diff --git a/apps/web/src/domains/runtime/session-stream/session-stream-render-scheduler.ts b/apps/web/src/domains/runtime/session-stream/session-stream-render-scheduler.ts
index 243ef132..d166e81f 100644
--- a/apps/web/src/domains/runtime/session-stream/session-stream-render-scheduler.ts
+++ b/apps/web/src/domains/runtime/session-stream/session-stream-render-scheduler.ts
@@ -70,8 +70,15 @@ const segmentGraphemes = createGraphemeSegmenter();
 // Only visible streaming text is paced. Tool-call args and lifecycle events
 // pass through so pacing never delays structural updates, and reasoning-chunk /
 // thinking deltas are live-state reducer no-ops not worth throttling.
-function getPaceableTextDelta(event: AgUiSessionEvent): string | null {
+function getPaceableTextDelta(
+  event: AgUiSessionEvent,
+  messageRole: "assistant" | "user" | undefined,
+): string | null {
   if (event.type === "REASONING_MESSAGE_CONTENT" || event.type === "TEXT_MESSAGE_CONTENT") {
+    if (event.type === "TEXT_MESSAGE_CONTENT" && messageRole === "user") {
+      return null;
+    }
+
     return event.delta.length > 0 ? event.delta : null;
   }
 
@@ -150,6 +157,7 @@ export class SessionStreamRenderScheduler {
   #frameHandle: number | null = null;
   readonly #host: SessionStreamRenderSchedulerHost;
   #lastDrainAt: number | null = null;
+  readonly #messageRoles = new Map<string, "assistant" | "user">();
   #pacingCarry = 0;
   #queue: QueuedSessionEvent[] = [];
   #queueOffset = 0;
@@ -168,6 +176,7 @@ export class SessionStreamRenderScheduler {
     this.#queue = [];
     this.#queueOffset = 0;
     this.#lastDrainAt = null;
+    this.#messageRoles.clear();
     this.#pacingCarry = 0;
   }
 
@@ -181,7 +190,23 @@ export class SessionStreamRenderScheduler {
     }
 
     for (const event of events) {
+      const messageKey =
+        "messageId" in event && typeof event.messageId === "string"
+          ? `${sessionId}\0${event.messageId}`
+          : null;
+
+      if (
+        event.type === "TEXT_MESSAGE_START" &&
+        (event.role === "assistant" || event.role === "user")
+      ) {
+        this.#messageRoles.set(`${sessionId}\0${event.messageId}`, event.role);
+      }
+
       this.#queue.push(this.#toQueueItem(sessionId, event));
+
+      if (event.type === "TEXT_MESSAGE_END" && messageKey !== null) {
+        this.#messageRoles.delete(messageKey);
+      }
     }
 
     this.#schedule();
@@ -411,7 +436,11 @@ export class SessionStreamRenderScheduler {
   }
 
   #toQueueItem(sessionId: string, event: AgUiSessionEvent): QueuedSessionEvent {
-    const delta = getPaceableTextDelta(event);
+    const messageRole =
+      "messageId" in event && typeof event.messageId === "string"
+        ? this.#messageRoles.get(`${sessionId}\0${event.messageId}`)
+        : undefined;
+    const delta = getPaceableTextDelta(event, messageRole);
 
     return {
       event,
diff --git a/apps/web/src/features/session-chat/assistant-ui/convert-session-message.ts b/apps/web/src/features/session-chat/assistant-ui/convert-session-message.ts
index 4d01ef65..6ad0d016 100644
--- a/apps/web/src/features/session-chat/assistant-ui/convert-session-message.ts
+++ b/apps/web/src/features/session-chat/assistant-ui/convert-session-message.ts
@@ -109,9 +109,16 @@ function sessionSegmentsToParts(segments: readonly SessionViewSegment[]): Assist
 // keeps assistant-ui's per-message memo + keys aligned with the live stream.
 // Permission/needs_approval is intentionally NOT folded here (this conversion is
 // identity-cached and permissionRequests live in a separate array).
-export function convertSessionMessage(message: SessionViewMessage): ThreadMessageLike {
+export function convertSessionMessage(
+  message: SessionViewMessage,
+  messageIndex?: number,
+): ThreadMessageLike {
   if (message.role === "user") {
-    return { role: "user", id: message.id, content: message.content };
+    // The optimistic message and its server echo have different source ids but
+    // occupy the same append-only transcript slot. Keep the UI id stable so
+    // assistant-ui updates the bubble instead of deleting and remounting it.
+    const id = messageIndex === undefined ? message.id : `user:${messageIndex}`;
+    return { role: "user", id, content: message.content };
   }
 
   const parts = sessionSegmentsToParts(message.segments);
diff --git a/apps/web/tests/session-message-convert.test.ts b/apps/web/tests/session-message-convert.test.ts
index d844c2d8..6e5894ac 100644
--- a/apps/web/tests/session-message-convert.test.ts
+++ b/apps/web/tests/session-message-convert.test.ts
@@ -29,6 +29,34 @@ describe("convertSessionMessage", () => {
     expect(message.role).toBe("user");
   });
 
+  test("keeps a user bubble identity when the optimistic message is replaced by its echo", () => {
+    const optimistic = convertSessionMessage(
+      {
+        content: "hello",
+        createdAt: "",
+        id: "pending:request-1",
+        plan: [],
+        role: "user",
+        segments: [],
+      },
+      2,
+    );
+    const echoed = convertSessionMessage(
+      {
+        content: "hello",
+        createdAt: "",
+        id: "01J0000000000000000000000U",
+        plan: [],
+        role: "user",
+        segments: [],
+      },
+      2,
+    );
+
+    expect(optimistic.id).toBe("user:2");
+    expect(echoed.id).toBe(optimistic.id);
+  });
+
   test("merges consecutive text segments into one markdown part", () => {
     const like = convertSessionMessage(
       assistantMessage([
diff --git a/apps/web/tests/session-stream-render-scheduler.test.ts b/apps/web/tests/session-stream-render-scheduler.test.ts
index c2d9f66f..d8c73035 100644
--- a/apps/web/tests/session-stream-render-scheduler.test.ts
+++ b/apps/web/tests/session-stream-render-scheduler.test.ts
@@ -354,6 +354,25 @@ describe("session stream render scheduler", () => {
     expect(drainFrames(manual)).toBe(0);
   });
 
+  test("delivers user start/content events atomically instead of pacing the server echo", () => {
+    const manual = createManualHost();
+    const batches: AgUiSessionEvent[][] = [];
+    const scheduler = new SessionStreamRenderScheduler((_sessionId, events) => {
+      batches.push(events);
+      return true;
+    }, manual.host);
+    const events = [
+      { messageId: "message-user", role: "user", type: "TEXT_MESSAGE_START" },
+      textEvent("用".repeat(1000), "message-user"),
+    ] as const satisfies readonly AgUiSessionEvent[];
+
+    scheduler.enqueueMany("session-1", [...events]);
+    manual.fireFrame();
+
+    expect(batches).toEqual([[...events]]);
+    expect(drainFrames(manual)).toBe(0);
+  });
+
   test("flushNow delivers the paced remainder exactly once", () => {
     const manual = createManualHost();
     const batches: AgUiSessionEvent[][] = [];