diff --git a/packages/app/src/types/stream.test.ts b/packages/app/src/types/stream.test.ts
index 20d953a5d..6d3cc93e2 100644
--- a/packages/app/src/types/stream.test.ts
+++ b/packages/app/src/types/stream.test.ts
@@ -1457,6 +1457,52 @@ describe("turn lifecycle events", () => {
     );
   });
 
+  it("does not shift later prompts when an earlier optimistic prompt has no canonical echo", () => {
+    const staleTimestamp = new Date("2025-01-01T15:04:00Z");
+    const submittedTimestamp = new Date("2025-01-01T15:04:01Z");
+    const stalePrompt: StreamItem = {
+      kind: "user_message",
+      id: "msg_stale",
+      text: "first prompt without an echo",
+      timestamp: staleTimestamp,
+      optimistic: true,
+    };
+    const submittedPrompt: StreamItem = {
+      kind: "user_message",
+      id: "msg_submitted",
+      text: "later submitted prompt",
+      timestamp: submittedTimestamp,
+      optimistic: true,
+    };
+
+    const state = reduceStreamUpdate(
+      [stalePrompt, submittedPrompt],
+      {
+        type: "timeline",
+        provider: "codex",
+        item: {
+          type: "user_message",
+          text: "canonical rendered prompt",
+          messageId: "provider-owned-submitted",
+          clientMessageId: submittedPrompt.id,
+        },
+      },
+      new Date("2025-01-01T15:04:02Z"),
+      { source: "live" },
+    );
+
+    assert.deepStrictEqual(state, [
+      stalePrompt,
+      {
+        kind: "user_message",
+        id: "provider-owned-submitted",
+        clientMessageId: submittedPrompt.id,
+        text: submittedPrompt.text,
+        timestamp: submittedPrompt.timestamp,
+      },
+    ]);
+  });
+
   it("appends a live server user message when no optimistic user message is pending", () => {
     const state = reduceStreamUpdate(
       [],
diff --git a/packages/app/src/types/stream.ts b/packages/app/src/types/stream.ts
index 21d2726d3..edd1ab36d 100644
--- a/packages/app/src/types/stream.ts
+++ b/packages/app/src/types/stream.ts
@@ -365,7 +365,10 @@ function appendUserMessage(
   const chunkSeed = chunk.trim() || chunk;
   const entryId = messageId ?? createUniqueTimelineId(state, "user", chunkSeed, timestamp);
   const optimisticIndex = state.findIndex(
-    (entry) => entry.kind === "user_message" && entry.optimistic,
+    (entry) =>
+      entry.kind === "user_message" &&
+      entry.optimistic &&
+      (clientMessageId === undefined || entry.id === clientMessageId),
   );
   const optimistic = optimisticIndex >= 0 ? (state[optimisticIndex] as UserMessageItem) : null;