feat(app-server): add optional turn_id to thread/fork (#30277)

## Description

This adds stable optional `turnId` support to `thread/fork`. When
supplied, the fork copies persisted history through that terminal turn,
inclusive, and drops later turns from the new thread.

Omitting or passing `null` preserves the existing full-history fork
behavior, including the interruption marker when the stored source
history ends mid-turn.

## Why

We're deprecating `thread/rollback` and this will help certain UX use
cases work around it by using `thread/fork` + `turn_id` instead.

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -3673,6 +3673,13 @@
             }
           ],
           "description": "Optional client-supplied analytics source classification for this forked thread."
+        },
+        "lastTurnId": {
+          "description": "Optional last turn id to fork through, inclusive.\n\nWhen specified, turns after `last_turn_id` are omitted from the fork. The referenced turn cannot be in progress.",
+          "type": [
+            "string",
+            "null"
+          ]
         }
       },
       "required": [
@@ -6866,4 +6873,4 @@
     }
   ],
   "title": "ClientRequest"
-}
+}

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -17287,6 +17287,13 @@
               }
             ],
             "description": "Optional client-supplied analytics source classification for this forked thread."
+          },
+        "lastTurnId": {
+          "description": "Optional last turn id to fork through, inclusive.\n\nWhen specified, turns after `last_turn_id` are omitted from the fork. The referenced turn cannot be in progress.",
+            "type": [
+              "string",
+              "null"
+            ]
           }
         },
         "required": [
@@ -20833,4 +20840,4 @@
   },
   "title": "CodexAppServerProtocol",
   "type": "object"
-}
+}

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -15066,6 +15066,13 @@
             }
           ],
           "description": "Optional client-supplied analytics source classification for this forked thread."
+        },
+        "lastTurnId": {
+          "description": "Optional last turn id to fork through, inclusive.\n\nWhen specified, turns after `last_turn_id` are omitted from the fork. The referenced turn cannot be in progress.",
+          "type": [
+            "string",
+            "null"
+          ]
         }
       },
       "required": [
@@ -18611,4 +18618,4 @@
   },
   "title": "CodexAppServerProtocolV2",
   "type": "object"
-}
+}

codex-rs/app-server-protocol/schema/json/v2/ThreadForkParams.json

@@ -168,6 +168,13 @@
         }
       ],
       "description": "Optional client-supplied analytics source classification for this forked thread."
+    },
+    "lastTurnId": {
+      "description": "Optional last turn id to fork through, inclusive.\n\nWhen specified, turns after `last_turn_id` are omitted from the fork. The referenced turn cannot be in progress.",
+      "type": [
+        "string",
+        "null"
+      ]
     }
   },
   "required": [
@@ -175,4 +182,4 @@
   ],
   "title": "ThreadForkParams",
   "type": "object"
-}
+}

codex-rs/app-server-protocol/schema/typescript/v2/ThreadForkParams.ts

@@ -18,6 +18,12 @@ import type { ThreadSource } from "./ThreadSource";
  * Prefer using thread_id whenever possible.
  */
 export type ThreadForkParams = {threadId: string, /**
+ * Optional last turn id to fork through, inclusive.
+ *
+ * When specified, turns after `last_turn_id` are omitted from the fork.
+ * The referenced turn cannot be in progress.
+ */
+lastTurnId?: string | null, /**
  * Configuration overrides for the forked thread, if any.
  */
 model?: string | null, modelProvider?: string | null, serviceTier?: string | null | null, cwd?: string | null, approvalPolicy?: AskForApproval | null, /**

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -839,6 +839,30 @@ fn thread_path_params_deserialize_empty_path_as_none() {
     );
 }
 
+#[test]
+fn thread_fork_last_turn_id_round_trips() {
+    let params: ThreadForkParams = serde_json::from_value(json!({
+        "threadId": "thread-1",
+        "lastTurnId": "turn-2",
+    }))
+    .expect("thread/fork params deserialize");
+
+    assert_eq!(params.last_turn_id, Some("turn-2".to_string()));
+    let serialized = serde_json::to_value(params).expect("thread/fork params serialize");
+    assert_eq!(serialized["lastTurnId"], json!("turn-2"));
+
+    let omitted = serde_json::to_value(ThreadForkParams {
+        thread_id: "thread-1".to_string(),
+        ..Default::default()
+    })
+    .expect("thread/fork params without last turn id serialize");
+    assert_eq!(
+        omitted["lastTurnId"],
+        serde_json::Value::Null,
+        "optional lastTurnId should serialize as null when omitted"
+    );
+}
+
 #[test]
 fn fs_get_metadata_response_round_trips_minimal_fields() {
     let response = FsGetMetadataResponse {

codex-rs/app-server-protocol/src/protocol/v2/thread.rs

@@ -494,6 +494,13 @@ impl From<ThreadTurnsListResponse> for TurnsPage {
 pub struct ThreadForkParams {
     pub thread_id: String,
 
+    /// Optional last turn id to fork through, inclusive.
+    ///
+    /// When specified, turns after `last_turn_id` are omitted from the fork.
+    /// The referenced turn cannot be in progress.
+    #[ts(optional = nullable)]
+    pub last_turn_id: Option<String>,
+
     /// [UNSTABLE] Specify the rollout path to fork from.
     /// If specified, the thread_id param will be ignored.
     #[experimental("thread/fork.path")]