diff --git a/webviews/src/agent-session/acpmux/App.tsx b/webviews/src/agent-session/acpmux/App.tsx
index 8bcc647fcbdd..e7b62149ec1f 100644
--- a/webviews/src/agent-session/acpmux/App.tsx
+++ b/webviews/src/agent-session/acpmux/App.tsx
@@ -489,6 +489,7 @@ import {
   AcpmuxRestartGate,
   hostErrorShows,
   hostIsReconnecting,
+  hostRefusedTooLarge,
   nextRetryDelay,
   offersAcpmuxRestart,
 } from "./hostRetry";
@@ -9581,7 +9582,9 @@ function AcpmuxPane() {
         if (!cancelled) {
           acpWire.lifecycle("handshake failed", { message: String(error) });
           setSnapshot((current) => ({ ...current, connection: `connecting: ${String(error)}` }));
-          setHostError(errorMessage(error) || String(error));
+          // A chat too large to open says so in plain words; the reply's byte count stays in the log.
+          const tooLarge = hostRefusedTooLarge(error);
+          setHostError(tooLarge ? translate("host.tooLarge") : errorMessage(error) || String(error));
           failingSince ??= Date.now();
           const recovering = hostIsReconnecting(error);
           setHostErrorShown(hostErrorShows(error, Date.now() - failingSince));
@@ -9591,7 +9594,8 @@ function AcpmuxPane() {
           setAcpmuxRestartOffered(offersAcpmuxRestart(error));
           // Back off so a host without a daemon is not asked four times a second. A Restart acpmux
           // in flight asks again itself when it ends.
-          if (restartGate.mayAsk()) {
+          // A reply too large to send is the same at every ask: only the line's Retry asks again.
+          if (!tooLarge && restartGate.mayAsk()) {
             retryTimer = window.setTimeout(() => void connectHost(), retryDelay);
             retryDelay = nextRetryDelay(retryDelay, reconnect || recovering);
           }
diff --git a/webviews/src/agent-session/acpmux/HostError.gallery.tsx b/webviews/src/agent-session/acpmux/HostError.gallery.tsx
index e665a48599ad..80cfefc9f816 100644
--- a/webviews/src/agent-session/acpmux/HostError.gallery.tsx
+++ b/webviews/src/agent-session/acpmux/HostError.gallery.tsx
@@ -153,6 +153,14 @@ export default componentEntry<Props>({
       },
       note: "A refusal whose holder is no app bundle offers the same Restart acpmux.",
     },
+    "too-large": {
+      props: {
+        message: "This chat is too large to open. Update acpmux, then Retry.",
+        initialRetrying: false,
+        withRetry: true,
+      },
+      note: 'acpmux refused the chat\'s open as larger than one message (message.too_large, an acpmux before #28754): said at once in plain words, never the byte count, and the pane does not ask again on its own; Retry still asks (Leo, 2026-10-31: "this is completely hardlocked bc of message size").',
+    },
     waiting: {
       props: { message: "", initialRetrying: false, withRetry: false, waitedMs: 95_000 },
       play: restartService,
diff --git a/webviews/src/agent-session/acpmux/Localizable.xcstrings b/webviews/src/agent-session/acpmux/Localizable.xcstrings
index 123aa9aab908..a662539d2113 100644
--- a/webviews/src/agent-session/acpmux/Localizable.xcstrings
+++ b/webviews/src/agent-session/acpmux/Localizable.xcstrings
@@ -1,6 +1,9 @@
 {
   "sourceLanguage": "en",
   "strings": {
+    "prompt.failure.tooLarge": {"comment":"Agent pane: under a prompt refused because it was larger than one message can carry (inline attachments); Retry is not offered, Edit is","extractionState":"manual","localizations":{"en":{"stringUnit":{"state":"translated","value":"Not sent: this message is too large to send at once. Take out an attachment with Edit, or send the attachments in separate messages."}},"ar":{"stringUnit":{"state":"needs_review","value":"لم تُرسَل: هذه الرسالة أكبر من أن تُرسَل دفعة واحدة. أزِل مرفقًا عبر «تعديل»، أو أرسل المرفقات في رسائل منفصلة."}},"bs":{"stringUnit":{"state":"needs_review","value":"Nije poslano: ova poruka je prevelika da bi se poslala odjednom. Uklonite prilog pomoću Uredi ili pošaljite priloge u zasebnim porukama."}},"da":{"stringUnit":{"state":"needs_review","value":"Ikke sendt: denne besked er for stor til at blive sendt på én gang. Fjern en vedhæftning med Rediger, eller send vedhæftningerne i separate beskeder."}},"de":{"stringUnit":{"state":"needs_review","value":"Nicht gesendet: Diese Nachricht ist zu groß, um auf einmal gesendet zu werden. Entferne mit Bearbeiten einen Anhang oder sende die Anhänge in getrennten Nachrichten."}},"es":{"stringUnit":{"state":"needs_review","value":"No enviado: este mensaje es demasiado grande para enviarlo de una vez. Quita un adjunto con Editar o envía los adjuntos en mensajes separados."}},"fr":{"stringUnit":{"state":"needs_review","value":"Non envoyé : ce message est trop volumineux pour être envoyé en une fois. Retirez une pièce jointe avec Modifier, ou envoyez les pièces jointes dans des messages séparés."}},"it":{"stringUnit":{"state":"needs_review","value":"Non inviato: questo messaggio è troppo grande per essere inviato in una volta. Rimuovi un allegato con Modifica o invia gli allegati in messaggi separati."}},"ja":{"stringUnit":{"state":"needs_review","value":"未送信：このメッセージは一度に送信するには大きすぎます。「編集」で添付ファイルを外すか、添付ファイルを別々のメッセージで送信してください。"}},"km":{"stringUnit":{"state":"needs_review","value":"មិនបានផ្ញើ៖ សារនេះធំពេកមិនអាចផ្ញើក្នុងពេលតែមួយបានទេ។ ដកឯកសារភ្ជាប់មួយចេញដោយប្រើ កែសម្រួល ឬផ្ញើឯកសារភ្ជាប់ក្នុងសារដាច់ដោយឡែក។"}},"ko":{"stringUnit":{"state":"needs_review","value":"보내지 않음: 이 메시지는 한 번에 보내기에 너무 큽니다. 편집으로 첨부 파일을 빼거나 첨부 파일을 별도의 메시지로 보내세요."}},"nb":{"stringUnit":{"state":"needs_review","value":"Ikke sendt: denne meldingen er for stor til å sendes på én gang. Fjern et vedlegg med Rediger, eller send vedleggene i separate meldinger."}},"pl":{"stringUnit":{"state":"needs_review","value":"Nie wysłano: ta wiadomość jest za duża, by wysłać ją naraz. Usuń załącznik przez Edytuj albo wyślij załączniki w osobnych wiadomościach."}},"pt-BR":{"stringUnit":{"state":"needs_review","value":"Não enviada: esta mensagem é grande demais para ser enviada de uma vez. Remova um anexo com Editar ou envie os anexos em mensagens separadas."}},"ru":{"stringUnit":{"state":"needs_review","value":"Не отправлено: это сообщение слишком велико, чтобы отправить его за раз. Уберите вложение через «Изменить» или отправьте вложения отдельными сообщениями."}},"th":{"stringUnit":{"state":"needs_review","value":"ไม่ได้ส่ง: ข้อความนี้ใหญ่เกินกว่าจะส่งในครั้งเดียว นำไฟล์แนบออกด้วย แก้ไข หรือส่งไฟล์แนบแยกเป็นหลายข้อความ"}},"tr":{"stringUnit":{"state":"needs_review","value":"Gönderilmedi: bu ileti tek seferde gönderilemeyecek kadar büyük. Düzenle ile bir eki çıkarın ya da ekleri ayrı iletilerde gönderin."}},"uk":{"stringUnit":{"state":"needs_review","value":"Не надіслано: це повідомлення завелике, щоб надіслати його за раз. Приберіть вкладення через «Редагувати» або надішліть вкладення окремими повідомленнями."}},"vi":{"stringUnit":{"state":"needs_review","value":"Chưa gửi: tin nhắn này quá lớn để gửi một lần. Gỡ một tệp đính kèm bằng Sửa, hoặc gửi các tệp đính kèm trong các tin nhắn riêng."}},"zh-Hans":{"stringUnit":{"state":"needs_review","value":"未发送：此消息太大，无法一次发送。请用“编辑”移除一个附件，或将附件分成多条消息发送。"}},"zh-Hant":{"stringUnit":{"state":"needs_review","value":"未傳送：此訊息太大，無法一次傳送。請用「編輯」移除一個附件，或將附件分成多則訊息傳送。"}}}},
+    "prompt.failure.tooLargeFile": {"comment":"Agent pane: under a prompt refused because it was larger than one message can carry; {name} is the attachment that takes the most of it. Retry is not offered, Edit is","extractionState":"manual","localizations":{"en":{"stringUnit":{"state":"translated","value":"Not sent: this message is too large to send at once. Take out {name} with Edit, or send it in a message of its own."}},"ar":{"stringUnit":{"state":"needs_review","value":"لم تُرسَل: هذه الرسالة أكبر من أن تُرسَل دفعة واحدة. أزِل {name} عبر «تعديل»، أو أرسله في رسالة مستقلة."}},"bs":{"stringUnit":{"state":"needs_review","value":"Nije poslano: ova poruka je prevelika da bi se poslala odjednom. Uklonite {name} pomoću Uredi ili ga pošaljite u zasebnoj poruci."}},"da":{"stringUnit":{"state":"needs_review","value":"Ikke sendt: denne besked er for stor til at blive sendt på én gang. Fjern {name} med Rediger, eller send den i en besked for sig."}},"de":{"stringUnit":{"state":"needs_review","value":"Nicht gesendet: Diese Nachricht ist zu groß, um auf einmal gesendet zu werden. Entferne {name} mit Bearbeiten oder sende es in einer eigenen Nachricht."}},"es":{"stringUnit":{"state":"needs_review","value":"No enviado: este mensaje es demasiado grande para enviarlo de una vez. Quita {name} con Editar o envíalo en un mensaje aparte."}},"fr":{"stringUnit":{"state":"needs_review","value":"Non envoyé : ce message est trop volumineux pour être envoyé en une fois. Retirez {name} avec Modifier, ou envoyez-le dans un message à part."}},"it":{"stringUnit":{"state":"needs_review","value":"Non inviato: questo messaggio è troppo grande per essere inviato in una volta. Rimuovi {name} con Modifica o invialo in un messaggio a parte."}},"ja":{"stringUnit":{"state":"needs_review","value":"未送信：このメッセージは一度に送信するには大きすぎます。「編集」で {name} を外すか、別のメッセージで送信してください。"}},"km":{"stringUnit":{"state":"needs_review","value":"មិនបានផ្ញើ៖ សារនេះធំពេកមិនអាចផ្ញើក្នុងពេលតែមួយបានទេ។ ដក {name} ចេញដោយប្រើ កែសម្រួល ឬផ្ញើវាក្នុងសារដាច់ដោយឡែក។"}},"ko":{"stringUnit":{"state":"needs_review","value":"보내지 않음: 이 메시지는 한 번에 보내기에 너무 큽니다. 편집으로 {name}을(를) 빼거나 별도의 메시지로 보내세요."}},"nb":{"stringUnit":{"state":"needs_review","value":"Ikke sendt: denne meldingen er for stor til å sendes på én gang. Fjern {name} med Rediger, eller send den i en egen melding."}},"pl":{"stringUnit":{"state":"needs_review","value":"Nie wysłano: ta wiadomość jest za duża, by wysłać ją naraz. Usuń {name} przez Edytuj albo wyślij to w osobnej wiadomości."}},"pt-BR":{"stringUnit":{"state":"needs_review","value":"Não enviada: esta mensagem é grande demais para ser enviada de uma vez. Remova {name} com Editar ou envie em uma mensagem separada."}},"ru":{"stringUnit":{"state":"needs_review","value":"Не отправлено: это сообщение слишком велико, чтобы отправить его за раз. Уберите {name} через «Изменить» или отправьте отдельным сообщением."}},"th":{"stringUnit":{"state":"needs_review","value":"ไม่ได้ส่ง: ข้อความนี้ใหญ่เกินกว่าจะส่งในครั้งเดียว นำ {name} ออกด้วย แก้ไข หรือส่งแยกเป็นข้อความของมันเอง"}},"tr":{"stringUnit":{"state":"needs_review","value":"Gönderilmedi: bu ileti tek seferde gönderilemeyecek kadar büyük. Düzenle ile {name} ögesini çıkarın ya da ayrı bir iletide gönderin."}},"uk":{"stringUnit":{"state":"needs_review","value":"Не надіслано: це повідомлення завелике, щоб надіслати його за раз. Приберіть {name} через «Редагувати» або надішліть окремим повідомленням."}},"vi":{"stringUnit":{"state":"needs_review","value":"Chưa gửi: tin nhắn này quá lớn để gửi một lần. Gỡ {name} bằng Sửa, hoặc gửi nó trong một tin nhắn riêng."}},"zh-Hans":{"stringUnit":{"state":"needs_review","value":"未发送：此消息太大，无法一次发送。请用“编辑”移除 {name}，或单独发送它。"}},"zh-Hant":{"stringUnit":{"state":"needs_review","value":"未傳送：此訊息太大，無法一次傳送。請用「編輯」移除 {name}，或單獨傳送它。"}}}},
+    "host.tooLarge": {"comment":"Agent pane: the line above the composer when a chat could not be opened because acpmux's reply was larger than one message; the pane no longer retries on its own, the line's Retry does","extractionState":"manual","localizations":{"en":{"stringUnit":{"state":"translated","value":"This chat is too large to open. Update acpmux, then Retry."}},"ar":{"stringUnit":{"state":"needs_review","value":"هذه المحادثة أكبر من أن تُفتح. حدّث acpmux، ثم أعد المحاولة."}},"bs":{"stringUnit":{"state":"needs_review","value":"Ovaj chat je prevelik za otvaranje. Ažurirajte acpmux, pa pokušajte ponovo."}},"da":{"stringUnit":{"state":"needs_review","value":"Denne chat er for stor til at åbne. Opdater acpmux, og prøv igen."}},"de":{"stringUnit":{"state":"needs_review","value":"Dieser Chat ist zu groß zum Öffnen. Aktualisiere acpmux und versuche es erneut."}},"es":{"stringUnit":{"state":"needs_review","value":"Este chat es demasiado grande para abrirlo. Actualiza acpmux y vuelve a intentarlo."}},"fr":{"stringUnit":{"state":"needs_review","value":"Cette discussion est trop volumineuse pour s’ouvrir. Mettez à jour acpmux, puis réessayez."}},"it":{"stringUnit":{"state":"needs_review","value":"Questa chat è troppo grande per essere aperta. Aggiorna acpmux, poi riprova."}},"ja":{"stringUnit":{"state":"needs_review","value":"このチャットは大きすぎて開けません。acpmux を更新してから再試行してください。"}},"km":{"stringUnit":{"state":"needs_review","value":"ការជជែកនេះធំពេកមិនអាចបើកបានទេ។ ធ្វើបច្ចុប្បន្នភាព acpmux រួចព្យាយាមម្ដងទៀត។"}},"ko":{"stringUnit":{"state":"needs_review","value":"이 채팅은 너무 커서 열 수 없습니다. acpmux를 업데이트한 다음 다시 시도하세요."}},"nb":{"stringUnit":{"state":"needs_review","value":"Denne chatten er for stor til å åpnes. Oppdater acpmux, og prøv igjen."}},"pl":{"stringUnit":{"state":"needs_review","value":"Ten czat jest za duży, by go otworzyć. Zaktualizuj acpmux, a potem spróbuj ponownie."}},"pt-BR":{"stringUnit":{"state":"needs_review","value":"Este chat é grande demais para abrir. Atualize o acpmux e tente novamente."}},"ru":{"stringUnit":{"state":"needs_review","value":"Этот чат слишком велик, чтобы его открыть. Обновите acpmux и повторите попытку."}},"th":{"stringUnit":{"state":"needs_review","value":"แชทนี้ใหญ่เกินกว่าจะเปิดได้ อัปเดต acpmux แล้วลองอีกครั้ง"}},"tr":{"stringUnit":{"state":"needs_review","value":"Bu sohbet açılamayacak kadar büyük. acpmux'u güncelleyip yeniden deneyin."}},"uk":{"stringUnit":{"state":"needs_review","value":"Цей чат завеликий, щоб його відкрити. Оновіть acpmux, а тоді повторіть спробу."}},"vi":{"stringUnit":{"state":"needs_review","value":"Cuộc trò chuyện này quá lớn để mở. Hãy cập nhật acpmux rồi thử lại."}},"zh-Hans":{"stringUnit":{"state":"needs_review","value":"此聊天太大，无法打开。请更新 acpmux，然后重试。"}},"zh-Hant":{"stringUnit":{"state":"needs_review","value":"此聊天太大，無法開啟。請更新 acpmux，然後重試。"}}}},
     "featureTip.new": {"comment":"Agent pane feature tip: the pill beside a new feature's title","extractionState":"manual","localizations":{"en":{"stringUnit":{"state":"translated","value":"New"}},"ar":{"stringUnit":{"state":"needs_review","value":"جديد"}},"bs":{"stringUnit":{"state":"needs_review","value":"Novo"}},"da":{"stringUnit":{"state":"needs_review","value":"Ny"}},"de":{"stringUnit":{"state":"needs_review","value":"Neu"}},"es":{"stringUnit":{"state":"needs_review","value":"Nuevo"}},"fr":{"stringUnit":{"state":"needs_review","value":"Nouveau"}},"it":{"stringUnit":{"state":"needs_review","value":"Novità"}},"ja":{"stringUnit":{"state":"needs_review","value":"新機能"}},"km":{"stringUnit":{"state":"needs_review","value":"ថ្មី"}},"ko":{"stringUnit":{"state":"needs_review","value":"새 기능"}},"nb":{"stringUnit":{"state":"needs_review","value":"Ny"}},"pl":{"stringUnit":{"state":"needs_review","value":"Nowość"}},"pt-BR":{"stringUnit":{"state":"needs_review","value":"Novo"}},"ru":{"stringUnit":{"state":"needs_review","value":"Новое"}},"th":{"stringUnit":{"state":"needs_review","value":"ใหม่"}},"tr":{"stringUnit":{"state":"needs_review","value":"Yeni"}},"uk":{"stringUnit":{"state":"needs_review","value":"Нове"}},"vi":{"stringUnit":{"state":"needs_review","value":"Mới"}},"zh-Hans":{"stringUnit":{"state":"needs_review","value":"新"}},"zh-Hant":{"stringUnit":{"state":"needs_review","value":"新"}}}},
     "featureTip.close": {"comment":"Agent pane feature tip: the close button's accessible name","extractionState":"manual","localizations":{"en":{"stringUnit":{"state":"translated","value":"Close tip"}},"ar":{"stringUnit":{"state":"needs_review","value":"إغلاق التلميح"}},"bs":{"stringUnit":{"state":"needs_review","value":"Zatvori savjet"}},"da":{"stringUnit":{"state":"needs_review","value":"Luk tip"}},"de":{"stringUnit":{"state":"needs_review","value":"Tipp schließen"}},"es":{"stringUnit":{"state":"needs_review","value":"Cerrar consejo"}},"fr":{"stringUnit":{"state":"needs_review","value":"Fermer l’astuce"}},"it":{"stringUnit":{"state":"needs_review","value":"Chiudi suggerimento"}},"ja":{"stringUnit":{"state":"needs_review","value":"ヒントを閉じる"}},"km":{"stringUnit":{"state":"needs_review","value":"បិទគន្លឹះ"}},"ko":{"stringUnit":{"state":"needs_review","value":"팁 닫기"}},"nb":{"stringUnit":{"state":"needs_review","value":"Lukk tips"}},"pl":{"stringUnit":{"state":"needs_review","value":"Zamknij wskazówkę"}},"pt-BR":{"stringUnit":{"state":"needs_review","value":"Fechar dica"}},"ru":{"stringUnit":{"state":"needs_review","value":"Закрыть подсказку"}},"th":{"stringUnit":{"state":"needs_review","value":"ปิดเคล็ดลับ"}},"tr":{"stringUnit":{"state":"needs_review","value":"İpucunu kapat"}},"uk":{"stringUnit":{"state":"needs_review","value":"Закрити підказку"}},"vi":{"stringUnit":{"state":"needs_review","value":"Đóng mẹo"}},"zh-Hans":{"stringUnit":{"state":"needs_review","value":"关闭提示"}},"zh-Hant":{"stringUnit":{"state":"needs_review","value":"關閉提示"}}}},
     "access.addCredits": {"comment":"Agent pane: a failed turn's action that opens the cmux paid-plan Checkout (cmux credits), when the cmux model router refused the turn for want of credits or a plan","extractionState":"manual","localizations":{"en":{"stringUnit":{"state":"translated","value":"Add Credits…"}},"ar":{"stringUnit":{"state":"needs_review","value":"إضافة رصيد…"}},"bs":{"stringUnit":{"state":"needs_review","value":"Dodaj kredite…"}},"da":{"stringUnit":{"state":"needs_review","value":"Tilføj kreditter…"}},"de":{"stringUnit":{"state":"needs_review","value":"Guthaben hinzufügen…"}},"es":{"stringUnit":{"state":"needs_review","value":"Añadir créditos…"}},"fr":{"stringUnit":{"state":"needs_review","value":"Ajouter des crédits…"}},"it":{"stringUnit":{"state":"needs_review","value":"Aggiungi crediti…"}},"ja":{"stringUnit":{"state":"needs_review","value":"クレジットを追加…"}},"km":{"stringUnit":{"state":"needs_review","value":"បន្ថែមឥណទាន…"}},"ko":{"stringUnit":{"state":"needs_review","value":"크레딧 추가…"}},"nb":{"stringUnit":{"state":"needs_review","value":"Legg til kreditter…"}},"pl":{"stringUnit":{"state":"needs_review","value":"Dodaj kredyty…"}},"pt-BR":{"stringUnit":{"state":"needs_review","value":"Adicionar créditos…"}},"ru":{"stringUnit":{"state":"needs_review","value":"Добавить кредиты…"}},"th":{"stringUnit":{"state":"needs_review","value":"เพิ่มเครดิต…"}},"tr":{"stringUnit":{"state":"needs_review","value":"Kredi ekle…"}},"uk":{"stringUnit":{"state":"needs_review","value":"Додати кредити…"}},"vi":{"stringUnit":{"state":"needs_review","value":"Thêm tín dụng…"}},"zh-Hans":{"stringUnit":{"state":"needs_review","value":"添加额度…"}},"zh-Hant":{"stringUnit":{"state":"needs_review","value":"新增額度…"}}}},
diff --git a/webviews/src/agent-session/acpmux/conversation/PromptDelivery.test.tsx b/webviews/src/agent-session/acpmux/conversation/PromptDelivery.test.tsx
index 6518ea0bf8d1..63156ad7cf75 100644
--- a/webviews/src/agent-session/acpmux/conversation/PromptDelivery.test.tsx
+++ b/webviews/src/agent-session/acpmux/conversation/PromptDelivery.test.tsx
@@ -8,8 +8,10 @@ import { act, type ReactNode } from "react";
 import { createRoot, type Root } from "react-dom/client";
 import { JSDOM } from "jsdom";
 import { translate } from "../i18n";
-import type { AcpmuxRow, PromptDelivery } from "../model";
-import { failedRuns } from "../sendFailure";
+import type { AcpmuxRow, AcpmuxSnapshot, PromptDelivery } from "../model";
+import type { ComposerAttachment } from "../attachments";
+import { failedRuns, failedSendText } from "../sendFailure";
+import { messageMenuTarget } from "./messageMenu";
 import { DeliveryFailure, FailedSendsContext, type FailedSends } from "./PromptDelivery";
 
 let dom: JSDOM | undefined;
@@ -159,3 +161,45 @@ test("Retry waits while the folder's trust question is open, and comes back once
   expect(button(container, "Retry")!.disabled).toBe(false);
   expect(button(container, "Trust folder")).toBeUndefined();
 });
+
+/// A prompt past one message (`message.too_large`, acpmux's `rpc::MAX_MESSAGE_BYTES`) is refused the
+/// same way at every Retry (Leo, 2026-10-31: "this is completely hardlocked bc of message size"): its
+/// card names the attachment to take out and offers Edit, never Retry.
+test("a prompt too large to send names its largest attachment and offers Edit, not Retry", async () => {
+  const tooLarge = { code: -32000, reason: "message.too_large", message: "message too large to send: 16907300 bytes" };
+  const attachment = (id: string, name: string, bytes: number): ComposerAttachment => ({
+    id,
+    kind: "image",
+    name,
+    mimeType: "image/png",
+    size: bytes,
+    data: "A".repeat(bytes),
+  });
+  const attachments = [
+    attachment("a", "small.png", 10),
+    attachment("b", "screen-recording.png", 4_000),
+    attachment("c", "notes.png", 20),
+  ];
+  const sentence = failedSendText(tooLarge, false, attachments);
+  expect(sentence).toBe(translate("prompt.failure.tooLargeFile", { name: "screen-recording.png" }));
+  expect(sentence).toContain("screen-recording.png");
+  expect(sentence).not.toContain("16907300");
+  // With no attachment to name (a long pasted text), it still says how to send it.
+  expect(failedSendText(tooLarge, false)).toBe(translate("prompt.failure.tooLarge"));
+
+  const row: AcpmuxRow = {
+    ...failed("a", "look at these", { reason: "message.too_large", code: "-32000", error: tooLarge.message }),
+    error: sentence,
+  };
+  const container = await render(transcript([row], {}));
+  const card = container.querySelector<HTMLElement>(".cv-user__status--failed")!;
+  expect(card.dataset.failureCause).toBe("too-large");
+  expect(card.querySelector(".cv-user__failure-text")!.textContent).toBe(sentence);
+  expect(button(card, "Retry")).toBeUndefined();
+  expect(button(card, "Edit")).toBeDefined();
+  // Nor from the message's menu (the native context menu offers Retry for `retryRowId`).
+  const other = failed("b", "deploy", { code: "timeout", error: "no answer" });
+  const snapshot = { rows: [row, other], connection: "connected" } as unknown as AcpmuxSnapshot;
+  expect(messageMenuTarget(snapshot, "a")?.retryRowId).toBeUndefined();
+  expect(messageMenuTarget(snapshot, "b")?.retryRowId).toBe("b");
+});
diff --git a/webviews/src/agent-session/acpmux/conversation/PromptDelivery.tsx b/webviews/src/agent-session/acpmux/conversation/PromptDelivery.tsx
index 2aeaa185c1b5..7736e0131527 100644
--- a/webviews/src/agent-session/acpmux/conversation/PromptDelivery.tsx
+++ b/webviews/src/agent-session/acpmux/conversation/PromptDelivery.tsx
@@ -7,7 +7,7 @@
 import { createContext, useContext, useEffect, useState } from "react";
 import type { AcpmuxRow, PromptDelivery } from "../model";
 import { useT, type StringKey } from "../i18n";
-import { isTrustCause, sendCause, type FailedRun } from "../sendFailure";
+import { isTrustCause, retryHelps, sendCause, type FailedRun } from "../sendFailure";
 import { copyText } from "./clipboard";
 import { Clock } from "./icons";
 
@@ -97,7 +97,8 @@ export function DeliveryFailure({ row }: { row: AcpmuxRow }) {
   if (!row.failed) return null;
   const rowIds = sends?.runs.get(row.id)?.rowIds ?? [row.id];
   if (rowIds[rowIds.length - 1] !== row.id) return null;
-  const trust = isTrustCause(sendCause(delivery));
+  const cause = sendCause(delivery);
+  const trust = isTrustCause(cause);
   const retryWaits = trust ? sends?.trustBlock : undefined;
   const fix = trust ? sends?.trustFolder : undefined;
   const act = (name: string) => void window.cmuxAcpmuxActions?.[name]?.({ rowId: row.id, rowIds });
@@ -107,7 +108,7 @@ export function DeliveryFailure({ row }: { row: AcpmuxRow }) {
   return (
     <div
       className="cv-user__status cv-user__status--failed"
-      data-failure-cause={sendCause(delivery)}
+      data-failure-cause={cause}
       data-failed-count={rowIds.length}
       aria-live="polite"
     >
@@ -120,15 +121,18 @@ export function DeliveryFailure({ row }: { row: AcpmuxRow }) {
             {t("trust.trustFolder")}
           </button>
         )}
-        <button
-          type="button"
-          className="cv-user__cancel"
-          disabled={retryWaits !== undefined}
-          title={retryWaits && t(retryWaits)}
-          onClick={() => act("chat.retryPrompt")}
-        >
-          {t("turn.retry")}
-        </button>
+        {/* A prompt too large to send is refused the same way again: Edit takes an attachment out. */}
+        {retryHelps(cause) && (
+          <button
+            type="button"
+            className="cv-user__cancel"
+            disabled={retryWaits !== undefined}
+            title={retryWaits && t(retryWaits)}
+            onClick={() => act("chat.retryPrompt")}
+          >
+            {t("turn.retry")}
+          </button>
+        )}
         {delivery && (
           <>
             <button type="button" className="cv-user__cancel" onClick={() => act("chat.editFailedPrompt")}>
diff --git a/webviews/src/agent-session/acpmux/conversation/messageMenu.ts b/webviews/src/agent-session/acpmux/conversation/messageMenu.ts
index ec06043dfb88..21aedb70ebd5 100644
--- a/webviews/src/agent-session/acpmux/conversation/messageMenu.ts
+++ b/webviews/src/agent-session/acpmux/conversation/messageMenu.ts
@@ -10,6 +10,7 @@ import { Lexer, lexer, type Token, walkTokens } from "marked";
 import { turnRows } from "../diff";
 import type { AcpmuxSnapshot } from "../model";
 import { menuPartAt, type MenuPart } from "./menuParts";
+import { retryHelps, sendCause } from "../sendFailure";
 
 export const MESSAGE_MENU_HANDLER = "cmuxAgentContextMenu";
 
@@ -140,7 +141,8 @@ export function messageMenuTarget(
     row.kind === "user" ? { text: row.text } : { text: plainText(row.text), markdown: row.text };
   if (forkSeq !== undefined) target.forkSeq = forkSeq;
   if (forkSeq !== undefined && sideChat) target.sideChat = true;
-  if (row.kind === "user" && row.failed) target.retryRowId = row.id;
+  // A prompt too large to send is refused the same way again: no Retry, Edit takes an attachment out.
+  if (row.kind === "user" && row.failed && retryHelps(sendCause(row.delivery))) target.retryRowId = row.id;
   const links = webLinks(row.text);
   if (links.length > 0) target.links = links;
   return target;
diff --git a/webviews/src/agent-session/acpmux/conversation/transcript.gallery.ts b/webviews/src/agent-session/acpmux/conversation/transcript.gallery.ts
index bd8630429d73..3f11ad13eca6 100644
--- a/webviews/src/agent-session/acpmux/conversation/transcript.gallery.ts
+++ b/webviews/src/agent-session/acpmux/conversation/transcript.gallery.ts
@@ -2794,6 +2794,33 @@ export default agentPaneEntry({
         );
       },
     },
+    "failed-send-too-large": {
+      note: 'A prompt larger than one message (message.too_large): acpmux refuses it the same way at every Retry, so the card names the attachment to take out and offers Edit, Remove and Details, never Retry (Leo, 2026-10-31: "this is completely hardlocked bc of message size").',
+      snapshot: chat([
+        user(prompt, 6),
+        assistant("Added retries with backoff.", 5.5),
+        summary(5.5, { status: "completed" }),
+        user("Why does the dashboard look like this on the staging build?", 1, {
+          failed: true,
+          error:
+            "Not sent: this message is too large to send at once. Take out dashboard-recording.png with Edit, or send it in a message of its own.",
+          delivery: {
+            state: "failed",
+            promptId: "p1",
+            sessionId: "s",
+            sentAt: 0,
+            failedAt: 900,
+            hop: "acpmux",
+            code: "-32000",
+            reason: "message.too_large",
+            error: "message too large to send: 16907300 bytes, the limit is 15728640",
+          },
+        }),
+      ]),
+      play: async (ctx) => {
+        await ctx.waitFor(() => ctx.document.querySelector('[data-failure-cause="too-large"]'));
+      },
+    },
     "edited-files": {
       note: "A turn that edited three files: the edited-files card with Undo.",
       snapshot: chat([...workedTurnRows(minutesAgo(15)), summary(14, { status: "completed", toolCount: 3 })]),
diff --git a/webviews/src/agent-session/acpmux/direct.ts b/webviews/src/agent-session/acpmux/direct.ts
index 05673c59511b..957c23ad6b31 100644
--- a/webviews/src/agent-session/acpmux/direct.ts
+++ b/webviews/src/agent-session/acpmux/direct.ts
@@ -71,7 +71,7 @@ import { cliHost, incomingPrompt, type IncomingContext, type IncomingMessage } f
 import { startedCardsShown } from "./started/startedSessions";
 import { OfferChildren } from "./offer/offerChildren";
 import { errorMessage } from "./transportErrors";
-import { failedSendText } from "./sendFailure";
+import { failedSendText, retryHelps, sendCause } from "./sendFailure";
 import { SessionPager, type SessionPage } from "./sessionPages";
 import { isWarmableCwd } from "./warmFolders";
 import { isPlumbingCall, showsNothing } from "./hiddenTools";
@@ -6073,7 +6073,9 @@ export class AcpmuxDirectClient {
         // says just that, not the generic refusal's pick wording (cx-w2p8r).
         if (held && !isTrustRefusal(error))
           this.notice(
-            code === "transport.gesture_required" ? translate("prompt.notSentAgain") : failedSendText(error, false),
+            code === "transport.gesture_required"
+              ? translate("prompt.notSentAgain")
+              : failedSendText(error, false, sendAttachments),
           );
         else this.emit();
         throw error;
@@ -6103,7 +6105,7 @@ export class AcpmuxDirectClient {
           ...row,
           pending: false,
           failed: true,
-          error: failedSendText(error, this.retriedPrompts.has(promptId)),
+          error: failedSendText(error, this.retriedPrompts.has(promptId), sendAttachments),
           version: row.version + 1,
         });
         this.failedPrompts.set(rowId, { input, attachments, promptId });
@@ -6230,7 +6232,9 @@ export class AcpmuxDirectClient {
   /// prompt id (a trust refusal's kept gesture is bound to that id, heldPrompt.ts). The run's other
   /// rows go.
   async retryPrompts(rowIds: readonly string[], ticket?: string, anchor?: string): Promise<string | undefined> {
-    const run = rowIds.filter((id) => this.failedPrompts.has(id));
+    // A prompt too large to send is refused the same way again, from any Retry (the card, the
+    // message menu): it stays failed until Edit takes an attachment out.
+    const run = rowIds.filter((id) => this.failedPrompts.has(id) && retryHelps(sendCause(this.rows.get(id)?.delivery)));
     const kept = anchor && run.includes(anchor) ? anchor : run[run.length - 1];
     if (!kept) return undefined;
     if (run.length > 1) {
diff --git a/webviews/src/agent-session/acpmux/hostRetry.test.ts b/webviews/src/agent-session/acpmux/hostRetry.test.ts
index bce1bbaea830..7a860b1ee09d 100644
--- a/webviews/src/agent-session/acpmux/hostRetry.test.ts
+++ b/webviews/src/agent-session/acpmux/hostRetry.test.ts
@@ -7,6 +7,7 @@ import {
   HOST_ERROR_DEADLINE_MS,
   hostErrorShows,
   hostIsReconnecting,
+  hostRefusedTooLarge,
   nextRetryDelay,
   offersAcpmuxRestart,
 } from "./hostRetry";
@@ -171,3 +172,16 @@ test("a recovering pane asks again within 2 s; a refusal backs off to 30 s", ()
   expect(waits(true)).toEqual([250, 500, 1000, 2000, 2000, 2000, 2000, 2000]);
   expect(Math.max(...waits(false))).toBe(30_000);
 });
+
+/// A chat whose open reply passes one message is refused the same way at every ask (Leo,
+/// 2026-10-31: "this is completely hardlocked bc of message size"): the pane stops asking on its own.
+test("a handshake refused as message.too_large is told apart from every other refusal", () => {
+  const tooLarge = { code: -32000, reason: "message.too_large", message: "message too large to send: 16907300 bytes" };
+  expect(hostRefusedTooLarge(tooLarge)).toBe(true);
+  // Said at once, as any refusal: no waiting out the recovery deadline first.
+  expect(hostErrorShows(tooLarge, 0)).toBe(true);
+  expect(hostRefusedTooLarge({ code: "native.timed_out" })).toBe(false);
+  expect(hostRefusedTooLarge({ code: -32000, reason: "trust.pending" })).toBe(false);
+  expect(hostRefusedTooLarge(new Error("message too large to send"))).toBe(false);
+  expect(hostRefusedTooLarge(null)).toBe(false);
+});
diff --git a/webviews/src/agent-session/acpmux/hostRetry.ts b/webviews/src/agent-session/acpmux/hostRetry.ts
index 70d88d64d333..f2d9f3e92be3 100644
--- a/webviews/src/agent-session/acpmux/hostRetry.ts
+++ b/webviews/src/agent-session/acpmux/hostRetry.ts
@@ -18,6 +18,14 @@ export function hostIsReconnecting(error: unknown): boolean {
   return fields?.code === undefined;
 }
 
+/// A handshake refused because a reply would pass one message (`message.too_large`: a chat
+/// whose records together were too large to open, on an acpmux before #28754). Asking again gets
+/// the same reply, so the pane never asks on its own; the line's Retry still asks (Leo,
+/// 2026-10-31: "this is completely hardlocked bc of message size").
+export function hostRefusedTooLarge(error: unknown): boolean {
+  return (error as { reason?: unknown } | null)?.reason === "message.too_large";
+}
+
 /// How long a pane's handshakes may keep failing while the host is still recovering (an update's
 /// handoff, a daemon starting or restarting, a start that lost the lock race) before the pane says
 /// so. Until then the pane shows nothing: no card, no header status, no button (Leo, 2026-10-29:
diff --git a/webviews/src/agent-session/acpmux/sendFailure.ts b/webviews/src/agent-session/acpmux/sendFailure.ts
index b1a07f214a82..6675b1a3b823 100644
--- a/webviews/src/agent-session/acpmux/sendFailure.ts
+++ b/webviews/src/agent-session/acpmux/sendFailure.ts
@@ -5,11 +5,12 @@
 // prompt's delivery), never from its words; the words and the code stay for Details.
 import { translate } from "./i18n";
 import { DATE } from "./conversation/turns";
+import type { ComposerAttachment } from "./attachments";
 import type { AcpmuxRow } from "./model";
 import { errorMessage } from "./transportErrors";
 
 /// The causes the failure card names in its own words, with their own way out.
-export type SendCause = "trust-pending" | "trust-untrusted" | "pane" | "other";
+export type SendCause = "trust-pending" | "trust-untrusted" | "pane" | "too-large" | "other";
 
 type CauseFields = { code?: unknown; reason?: unknown } | null | undefined;
 
@@ -21,6 +22,8 @@ export function sendCause(fields: CauseFields): SendCause {
   // The reasons acpmux's trust gate writes (trust_gate.rs), as isTrustRefusal reads them.
   if (reason === "trust.pending") return "trust-pending";
   if (reason === "trust.untrusted") return "trust-untrusted";
+  // More than one message carries (`rpc::MAX_MESSAGE_BYTES`): the same prompt is refused again.
+  if (reason === "message.too_large") return "too-large";
   // A request the pane's host would not take is the pane's bug, not something the person did.
   if (text(fields?.code) === "transport.intent_invalid") return "pane";
   return "other";
@@ -29,9 +32,32 @@ export function sendCause(fields: CauseFields): SendCause {
 /// A trust cause: Trust folder fixes it, and Retry waits while the folder's question is open.
 export const isTrustCause = (cause: SendCause) => cause === "trust-pending" || cause === "trust-untrusted";
 
+/// Whether Retry can help: a prompt too large to send is refused the same way again, so its way
+/// out is Edit (Leo, 2026-10-31: "this is completely hardlocked bc of message size").
+export const retryHelps = (cause: SendCause) => cause !== "too-large";
+
+/// The bytes an attachment puts in the prompt: its base64 or its text.
+const promptBytes = (attachment: ComposerAttachment) => (attachment.data?.length ?? 0) + (attachment.text?.length ?? 0);
+
+/// A prompt too large to send: the attachment that takes the most of it, by name, when one does.
+function tooLargeText(attachments: readonly ComposerAttachment[]): string {
+  const largest = attachments.reduce<ComposerAttachment | undefined>(
+    (most, attachment) => (promptBytes(attachment) > (most ? promptBytes(most) : 0) ? attachment : most),
+    undefined,
+  );
+  return largest
+    ? translate("prompt.failure.tooLargeFile", { name: largest.name })
+    : translate("prompt.failure.tooLarge");
+}
+
 /// The failed prompt's one sentence. `retried`: a Retry that was refused again says "Still"
-/// (hq#2569 item 5), so the click visibly did something.
-export function failedSendText(error: unknown, retried: boolean): string {
+/// (hq#2569 item 5), so the click visibly did something. `attachments`: the prompt's, so a prompt
+/// too large to send names the one to take out.
+export function failedSendText(
+  error: unknown,
+  retried: boolean,
+  attachments: readonly ComposerAttachment[] = [],
+): string {
   const fields = error && typeof error === "object" ? (error as { code?: unknown; reason?: unknown }) : undefined;
   switch (sendCause(fields)) {
     case "trust-pending":
@@ -42,6 +68,8 @@ export function failedSendText(error: unknown, retried: boolean): string {
       // The pane's bug, not the person's: logged for its report, as errorMessage logs it.
       console.error("agent pane: the host refused a request", error);
       return translate(retried ? "prompt.failure.paneAgain" : "prompt.failure.pane");
+    case "too-large":
+      return tooLargeText(attachments);
     default:
       break;
   }