diff --git a/webui/src/components/MessageBubble.tsx b/webui/src/components/MessageBubble.tsx
index 7e9ae0ca4..f81966dc3 100644
--- a/webui/src/components/MessageBubble.tsx
+++ b/webui/src/components/MessageBubble.tsx
@@ -9,6 +9,7 @@ import {
 import {
   Check,
   ChevronRight,
+  CircleAlert,
   Clock3,
   Copy,
   ImageIcon,
@@ -44,6 +45,8 @@ import type {
   UIImage,
   UIMediaAttachment,
   UIMessage,
+  MessageDeliveryErrorKind,
+  MessageDeliveryStatus,
 } from "@/lib/types";
 
 interface MessageBubbleProps {
@@ -130,6 +133,91 @@ function MessageCopyButton({ content }: { content: string }) {
   );
 }
 
+function deliveryErrorCopy(
+  kind: MessageDeliveryErrorKind | undefined,
+  t: (key: string) => string,
+): { title: string; body: string } {
+  switch (kind) {
+    case "message_too_big":
+      return {
+        title: t("errors.messageTooBig.title"),
+        body: t("errors.messageTooBig.body"),
+      };
+    case "workspace_scope_rejected":
+      return {
+        title: t("errors.workspaceScopeRejected.title"),
+        body: t("errors.workspaceScopeRejected.body"),
+      };
+    case "turn_rejected":
+    case undefined:
+      return {
+        title: t("errors.turnRejected.title"),
+        body: t("errors.turnRejected.body"),
+      };
+    default: {
+      const _exhaustive: never = kind;
+      return { title: String(_exhaustive), body: "" };
+    }
+  }
+}
+
+function UserDeliveryStatus({
+  status,
+  errorKind,
+}: {
+  status: MessageDeliveryStatus | undefined;
+  errorKind: MessageDeliveryErrorKind | undefined;
+}) {
+  const { t } = useTranslation();
+  if (status !== "sending" && status !== "failed") return null;
+  if (status === "sending") {
+    return (
+      <span
+        role="status"
+        className="inline-flex items-center gap-1 text-[12px] leading-none text-muted-foreground"
+      >
+        <Clock3 className="h-3.5 w-3.5" aria-hidden />
+        {t("message.delivery.sending")}
+      </span>
+    );
+  }
+
+  const label = t("message.delivery.failed");
+  const { title, body } = deliveryErrorCopy(errorKind, t);
+  return (
+    <>
+      <Tooltip>
+        <TooltipTrigger asChild>
+          <button
+            type="button"
+            aria-label={`${label}: ${title}`}
+            className={cn(
+              "inline-flex items-center gap-1 rounded-sm text-[12px] leading-none",
+              "text-destructive/80 transition-colors hover:text-destructive",
+              "focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring",
+              "dark:text-red-400/80 dark:hover:text-red-400",
+            )}
+          >
+            <CircleAlert className="h-3.5 w-3.5" aria-hidden />
+            {label}
+          </button>
+        </TooltipTrigger>
+        <TooltipContent
+          side="top"
+          align="end"
+          className="max-w-72 px-3 py-2.5 text-left"
+        >
+          <p className="font-medium text-popover-foreground">{title}</p>
+          <p className="mt-1 leading-relaxed text-muted-foreground">{body}</p>
+        </TooltipContent>
+      </Tooltip>
+      <span role="alert" aria-live="assertive" className="sr-only">
+        {title}. {body}
+      </span>
+    </>
+  );
+}
+
 /** Render user turns as compact bubbles and assistant turns as document-like prose. */
 export function MessageBubble({
   message,
@@ -163,6 +251,8 @@ export function MessageBubble({
     const parsedMessage = parseQuotedUserMessage(message.content);
     const userContent = parsedMessage.content;
     const hasText = userContent.trim().length > 0;
+    const showDeliveryStatus =
+      message.deliveryStatus === "sending" || message.deliveryStatus === "failed";
     const quotedContext = parsedMessage.quotedContext;
     const slashCommand = matchingSlashCommand(userContent, slashCommands);
     const messageText = slashCommand ? (
@@ -208,10 +298,14 @@ export function MessageBubble({
             {messageText}
           </p>
         ) : null}
-        {hasText && showCopyAction ? (
+        {showDeliveryStatus || (hasText && showCopyAction) ? (
           <TooltipProvider delayDuration={220} skipDelayDuration={80}>
-            <div className="flex min-h-8 items-center justify-end text-muted-foreground">
-              <MessageCopyButton content={message.content} />
+            <div className="flex min-h-8 items-center justify-end gap-1.5 text-muted-foreground">
+              <UserDeliveryStatus
+                status={message.deliveryStatus}
+                errorKind={message.deliveryErrorKind}
+              />
+              {hasText && showCopyAction ? <MessageCopyButton content={message.content} /> : null}
             </div>
           </TooltipProvider>
         ) : null}
diff --git a/webui/src/components/thread/StreamErrorNotice.tsx b/webui/src/components/thread/StreamErrorNotice.tsx
index 4875b7353..541fc45d9 100644
--- a/webui/src/components/thread/StreamErrorNotice.tsx
+++ b/webui/src/components/thread/StreamErrorNotice.tsx
@@ -11,9 +11,8 @@ interface StreamErrorNoticeProps {
 }
 
 /**
- * Dismissible banner that surfaces transport-level faults the user needs to
- * know about. Rendered above the composer so the message the fault referred
- * to remains in view just above. ``role="alert"`` + ``aria-live="assertive"``
+ * Fallback banner for transport-level faults that cannot be attached to a
+ * visible failed message. ``role="alert"`` + ``aria-live="assertive"``
  * ensures screen readers announce the failure.
  */
 export function StreamErrorNotice({ error, onDismiss }: StreamErrorNoticeProps) {
diff --git a/webui/src/components/thread/ThreadMessages.tsx b/webui/src/components/thread/ThreadMessages.tsx
index ad7cd18ca..a9464c316 100644
--- a/webui/src/components/thread/ThreadMessages.tsx
+++ b/webui/src/components/thread/ThreadMessages.tsx
@@ -107,7 +107,11 @@ export function ThreadMessages({
           unit.type === "message" && unit.message.role === "assistant" && forkFlags[index]
             ? nextUserIndex
             : undefined;
-        if (unit.type === "message" && unit.message.role === "user") nextUserIndex += 1;
+        if (
+          unit.type === "message"
+          && unit.message.role === "user"
+          && unit.message.deliveryStatus !== "failed"
+        ) nextUserIndex += 1;
 
         return (
           <ThreadDisplayUnit
diff --git a/webui/src/components/thread/ThreadShell.tsx b/webui/src/components/thread/ThreadShell.tsx
index 9219ef6d3..daf841a3f 100644
--- a/webui/src/components/thread/ThreadShell.tsx
+++ b/webui/src/components/thread/ThreadShell.tsx
@@ -31,7 +31,7 @@ import {
   installedMcpPresetsFromPayload,
   isMcpPresetsPayload,
 } from "@/lib/mcp-preset-events";
-import type { CanonicalRunSnapshot } from "@/lib/nanobot-client";
+import type { CanonicalRunSnapshot, StreamError } from "@/lib/nanobot-client";
 import { inferProviderFromModelName, providerDisplayLabel } from "@/lib/provider-brand";
 import type {
   ChatSummary,
@@ -222,11 +222,28 @@ function latestActiveTurnId(messages: UIMessage[]): string | null {
   }
   for (let index = messages.length - 1; index >= 0; index -= 1) {
     const message = messages[index];
-    if (message.role === "user" && message.turnId) return message.turnId;
+    if (
+      message.role === "user"
+      && message.deliveryStatus !== "failed"
+      && message.turnId
+    ) return message.turnId;
   }
   return null;
 }
 
+function hasInlineDeliveryError(
+  messages: UIMessage[],
+  error: StreamError | null,
+): boolean {
+  if (!error?.turnId) return false;
+  return messages.some((message) => (
+    message.role === "user"
+    && message.turnId === error.turnId
+    && message.deliveryStatus === "failed"
+    && message.deliveryErrorKind === error.kind
+  ));
+}
+
 function completedAssistantTurnIds(messages: UIMessage[]): string[] {
   return Array.from(new Set(
     messages
@@ -1283,7 +1300,7 @@ export function ThreadShell({
 
   const composer = (
     <>
-      {streamError ? (
+      {streamError && !hasInlineDeliveryError(messages, streamError) ? (
         <StreamErrorNotice
           error={streamError}
           onDismiss={dismissStreamError}
diff --git a/webui/src/components/thread/ThreadViewport.tsx b/webui/src/components/thread/ThreadViewport.tsx
index 710e948a7..5bf74e59a 100644
--- a/webui/src/components/thread/ThreadViewport.tsx
+++ b/webui/src/components/thread/ThreadViewport.tsx
@@ -250,7 +250,9 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
   const hiddenUserMessageCount =
     userMessageOffset
     + (hiddenMessageCount > 0
-      ? messages.slice(0, hiddenMessageCount).filter((message) => message.role === "user").length
+      ? messages.slice(0, hiddenMessageCount).filter(
+        (message) => message.role === "user" && message.deliveryStatus !== "failed",
+      ).length
       : 0);
   const visibleForkBoundaryMessageCount =
     forkBoundaryMessageCount !== null && forkBoundaryMessageCount > hiddenMessageCount
diff --git a/webui/src/hooks/useNanobotStream.ts b/webui/src/hooks/useNanobotStream.ts
index e0bcde04a..a81b3a221 100644
--- a/webui/src/hooks/useNanobotStream.ts
+++ b/webui/src/hooks/useNanobotStream.ts
@@ -17,6 +17,7 @@ import type {
   OutboundMcpPresetMention,
   OutboundMedia,
   GoalStateWsPayload,
+  MessageDeliveryStatus,
   ToolProgressEvent,
   UIMediaAttachment,
   UIFileEdit,
@@ -519,6 +520,27 @@ function eventTurnId(ev: InboundEvent): string | undefined {
   return "turn_id" in ev && typeof ev.turn_id === "string" ? ev.turn_id : undefined;
 }
 
+function transitionTurnDelivery(
+  messages: UIMessage[],
+  turnId: string,
+  status: MessageDeliveryStatus,
+): UIMessage[] {
+  let changed = false;
+  const next = messages.map((message) => {
+    if (
+      message.role !== "user"
+      || message.turnId !== turnId
+      || message.deliveryStatus === status
+      || (status === "accepted" && message.deliveryStatus !== "sending")
+    ) {
+      return message;
+    }
+    changed = true;
+    return { ...message, deliveryStatus: status };
+  });
+  return changed ? next : messages;
+}
+
 export function useNanobotStream(
   chatId: string | null,
   initialMessages: UIMessage[] = [],
@@ -697,7 +719,15 @@ export function useNanobotStream(
       ) {
         fileEditSegmentRef.current = null;
       }
-      return prev.filter((message) => message.turnId !== rejectedTurnId);
+      return prev.flatMap((message) => {
+        if (message.turnId !== rejectedTurnId) return [message];
+        if (message.role !== "user") return [];
+        return [{
+          ...message,
+          deliveryStatus: "failed",
+          deliveryErrorKind: err.kind,
+        }];
+      });
     });
 
     const remainingStartedAt = client.getRunStartedAt(chatId);
@@ -975,6 +1005,11 @@ export function useNanobotStream(
         }
         return;
       }
+      const turnId = eventTurnId(ev);
+      if (turnId) {
+        setMessages((prev) => transitionTurnDelivery(prev, turnId, "accepted"));
+      }
+      if (ev.event === "message_accepted") return;
       const sideChannelEvent = isSideChannelEvent(ev);
       if (
         streamEndTimerRef.current !== null
@@ -1354,6 +1389,7 @@ export function useNanobotStream(
             turnId,
             turnPhase: "user",
             turnSeq: 0,
+            deliveryStatus: "sending",
             createdAt: Date.now(),
             ...(previews ? { media: previews } : {}),
             ...(options?.cliApps?.length ? { cliApps: options.cliApps } : {}),
diff --git a/webui/src/i18n/locales/en/common.json b/webui/src/i18n/locales/en/common.json
index 417495294..1e8cc1a7a 100644
--- a/webui/src/i18n/locales/en/common.json
+++ b/webui/src/i18n/locales/en/common.json
@@ -1172,6 +1172,10 @@
   },
   "message": {
     "streaming": "streaming",
+    "delivery": {
+      "sending": "Sending…",
+      "failed": "Not sent"
+    },
     "assistantTyping": "Assistant is typing",
     "toolSingle": "Using a tool",
     "toolMany": "Used {{count}} tools",
diff --git a/webui/src/i18n/locales/es/common.json b/webui/src/i18n/locales/es/common.json
index 2a37f01a9..d8847971f 100644
--- a/webui/src/i18n/locales/es/common.json
+++ b/webui/src/i18n/locales/es/common.json
@@ -1159,6 +1159,10 @@
   },
   "message": {
     "streaming": "transmitiendo",
+    "delivery": {
+      "sending": "Enviando…",
+      "failed": "No enviado"
+    },
     "assistantTyping": "El asistente está escribiendo",
     "toolSingle": "Usando una herramienta",
     "toolMany": "Se usaron {{count}} herramientas",
diff --git a/webui/src/i18n/locales/fr/common.json b/webui/src/i18n/locales/fr/common.json
index 1e61365b2..d241c6fa4 100644
--- a/webui/src/i18n/locales/fr/common.json
+++ b/webui/src/i18n/locales/fr/common.json
@@ -1158,6 +1158,10 @@
   },
   "message": {
     "streaming": "en cours de génération",
+    "delivery": {
+      "sending": "Envoi…",
+      "failed": "Non envoyé"
+    },
     "assistantTyping": "L’assistant est en train d’écrire",
     "toolSingle": "Utilisation d’un outil",
     "toolMany": "{{count}} outils utilisés",
diff --git a/webui/src/i18n/locales/id/common.json b/webui/src/i18n/locales/id/common.json
index 60223028a..49b06fc00 100644
--- a/webui/src/i18n/locales/id/common.json
+++ b/webui/src/i18n/locales/id/common.json
@@ -1158,6 +1158,10 @@
   },
   "message": {
     "streaming": "sedang mengalir",
+    "delivery": {
+      "sending": "Mengirim…",
+      "failed": "Belum terkirim"
+    },
     "assistantTyping": "Asisten sedang mengetik",
     "toolSingle": "Menggunakan sebuah alat",
     "toolMany": "Menggunakan {{count}} alat",
diff --git a/webui/src/i18n/locales/ja/common.json b/webui/src/i18n/locales/ja/common.json
index 0e256f775..b85e9c225 100644
--- a/webui/src/i18n/locales/ja/common.json
+++ b/webui/src/i18n/locales/ja/common.json
@@ -1158,6 +1158,10 @@
   },
   "message": {
     "streaming": "生成中",
+    "delivery": {
+      "sending": "送信中…",
+      "failed": "未送信"
+    },
     "assistantTyping": "アシスタントが入力中",
     "toolSingle": "ツールを使用中",
     "toolMany": "{{count}} 個のツールを使用",
diff --git a/webui/src/i18n/locales/ko/common.json b/webui/src/i18n/locales/ko/common.json
index 716bea6da..fa8159d0b 100644
--- a/webui/src/i18n/locales/ko/common.json
+++ b/webui/src/i18n/locales/ko/common.json
@@ -1158,6 +1158,10 @@
   },
   "message": {
     "streaming": "생성 중",
+    "delivery": {
+      "sending": "전송 중…",
+      "failed": "전송되지 않음"
+    },
     "assistantTyping": "도우미가 입력 중",
     "toolSingle": "도구 사용 중",
     "toolMany": "도구 {{count}}개 사용됨",
diff --git a/webui/src/i18n/locales/pt-BR/common.json b/webui/src/i18n/locales/pt-BR/common.json
index f78f15218..e3bac6f03 100644
--- a/webui/src/i18n/locales/pt-BR/common.json
+++ b/webui/src/i18n/locales/pt-BR/common.json
@@ -1172,6 +1172,10 @@
   },
   "message": {
     "streaming": "transmitindo",
+    "delivery": {
+      "sending": "Enviando…",
+      "failed": "Não enviada"
+    },
     "assistantTyping": "Assistente está digitando",
     "toolSingle": "Usando uma ferramenta",
     "toolMany": "Foram usadas {{count}} ferramentas",
diff --git a/webui/src/i18n/locales/vi/common.json b/webui/src/i18n/locales/vi/common.json
index 6d633874b..5ae45f6f7 100644
--- a/webui/src/i18n/locales/vi/common.json
+++ b/webui/src/i18n/locales/vi/common.json
@@ -1158,6 +1158,10 @@
   },
   "message": {
     "streaming": "đang truyền",
+    "delivery": {
+      "sending": "Đang gửi…",
+      "failed": "Chưa gửi"
+    },
     "assistantTyping": "Trợ lý đang nhập",
     "toolSingle": "Đang dùng một công cụ",
     "toolMany": "Đã dùng {{count}} công cụ",
diff --git a/webui/src/i18n/locales/zh-CN/common.json b/webui/src/i18n/locales/zh-CN/common.json
index 40a6feafd..385f27aae 100644
--- a/webui/src/i18n/locales/zh-CN/common.json
+++ b/webui/src/i18n/locales/zh-CN/common.json
@@ -1172,6 +1172,10 @@
   },
   "message": {
     "streaming": "流式输出中",
+    "delivery": {
+      "sending": "发送中…",
+      "failed": "未发送"
+    },
     "assistantTyping": "助手正在输入",
     "toolSingle": "正在使用工具",
     "toolMany": "已使用 {{count}} 个工具",
diff --git a/webui/src/i18n/locales/zh-TW/common.json b/webui/src/i18n/locales/zh-TW/common.json
index 2d02c80da..2052e8ce2 100644
--- a/webui/src/i18n/locales/zh-TW/common.json
+++ b/webui/src/i18n/locales/zh-TW/common.json
@@ -1158,6 +1158,10 @@
   },
   "message": {
     "streaming": "串流輸出中",
+    "delivery": {
+      "sending": "傳送中…",
+      "failed": "未傳送"
+    },
     "assistantTyping": "助理正在輸入",
     "toolSingle": "正在使用工具",
     "toolMany": "已使用 {{count}} 個工具",
diff --git a/webui/src/lib/nanobot-client.ts b/webui/src/lib/nanobot-client.ts
index b7f2b2b5a..1c8d4630e 100644
--- a/webui/src/lib/nanobot-client.ts
+++ b/webui/src/lib/nanobot-client.ts
@@ -935,6 +935,9 @@ export class NanobotClient {
       : null;
     if (parsed.event === "message_accepted") {
       this.recordRunAcceptance(parsed.chat_id, parsed.turn_id);
+      if (!isSystemCommandTurnId(turnId)) {
+        this.dispatch(parsed.chat_id, parsed);
+      }
       return;
     }
     if (isSystemCommandTurnId(turnId)) {
diff --git a/webui/src/lib/types.ts b/webui/src/lib/types.ts
index a0ef20047..1f4cb1180 100644
--- a/webui/src/lib/types.ts
+++ b/webui/src/lib/types.ts
@@ -5,6 +5,11 @@ export type Role = "user" | "assistant" | "tool" | "system";
 export type MessageKind = "message" | "trace";
 
 export type UITurnPhase = "user" | "reasoning" | "activity" | "answer" | "complete";
+export type MessageDeliveryStatus = "sending" | "accepted" | "failed";
+export type MessageDeliveryErrorKind =
+  | "message_too_big"
+  | "workspace_scope_rejected"
+  | "turn_rejected";
 
 /** One image attached to a UIMessage.
  *
@@ -76,6 +81,10 @@ export interface UIMessage {
   turnId?: string;
   turnPhase?: UITurnPhase;
   turnSeq?: number;
+  /** Ephemeral delivery lifecycle for optimistic user messages. */
+  deliveryStatus?: MessageDeliveryStatus;
+  /** Structured rejection reason shown with a failed optimistic message. */
+  deliveryErrorKind?: MessageDeliveryErrorKind;
 }
 
 export interface UICliAppAttachment {
diff --git a/webui/src/tests/message-bubble.test.tsx b/webui/src/tests/message-bubble.test.tsx
index 9b82b8d88..62f73d164 100644
--- a/webui/src/tests/message-bubble.test.tsx
+++ b/webui/src/tests/message-bubble.test.tsx
@@ -113,6 +113,53 @@ describe("MessageBubble", () => {
     expect(screen.queryByRole("button", { name: "Fork" })).not.toBeInTheDocument();
   });
 
+  it("renders failed delivery details on focus without persistent accepted chrome", async () => {
+    const message: UIMessage = {
+      id: "u-delivery",
+      role: "user",
+      content: "hello",
+      createdAt: Date.now(),
+      deliveryStatus: "sending",
+    };
+
+    const { rerender } = render(<MessageBubble message={message} />);
+
+    expect(screen.getByRole("status")).toHaveTextContent("Sending…");
+
+    rerender(<MessageBubble message={{ ...message, deliveryStatus: "accepted" }} />);
+    expect(screen.queryByRole("status")).not.toBeInTheDocument();
+
+    rerender(
+      <MessageBubble
+        message={{
+          ...message,
+          deliveryStatus: "failed",
+          deliveryErrorKind: "message_too_big",
+        }}
+      />,
+    );
+    expect(screen.queryByRole("status")).not.toBeInTheDocument();
+    const failedStatus = screen.getByRole("button", {
+      name: "Not sent: Message too large",
+    });
+    expect(failedStatus).toHaveClass(
+      "text-destructive/80",
+      "dark:text-red-400/80",
+    );
+    expect(screen.getByText("hello")).not.toHaveClass("ring-1");
+    expect(screen.getByText("hello")).not.toHaveClass("ring-destructive/30");
+    expect(screen.queryByRole("tooltip")).not.toBeInTheDocument();
+
+    fireEvent.focus(failedStatus);
+
+    const tooltip = await screen.findByRole("tooltip");
+    expect(tooltip).toHaveTextContent("Message too large");
+    expect(tooltip).toHaveTextContent(
+      "The server rejected your last message because it exceeded the size limit.",
+    );
+    expect(screen.getByRole("alert")).toHaveClass("sr-only");
+  });
+
   it("styles only generated quoted context in user messages", () => {
     const message: UIMessage = {
       id: "u-quote",
diff --git a/webui/src/tests/nanobot-client.test.ts b/webui/src/tests/nanobot-client.test.ts
index bdecddc6d..c8e074ee8 100644
--- a/webui/src/tests/nanobot-client.test.ts
+++ b/webui/src/tests/nanobot-client.test.ts
@@ -90,6 +90,31 @@ describe("NanobotClient", () => {
     });
   });
 
+  it("routes message acceptance acknowledgements to the matching chat handler", () => {
+    const client = new NanobotClient({
+      url: "ws://test",
+      reconnect: false,
+      socketFactory: (url) => new FakeSocket(url) as unknown as WebSocket,
+    });
+    const handler = vi.fn();
+    client.onChat("chat-ack", handler);
+    client.connect();
+    lastSocket().fakeOpen();
+    client.sendMessage("chat-ack", "hello", undefined, { turnId: "turn-ack" });
+
+    lastSocket().fakeMessage({
+      event: "message_accepted",
+      chat_id: "chat-ack",
+      turn_id: "turn-ack",
+    });
+
+    expect(handler).toHaveBeenCalledWith({
+      event: "message_accepted",
+      chat_id: "chat-ack",
+      turn_id: "turn-ack",
+    });
+  });
+
   it("can swap the socket factory when the runtime URL changes", () => {
     const browserFactory = vi.fn(
       (url: string) => new FakeSocket(`browser:${url}`) as unknown as WebSocket,
diff --git a/webui/src/tests/thread-messages.test.tsx b/webui/src/tests/thread-messages.test.tsx
index 341d187a4..95cbb7aec 100644
--- a/webui/src/tests/thread-messages.test.tsx
+++ b/webui/src/tests/thread-messages.test.tsx
@@ -954,6 +954,34 @@ describe("ThreadMessages", () => {
     expect(screen.getAllByRole("button", { name: "Copy" })).toHaveLength(2);
   });
 
+  it("does not count failed optimistic messages in assistant fork indices", () => {
+    const onForkFromMessage = vi.fn();
+    const messages: UIMessage[] = [
+      { id: "u1", role: "user", content: "one", createdAt: 1 },
+      { id: "a1", role: "assistant", content: "answer one", createdAt: 2 },
+      {
+        id: "u-failed",
+        role: "user",
+        content: "not persisted",
+        deliveryStatus: "failed",
+        createdAt: 3,
+      },
+      { id: "u2", role: "user", content: "two", createdAt: 4 },
+      { id: "a2", role: "assistant", content: "answer two", createdAt: 5 },
+    ];
+
+    render(
+      <ThreadMessages
+        messages={messages}
+        isStreaming={false}
+        onForkFromMessage={onForkFromMessage}
+      />,
+    );
+
+    fireEvent.click(screen.getAllByRole("button", { name: "Fork" }).at(-1)!);
+    expect(onForkFromMessage).toHaveBeenCalledWith(2);
+  });
+
   it("uses turn ids as activity grouping boundaries when available", () => {
     const units = buildDisplayUnits([
       { id: "u1", role: "user", content: "one", turnId: "turn-1", createdAt: 1 },
diff --git a/webui/src/tests/thread-shell.test.tsx b/webui/src/tests/thread-shell.test.tsx
index 6954bb2b3..2d4b489c6 100644
--- a/webui/src/tests/thread-shell.test.tsx
+++ b/webui/src/tests/thread-shell.test.tsx
@@ -6,7 +6,7 @@ import { preloadMarkdownText } from "@/components/MarkdownText";
 import { ThreadCameraController } from "@/components/thread/thread-camera";
 import { ThreadShell } from "@/components/thread/ThreadShell";
 import { CLI_APPS_CHANGED_EVENT } from "@/lib/cli-app-events";
-import type { CanonicalRunSnapshot } from "@/lib/nanobot-client";
+import type { CanonicalRunSnapshot, StreamError } from "@/lib/nanobot-client";
 import { ClientProvider } from "@/providers/ClientProvider";
 import type { CliAppsPayload, ConnectionStatus, SettingsPayload, UIMessage } from "@/lib/types";
 
@@ -14,7 +14,7 @@ const HERO_GREETING_PATTERN =
   /What should we work on\?|Where should we start\?|What are we building today\?|What should we tackle together\?/;
 
 function makeClient() {
-  const errorHandlers = new Set<(err: { kind: string }) => void>();
+  const errorHandlers = new Set<(err: StreamError) => void>();
   const statusHandlers = new Set<(status: ConnectionStatus) => void>();
   const chatHandlers = new Map<string, Set<(ev: import("@/lib/types").InboundEvent) => void>>();
   const runtimeModelHandlers = new Set<
@@ -107,6 +107,7 @@ function makeClient() {
       };
     },
     getRunStartedAt: (chatId: string) => runStartedAtByChatId.get(chatId) ?? null,
+    hasUnsettledRun: () => false,
     getRunGeneration: (chatId: string) => runGenerationByChatId.get(chatId) ?? 0,
     canReconcileCanonicalCompletion,
     reconcileCanonicalCompletion,
@@ -122,7 +123,7 @@ function makeClient() {
         handlers?.delete(handler);
       };
     },
-    onError: (handler: (err: { kind: string }) => void) => {
+    onError: (handler: (err: StreamError) => void) => {
       errorHandlers.add(handler);
       return () => {
         errorHandlers.delete(handler);
@@ -134,7 +135,7 @@ function makeClient() {
         sessionUpdateHandlers.delete(handler);
       };
     },
-    _emitError(err: { kind: string }) {
+    _emitError(err: StreamError) {
       for (const h of errorHandlers) h(err);
     },
     _emitStatus(nextStatus: ConnectionStatus) {
@@ -3208,7 +3209,7 @@ describe("ThreadShell", () => {
     expect(screen.queryByText("Write code")).not.toBeInTheDocument();
   });
 
-  it("surfaces a dismissible banner when the stream reports message_too_big", async () => {
+  it("surfaces a dismissible banner for an uncorrelated message_too_big error", async () => {
     const client = makeClient();
     const onNewChat = vi.fn().mockResolvedValue("chat-a");
 
@@ -3243,6 +3244,52 @@ describe("ThreadShell", () => {
     });
   });
 
+  it("moves a correlated delivery error from the banner into the failed message tooltip", async () => {
+    const client = makeClient();
+
+    render(
+      wrap(
+        client,
+        <ThreadShell
+          session={session("chat-inline-error")}
+          title="Chat inline error"
+          onToggleSidebar={() => {}}
+          onGoHome={() => {}}
+          onNewChat={() => {}}
+        />,
+      ),
+    );
+
+    fireEvent.change(screen.getByLabelText("Message input"), {
+      target: { value: "oversized payload" },
+    });
+    fireEvent.click(screen.getByRole("button", { name: "Send message" }));
+    await waitFor(() => expect(client.sendMessage).toHaveBeenCalledTimes(1));
+    const turnId = client.sendMessage.mock.calls[0][3]?.turnId;
+    expect(turnId).toEqual(expect.any(String));
+
+    await act(async () => {
+      client._emitError({
+        kind: "message_too_big",
+        chatId: "chat-inline-error",
+        turnId,
+      });
+    });
+
+    expect(screen.queryByRole("button", { name: "Dismiss" })).not.toBeInTheDocument();
+    const status = screen.getByRole("button", {
+      name: "Not sent: Message too large",
+    });
+    expect(screen.getByRole("alert")).toHaveClass("sr-only");
+    expect(screen.queryByRole("tooltip")).not.toBeInTheDocument();
+
+    fireEvent.focus(status);
+
+    expect(await screen.findByRole("tooltip")).toHaveTextContent(
+      "The server rejected your last message because it exceeded the size limit.",
+    );
+  });
+
   it("clears the stream error banner when the user switches to another chat", async () => {
     const client = makeClient();
     const onNewChat = vi.fn().mockResolvedValue("chat-a");
diff --git a/webui/src/tests/useNanobotStream.test.tsx b/webui/src/tests/useNanobotStream.test.tsx
index 8ced5bccf..b320318d8 100644
--- a/webui/src/tests/useNanobotStream.test.tsx
+++ b/webui/src/tests/useNanobotStream.test.tsx
@@ -1652,6 +1652,7 @@ describe("useNanobotStream", () => {
     expect(result.current.messages[0].content).toBe("fine");
     expect(result.current.messages[0].turnId).toEqual(expect.any(String));
     expect(result.current.messages[0].turnPhase).toBe("user");
+    expect(result.current.messages[0].deliveryStatus).toBe("sending");
   });
 
   it("returns the submitted turn identity used by the optimistic row and wire frame", () => {
@@ -1681,7 +1682,34 @@ describe("useNanobotStream", () => {
     );
   });
 
-  it("removes only the optimistic turn named by a correlated rejection", () => {
+  it("marks an optimistic turn accepted when its acknowledgement arrives", () => {
+    const fake = fakeClient();
+    const { result } = renderHook(
+      () => useNanobotStream("chat-accept-one", EMPTY_MESSAGES),
+      { wrapper: wrap(fake.client) },
+    );
+    let submitted: ReturnType<typeof result.current.send> = null;
+    act(() => {
+      submitted = result.current.send("hello");
+    });
+
+    act(() => {
+      fake.emit("chat-accept-one", {
+        event: "message_accepted",
+        chat_id: "chat-accept-one",
+        turn_id: submitted!.turnId,
+      });
+    });
+
+    expect(result.current.messages).toEqual([
+      expect.objectContaining({
+        id: submitted!.userMessageId,
+        deliveryStatus: "accepted",
+      }),
+    ]);
+  });
+
+  it("marks only the optimistic turn named by a correlated rejection as failed", () => {
     const fake = fakeClient();
     const { result } = renderHook(
       () => useNanobotStream("chat-reject-one", EMPTY_MESSAGES),
@@ -1705,10 +1733,18 @@ describe("useNanobotStream", () => {
     });
 
     expect(result.current.messages).toEqual([
+      expect.objectContaining({
+        id: first!.userMessageId,
+        turnId: first!.turnId,
+        content: "first",
+        deliveryStatus: "failed",
+        deliveryErrorKind: "turn_rejected",
+      }),
       expect.objectContaining({
         id: second!.userMessageId,
         turnId: second!.turnId,
         content: "second",
+        deliveryStatus: "sending",
       }),
     ]);
     expect(result.current.isStreaming).toBe(true);
@@ -1751,6 +1787,12 @@ describe("useNanobotStream", () => {
       expect.objectContaining({
         id: first!.userMessageId,
         turnId: first!.turnId,
+        deliveryStatus: "accepted",
+      }),
+      expect.objectContaining({
+        id: second!.userMessageId,
+        turnId: second!.turnId,
+        deliveryStatus: "failed",
       }),
     ]);
     expect(result.current.runStartedAt).toBe(1234);
@@ -1784,7 +1826,13 @@ describe("useNanobotStream", () => {
     });
     await flushStreamFrame();
 
-    expect(result.current.messages).toEqual([]);
+    expect(result.current.messages).toEqual([
+      expect.objectContaining({
+        id: submitted!.userMessageId,
+        deliveryStatus: "failed",
+        deliveryErrorKind: "turn_rejected",
+      }),
+    ]);
     expect(result.current.runStartedAt).toBeNull();
     expect(result.current.isStreaming).toBe(false);
   });
@@ -1810,7 +1858,12 @@ describe("useNanobotStream", () => {
       });
     });
 
-    expect(result.current.messages).toEqual([]);
+    expect(result.current.messages).toEqual([
+      expect.objectContaining({
+        id: submitted!.userMessageId,
+        deliveryStatus: "failed",
+      }),
+    ]);
     expect(result.current.streamError).toMatchObject({
       kind: "turn_rejected",
       chatId: "chat-replayed-reject",
@@ -1860,7 +1913,7 @@ describe("useNanobotStream", () => {
     expect(result.current.streamError).toEqual({ kind: "message_too_big" });
   });
 
-  it("removes rejected side-channel guidance without stopping the main run", () => {
+  it("marks rejected side-channel guidance failed without stopping the main run", () => {
     const fake = fakeClient();
     const { result } = renderHook(
       () => useNanobotStream("chat-side-reject", EMPTY_MESSAGES),
@@ -1893,6 +1946,12 @@ describe("useNanobotStream", () => {
       expect.objectContaining({
         id: main!.userMessageId,
         turnId: main!.turnId,
+        deliveryStatus: "accepted",
+      }),
+      expect.objectContaining({
+        id: side!.userMessageId,
+        turnId: side!.turnId,
+        deliveryStatus: "failed",
       }),
     ]);
     expect(result.current.runStartedAt).toBe(9876);