feat: let plugins open a reply to the open message with text to start from

components/mail/mail-app.tsx

@@ -116,6 +116,7 @@ import { AppTopBannerSlot } from "@/components/plugins/app-top-banner-slot";
 import { useThemeStore } from "@/stores/theme-store";
 import { consumePendingMailto, subscribeToPendingMailto } from "@/lib/protocol-handlers/session";
 import { INTERNAL_MAILTO_EVENT, type ParsedMailto } from "@/lib/protocol-handlers/mailto";
+import { PLUGIN_REPLY_EVENT, type PluginReplyRequest } from "@/lib/plugin-sandbox/host-api-compose";
 import { mailtoComposerDraft } from "./mailto-draft";
 import { getQuoteBodies } from "@/lib/email-composer-utils";
 import { emailHooks } from "@/lib/plugin-hooks";
@@ -2268,7 +2269,7 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
   // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [cancelUndoSend, clearPendingUndoSend, client, fetchScheduledEmails, isScheduledView, pendingUndoSend?.submissionId, sendDelaySeconds, t]);
 
-  const handleReplyAll = async () => {
+  const handleReplyAll = async (draftText?: string) => {
     if (selectedEmail) {
       const formerSelected = { ...selectedEmail };
       const enrichedEmail = await emailHooks.onBeforeComposeOpenToReplyAll.transform(selectedEmail);
@@ -2286,14 +2287,31 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
     } else {
       setComposerQuoteHeader(null);
     }
-    await withComposeGate('replyAll', { source: selectedEmail }, () => {
+    const text = typeof draftText === 'string' ? draftText : '';
+    await withComposeGate('replyAll', { source: selectedEmail, ...(text ? { body: text } : {}) }, () => {
       startFreshComposerSession();
+      setComposerDraftText(text);
       setComposerMode('replyAll');
       setShowComposer(true);
       if (isMobile) setActiveView('viewer');
     });
   };
 
+  // A plugin's "reply with this" (api.compose.reply), for the open message only.
+  const pluginReplyRef = useRef({ handleReply, handleReplyAll, openId: selectedEmail?.id ?? null });
+  pluginReplyRef.current = { handleReply, handleReplyAll, openId: selectedEmail?.id ?? null };
+  useEffect(() => {
+    const onPluginReply = (event: Event) => {
+      const req = (event as CustomEvent<PluginReplyRequest>).detail;
+      const { openId, handleReply: reply, handleReplyAll: replyAll } = pluginReplyRef.current;
+      if (!req || !openId || req.emailId !== openId) return;
+      event.preventDefault();
+      void (req.all ? replyAll(req.text) : reply(req.text));
+    };
+    window.addEventListener(PLUGIN_REPLY_EVENT, onPluginReply);
+    return () => window.removeEventListener(PLUGIN_REPLY_EVENT, onPluginReply);
+  }, []);
+
   const handleForward = async () => {
     if (selectedEmail) {
       const formerSelected = { ...selectedEmail };

lib/__tests__/plugin-host-compose-drafts.test.ts

@@ -4,6 +4,7 @@ import type { IJMAPClient } from '../jmap/client-interface';
 import { useAuthStore } from '@/stores/auth-store';
 import { useIdentityStore } from '@/stores/identity-store';
 import { INTERNAL_MAILTO_EVENT, type ParsedMailto } from '../protocol-handlers/mailto';
+import { PLUGIN_REPLY_EVENT, type PluginReplyRequest } from '../plugin-sandbox/host-api-compose';
 import { consumePendingMailto } from '../protocol-handlers/session';
 import { resetPluginRateLimits } from '../plugin-sandbox/host-api-input';
 import { HIGH_RISK_PERMISSIONS } from '../plugin-sandbox/permission-text';
@@ -96,6 +97,39 @@ describe('api.compose.open', () => {
   });
 });
 
+describe('api.compose.reply', () => {
+  // The mail page, with message E1 open in the reader.
+  let taken: PluginReplyRequest[];
+  const mailPage = (event: Event) => {
+    const req = (event as CustomEvent<PluginReplyRequest>).detail;
+    if (req.emailId !== 'E1') return;
+    event.preventDefault();
+    taken.push(req);
+  };
+  beforeEach(() => {
+    taken = [];
+    resetPluginRateLimits();
+    window.addEventListener(PLUGIN_REPLY_EVENT, mailPage);
+  });
+  afterEach(() => window.removeEventListener(PLUGIN_REPLY_EVENT, mailPage));
+
+  it('asks the mail page to reply to the open message with the text', async () => {
+    expect(await call(COMPOSER, 'compose.reply', 'E1', { text: 'Thursday works for me.' })).toBe(true);
+    expect(await call(COMPOSER, 'compose.reply', 'E1', { text: 'Fine by all of us.', all: true })).toBe(true);
+    expect(taken).toEqual([
+      { emailId: 'E1', text: 'Thursday works for me.', all: false },
+      { emailId: 'E1', text: 'Fine by all of us.', all: true },
+    ]);
+  });
+
+  it('refuses a message that is not open, and needs compose:open', async () => {
+    await expect(call(COMPOSER, 'compose.reply', 'E2', { text: 'x' })).rejects.toThrow(/not open in the reader/);
+    await expect(call(DRAFTER, 'compose.reply', 'E1', { text: 'x' })).rejects.toThrow(/lacks permission "compose:open"/);
+    await expect(call(COMPOSER, 'compose.reply', 'E1', { text: 'x', to: ['eve@example.com'] })).rejects.toThrow(/unknown property "to"/);
+    expect(taken).toEqual([]);
+  });
+});
+
 describe('api.drafts.create', () => {
   let createDraft: ReturnType<typeof vi.fn>;
 

lib/plugin-sandbox/host-api-compose.ts

@@ -13,7 +13,7 @@ import { useIdentityStore } from '@/stores/identity-store';
 import { sanitizePluginBodyHtml } from '../email-sanitization';
 import { htmlToPlainText } from '../html-to-text';
 import {
-  readAddressList, readBlobRefs, readBoolean, readObject, readString, sessionClient, withinRate,
+  readAddressList, readBlobRefs, readBoolean, readId, readObject, readString, sessionClient, withinRate,
   type PluginBlobRef,
 } from './host-api-input';
 
@@ -100,6 +100,40 @@ export function doComposeOpen(plugin: InstalledPlugin, input: unknown): 'opened'
   return 'navigating';
 }
 
+// ─── compose.reply ────────────────────────────────────────────
+
+/** Asks the mail page to reply to the open message (components/mail/mail-app.tsx). */
+export const PLUGIN_REPLY_EVENT = 'bulwark:plugin-reply';
+
+export interface PluginReplyRequest {
+  emailId: string;
+  text: string;
+  all: boolean;
+}
+
+/**
+ * Reply (or reply to all) to the message open in the reader, with `text` as
+ * the start of the reply, the way the reader's own quick reply does. Only
+ * the open message: a plugin can't start replies to mail the user isn't
+ * looking at. Resolves to true when the composer opened.
+ */
+export function doComposeReply(plugin: InstalledPlugin, emailId: unknown, input: unknown): boolean {
+  const method = 'compose.reply';
+  const id = readId(emailId, method, 'emailId');
+  const data = readObject(input, method, 'reply', ['text', 'all'], true);
+  const text = data.text === undefined ? '' : readString(data.text, method, 'text', MAX_BODY);
+  const all = data.all === undefined ? false : readBoolean(data.all, method, 'all');
+  if (!withinRate('compose', plugin.id, COMPOSE_PER_MINUTE)) {
+    throw new Error(`${method}: at most ${COMPOSE_PER_MINUTE} times a minute`);
+  }
+  const uncancelled = window.dispatchEvent(new CustomEvent<PluginReplyRequest>(PLUGIN_REPLY_EVENT, {
+    detail: { emailId: id, text, all },
+    cancelable: true,
+  }));
+  if (uncancelled) throw new Error(`${method}: the message is not open in the reader`);
+  return true;
+}
+
 // ─── drafts.create ────────────────────────────────────────────
 
 const DRAFTS_PER_MINUTE = 10;

lib/plugin-sandbox/host-api.ts

@@ -50,7 +50,7 @@ import {
   doEmailArchive, doEmailGet, doEmailGetAttachment, doEmailGetRaw, doEmailList, doEmailMove, doMailboxesList,
 } from './host-api-mail';
 import { MAX_PLUGIN_FETCH_BYTES, readFileName, readMediaType, sessionClient } from './host-api-input';
-import { doComposeOpen, doDraftsCreate } from './host-api-compose';
+import { doComposeOpen, doComposeReply, doDraftsCreate } from './host-api-compose';
 import { doNotificationsShow } from './host-api-notify';
 import { doComposerAttach, doComposerGetBody, doComposerGetSelection, doComposerReplaceBody, doComposerReplaceSelection } from './host-api-composer';
 
@@ -253,6 +253,8 @@ export const PERM_PER_METHOD: Readonly<Record<string, Permission | null>> = {
   'email.archive': 'email:write',
   // Open the composer filled in; the user still decides whether to send.
   'compose.open': 'compose:open',
+  // Reply to the message open in the reader, with text to start from.
+  'compose.reply': 'compose:open',
   // Save a draft without opening the composer. Nothing is sent.
   'drafts.create': 'email:write',
   // A system notification (or a toast), naming the plugin, 5 a minute.
@@ -2013,6 +2015,7 @@ export async function dispatchApiCall(
     case 'email.archive': return doEmailArchive(plugin.id, args[0]);
 
     case 'compose.open': return doComposeOpen(plugin, args[0]);
+    case 'compose.reply': return doComposeReply(plugin, args[0], args[1]);
     case 'drafts.create': return doDraftsCreate(plugin, args[0]);
 
     case 'notifications.show': return doNotificationsShow(plugin, args[0]);

lib/plugin-sandbox/runtime.tsx

@@ -604,6 +604,13 @@ function buildPluginApi(manifest: PluginManifest) {
        * At most 5 times a minute.
        */
       open: (message: ComposeInput) => callApi('compose.open', [message]) as Promise<'opened' | 'navigating'>,
+      /**
+       * Reply to the message open in the reader (`all` for reply to all),
+       * starting the reply with `text`. Only the open message; resolves to
+       * true when the composer opened. Shares compose.open's 5 a minute.
+       */
+      reply: (emailId: string, reply?: { text?: string; all?: boolean }) =>
+        callApi('compose.reply', [emailId, reply]) as Promise<boolean>,
     },
     // Drafts (permission: email:write).
     drafts: {