diff --git a/dev/restate/service/protocol.proto b/dev/restate/service/protocol.proto
index 45aa417..699d005 100644
--- a/dev/restate/service/protocol.proto
+++ b/dev/restate/service/protocol.proto
@@ -431,6 +431,54 @@ message GetCallInvocationIdEntryMessage {
   string name = 12;
 }
 
+// Completable: Yes
+// Fallible: Yes
+// Type: 0x0C00 + 8
+message AttachInvocationEntryMessage {
+  oneof target {
+    // Target invocation id
+    string invocation_id = 1;
+    // Target index of the call/one way call journal entry in this journal.
+    uint32 call_entry_index = 2;
+    // Target idempotent request
+    IdempotentRequestTarget idempotent_request_target = 3;
+    // Target workflow target
+    WorkflowTarget workflow_target = 4;
+  }
+
+  oneof result {
+    string value = 14;
+    Failure failure = 15;
+  };
+
+  string name = 12;
+}
+
+// Completable: Yes
+// Fallible: Yes
+// Type: 0x0C00 + 9
+message GetInvocationOutputEntryMessage {
+  oneof target {
+    // Target invocation id
+    string invocation_id = 1;
+    // Target index of the call/one way call journal entry in this journal.
+    uint32 call_entry_index = 2;
+    // Target idempotent request
+    IdempotentRequestTarget idempotent_request_target = 3;
+    // Target workflow target
+    WorkflowTarget workflow_target = 4;
+  }
+
+  oneof result {
+    // Empty if no result is still available
+    Empty empty = 13;
+    string value = 14;
+    Failure failure = 15;
+  };
+
+  string name = 12;
+}
+
 // --- Nested messages
 
 // This failure object carries user visible errors,
@@ -447,5 +495,17 @@ message Header {
   string value = 2;
 }
 
+message WorkflowTarget {
+  string workflow_name = 1;
+  string workflow_key = 2;
+}
+
+message IdempotentRequestTarget {
+  string service_name = 1;
+  optional string service_key = 2;
+  string handler_name = 3;
+  string idempotency_key = 4;
+}
+
 message Empty {
 }
diff --git a/service-invocation-protocol.md b/service-invocation-protocol.md
index 89e23cb..1c1124e 100644
--- a/service-invocation-protocol.md
+++ b/service-invocation-protocol.md
@@ -350,6 +350,8 @@ descriptions in [`protocol.proto`](dev/restate/service/protocol.proto).
 | `CompletePromiseEntryMessage`     | `0x080A` | Yes         | No       | Complete the given promise. If the promise was completed already, this entry completes with a failure.                                                           |
 | `CancelInvocationEntryMessage`    | `0x0C06` | No          | Yes      | Cancel the target invocation id or the target journal entry.                                                                                                     |
 | `GetCallInvocationIdEntryMessage` | `0x0C07` | Yes         | Yes      | Get the invocation id of a previously created call/one way call.                                                                                                 |
+| `AttachInvocationEntryMessage`    | `0x0C08` | Yes         | Yes      | Attach to an existing invocation. If the invocation is still in-flight, this entry will be completed when the target invocation completes.                       |
+| `GetInvocationOutputEntryMessage` | `0x0C09` | Yes         | Yes      | Get output of an existing invocation. If the invocation is still in-flight, this entry will be completed with `empty` value.                                     |
 
 #### Awakeable identifier