+  string dts_sandbox_identifier = 4;
+  // Caller-defined profile/catalog ID shared by worker profiles and live worker registration.
+  string worker_profile_id = 5;
+  // Activity handlers registered by the worker process. DTS validates this
+  // matches the worker profile before advertising worker capacity.
+  repeated SandboxActivity activities = 6;
+}
+
+message SandboxActivityWorkerHeartbeat {
+  int32 active_activities_count = 1;
+}
+
+message SandboxActivityWorkerSessionResult {
+  string message = 1;
+}
+
+message SandboxWorkerProfile {
+  // Caller-defined profile/catalog ID shared by worker profiles and live worker registration.
+  string worker_profile_id = 1;
+  repeated SandboxActivity activities = 2;
+  SandboxActivityImage image = 3;
+  map<string, string> environment_variables = 4;
+  int32 max_concurrent_activities = 5;
+  SandboxActivityResources resources = 6;
+  string scheduler_managed_identity_client_id = 7;
+}
+
+message SandboxActivity {
+  string name = 1;
+  string version = 2;
+}
+
+message SandboxActivityImage {
+  string image_ref = 1;
+  string managed_identity_client_id = 2;
+  repeated string entrypoint = 3;
+  repeated string cmd = 4;
+}
+
+message SandboxActivityResources {
+  string cpu = 1;
+  string memory = 2;
+}
+
+message DeclareSandboxWorkerProfileResult {
+}
+
+message RemoveSandboxWorkerProfileRequest {
+  string worker_profile_id = 1;
+}
+
+message RemoveSandboxWorkerProfileResult {
+}
+
+// Sandbox provider executing the activity worker.
+enum SandboxProviderKind {
+  SANDBOX_PROVIDER_KIND_UNSPECIFIED = 0;
+  SANDBOX_PROVIDER_KIND_ACA_SESSION_POOL = 1;
+  SANDBOX_PROVIDER_KIND_SANDBOX = 2;
+}