fix(frontend): preserve interrupted uniform-run order (#4834)

frontend/src/core/threads/hooks.ts

@@ -675,7 +675,9 @@ export function restoreLocalTurnMessageOrder(
  * accepted transient, far safer than pulling a completed turn's answer
  * below the next user message. A resent turn after an interrupted run and
  * pagination orphans from older turns (#4399) fail the checks as well and
- * are left untouched.
+ * are left untouched. When an interrupted turn and the next turn share one
+ * synthetic run_id, the previous human anchor plus the missing terminal
+ * answer makes step ownership ambiguous, so that layout is also preserved.
  */
 export function restoreReconnectedTurnMessageOrder(
   messages: Message[],
@@ -721,6 +723,21 @@ export function restoreReconnectedTurnMessageOrder(
     }
   }
 
+  const previousHumanRunId =
+    segmentStart > 0 ? getMessageRunId(messages[segmentStart - 1]!) : undefined;
+  const hasUniformRunIdSincePreviousHuman =
+    previousHumanRunId !== undefined &&
+    messages
+      .slice(segmentStart - 1)
+      .every(
+        (message) =>
+          isHiddenFromUIMessage(message) ||
+          getMessageRunId(message) === previousHumanRunId,
+      );
+  if (candidateStart === segmentStart && hasUniformRunIdSincePreviousHuman) {
+    return messages;
+  }
+
   const runIdsAfter = new Set<string>();
   let hasLiveOnlyStepAfter = false;
   for (const message of messages.slice(humanIndex + 1)) {

frontend/tests/unit/core/threads/message-merge.test.ts

@@ -1688,6 +1688,46 @@ test("reconnected turn order leaves a resent turn after an interrupted run untou
   ).toEqual([interruptedStep, human, newStep]);
 });
 
+test("reconnected turn order keeps interrupted uniform-run history in its original turn", () => {
+  // Branch-seeded and mocked feeds can stamp every message with one run_id.
+  // Without a terminal answer, that synthetic id cannot prove that the older
+  // tool step belongs to the newer human turn.
+  const human1 = {
+    id: "human-1",
+    type: "human",
+    content: "First question",
+    run_id: "run-x",
+  } as Message;
+  const interruptedStep = {
+    id: "step-1",
+    type: "ai",
+    content: "Working on the first question",
+    tool_calls: [{ id: "tc-1", name: "web_search", args: {} }],
+    run_id: "run-x",
+  } as unknown as Message;
+  const human2 = {
+    id: "human-2",
+    type: "human",
+    content: "Second question",
+    run_id: "run-x",
+  } as Message;
+  const step2 = {
+    id: "step-2",
+    type: "ai",
+    content: "Working on the second question",
+    run_id: "run-x",
+  } as Message;
+
+  expect(
+    restoreReconnectedTurnMessageOrder([
+      human1,
+      interruptedStep,
+      human2,
+      step2,
+    ]),
+  ).toEqual([human1, interruptedStep, human2, step2]);
+});
+
 test("reconnected turn order only moves steps of the sandwiched run in multi-turn history", () => {
   const human1 = { id: "human-1", type: "human", content: "First" } as Message;
   const step1 = {