Fix: deduplicate localized reply/forward subject prefixes

Replying to a reply produced "Re: Re: foo" (and German used the English
"Re:"/"Fwd:" instead of "AW:"/"WG:"). Four code paths built reply/forward
subjects and only one deduplicated - and only for the English prefix, so
cross-locale threads accumulated chains.

New lib/subject-prefix.ts strips any leading run of reply/forward markers
across ~35 tokens from all supported languages (plus Outlook Re[2]: and
Eudora Re*2: counters), then prepends the locale-appropriate prefix. All four
call sites (composer getInitialSubject, the two page.tsx sites, and the three
pro-tab handlers) now use buildReplySubject/buildForwardSubject. German prefix
corrected to AW:/WG:.

app/(main)/[locale]/page.tsx

@@ -72,6 +72,7 @@ import { plainTextToComposerBody } from "@/lib/email-composer-utils";
 import { appLifecycleHooks, uiHooks, routerHooks, toastHooks, emailHooks } from "@/lib/plugin-hooks";
 import { emailToReadView } from "@/lib/plugin-projection";
 import { buildQuoteHeader } from "@/lib/quote-header";
+import { buildReplySubject, buildForwardSubject } from "@/lib/subject-prefix";
 import { useLocaleStore } from "@/stores/locale-store";
 import type { QuoteHeader } from "@/lib/plugin-types";
 
@@ -751,9 +752,9 @@ export default function Home() {
     let title = t('email_composer.new_message');
     if (baseSubject) {
       if (effectiveMode === 'reply' || effectiveMode === 'replyAll') {
-        title = baseSubject.startsWith('Re:') ? baseSubject : `Re: ${baseSubject}`;
+        title = buildReplySubject(baseSubject, t('email_composer.prefix.reply'));
       } else if (effectiveMode === 'forward') {
-        title = baseSubject.startsWith('Fwd:') ? baseSubject : `Fwd: ${baseSubject}`;
+        title = buildForwardSubject(baseSubject, t('email_composer.prefix.forward'));
       } else {
         title = baseSubject;
       }
@@ -2128,7 +2129,7 @@ export default function Home() {
     const result = await sendEmail(
       client,
       [sender.email],
-      `Re: ${selectedEmail.subject || "(no subject)"}`,
+      buildReplySubject(selectedEmail.subject || "(no subject)", t('email_composer.prefix.reply')),
       finalBody,
       undefined,
       undefined,

components/email/email-composer.tsx

@@ -10,6 +10,7 @@ import { cn, formatFileSize, formatDateTime, generateUUID } from "@/lib/utils";
 import { debug } from "@/lib/debug";
 import { toast } from "@/stores/toast-store";
 import { sanitizeSignatureHtml } from "@/lib/email-sanitization";
+import { buildReplySubject, buildForwardSubject } from "@/lib/subject-prefix";
 import { emailHooks, contactHooks } from "@/lib/plugin-hooks";
 import type { OutgoingEmail, RecipientSuggestion } from "@/lib/plugin-types";
 import { useAuthStore } from "@/stores/auth-store";
@@ -265,11 +266,9 @@ export function EmailComposer({
   const getInitialSubject = () => {
     if (!replyTo?.subject) return "";
     if (mode === 'forward') {
-      const fwdPrefix = t('prefix.forward');
+      return buildForwardSubject(replyTo.subject, t('prefix.forward'));
-      return `${fwdPrefix} ${replyTo.subject.replace(/^(Fwd:\s*|Tr:\s*)+/i, '')}`;
     } else if (mode === 'reply' || mode === 'replyAll') {
-      const rePrefix = t('prefix.reply');
+      return buildReplySubject(replyTo.subject, t('prefix.reply'));
-      return `${rePrefix} ${replyTo.subject.replace(/^(Re:\s*)+/i, '')}`;
     }
     return "";
   };

components/pro/pro-email-tab-body.tsx

@@ -10,6 +10,7 @@ import { useSettingsStore } from "@/stores/settings-store";
 import { toast } from "@/stores/toast-store";
 import { useProTabStore, type ProEmailTabData, type ProReplyContext } from "@/stores/pro-tab-store";
 import type { Email } from "@/lib/jmap/types";
+import { buildReplySubject, buildForwardSubject } from "@/lib/subject-prefix";
 
 interface ProEmailTabBodyProps {
   tabId: string;
@@ -104,7 +105,7 @@ export function ProEmailTabBody({ tabId, data }: ProEmailTabBodyProps) {
       replyTo: buildReplyContext(email),
       sourceEmailId: email.id,
       initialDraftText: draftText,
-      title: `Re: ${email.subject || t('email_composer.new_message')}`,
+      title: buildReplySubject(email.subject || t('email_composer.new_message'), t('email_composer.prefix.reply')),
     });
   }, [email, openComposeTab, t]);
 
@@ -116,7 +117,7 @@ export function ProEmailTabBody({ tabId, data }: ProEmailTabBodyProps) {
       mode: 'replyAll',
       replyTo: buildReplyContext(email),
       sourceEmailId: email.id,
-      title: `Re: ${email.subject || t('email_composer.new_message')}`,
+      title: buildReplySubject(email.subject || t('email_composer.new_message'), t('email_composer.prefix.reply')),
     });
   }, [email, openComposeTab, t]);
 
@@ -128,7 +129,7 @@ export function ProEmailTabBody({ tabId, data }: ProEmailTabBodyProps) {
       mode: 'forward',
       replyTo: buildReplyContext(email),
       sourceEmailId: email.id,
-      title: `Fwd: ${email.subject || t('email_composer.new_message')}`,
+      title: buildForwardSubject(email.subject || t('email_composer.new_message'), t('email_composer.prefix.forward')),
     });
   }, [email, openComposeTab, t]);
 

lib/subject-prefix.ts

@@ -0,0 +1,117 @@
+// Reply / forward subject prefix handling.
+//
+// Real-world email subjects accumulate prefixes across clients and languages:
+// "Re: AW: WG: Fwd: Re: foo". The deduplication regex needs to know ALL
+// commonly-used reply/forward markers - not just the current locale's, since
+// inbound messages may come from any locale. Failing to strip a foreign-locale
+// prefix means the user's locale prefix gets *added on top* and the subject
+// chain keeps growing.
+//
+// Sources: de-facto conventions in Outlook / Thunderbird / Apple Mail per
+// language. Includes a handful of legacy short-forms (R:, Fw:) that some
+// mobile clients still emit.
+
+const REPLY_TOKENS = [
+  "Re",      // English, Italian, French (also generic ISO)
+  "RE",      // Outlook variant
+  "AW",      // German (Antwort)
+  "Antw",    // German verbose
+  "Sv",      // Danish / Swedish / Norwegian (Svar)
+  "Yn",      // Turkish (Yanit)
+  "Yanit",   // Turkish verbose
+  "Odp",     // Polish (Odpowiedz)
+  "Ответ",   // Russian
+  "Resp",    // Spanish/Portuguese variant
+  "Vá",      // Hungarian
+  "回复",    // Chinese
+  "回覆",    // Chinese traditional
+  "답장",    // Korean
+  "R",       // Italian short form
+];
+
+const FORWARD_TOKENS = [
+  "Fwd",     // English standard
+  "Fw",      // English short / Polish / German short
+  "WG",      // German (Weitergeleitet)
+  "Tr",      // French (Transfert)
+  "Vs",      // Danish (Videresend)
+  "Enc",     // Portuguese (Encaminhar)
+  "ENC",     // Portuguese caps
+  "Rv",      // Spanish (Reenviar)
+  "RV",      // Spanish caps
+  "Rvf",     // Spanish variant
+  "Inol",    // Italian (Inoltro)
+  "I",       // Italian short
+  "PD",      // Polish (Przekazane Dalej)
+  "PR",      // Czech (Preposlat)
+  "İlt",     // Turkish (Ilet)
+  "Ilt",     // Turkish ASCII
+  "Пересл",  // Russian (Peresylka)
+  "Пер",     // Russian short
+  "转发",    // Chinese
+  "轉寄",    // Chinese traditional
+  "전달",    // Korean
+];
+
+// Match a single prefix token + optional [N] counter (Outlook) or *N (Eudora)
+// + colon + whitespace. Case-insensitive. The non-capturing groups keep the
+// regex composable for stripping multiple prefixes in a row.
+function buildPrefixRegex(tokens: string[]): RegExp {
+  // Escape regex specials in tokens (none currently, but be defensive)
+  const escaped = tokens.map((t) => t.replace(/[.*+?^${}()|[\]\\]/g, "\\$&"));
+  // Sort by length DESC so longer tokens (e.g. "Пересл") win over their
+  // shorter prefixes (e.g. "Пер") during alternation matching.
+  escaped.sort((a, b) => b.length - a.length);
+  return new RegExp(
+    `^\\s*(?:${escaped.join("|")})(?:\\[\\d+\\]|\\*\\d*)?\\s*:\\s*`,
+    "i",
+  );
+}
+
+const ANY_PREFIX_RE = buildPrefixRegex([...REPLY_TOKENS, ...FORWARD_TOKENS]);
+
+/**
+ * Strip any leading sequence of reply/forward prefixes (across languages) from
+ * a subject line. Idempotent and safe for empty input.
+ *
+ * Examples:
+ *   stripSubjectPrefixes("Re: AW: WG: foo")        === "foo"
+ *   stripSubjectPrefixes("Re[2]: foo")             === "foo"
+ *   stripSubjectPrefixes("RE: Re: foo")            === "foo"
+ *   stripSubjectPrefixes("foo")                    === "foo"
+ *   stripSubjectPrefixes("")                       === ""
+ */
+export function stripSubjectPrefixes(subject: string | undefined | null): string {
+  if (!subject) return "";
+  let s = subject;
+  // Bounded loop: in practice you never see more than ~10 prefixes; the bound
+  // protects against pathological input. Each iteration must consume input.
+  for (let i = 0; i < 20; i++) {
+    const next = s.replace(ANY_PREFIX_RE, "");
+    if (next === s) break;
+    s = next;
+  }
+  return s;
+}
+
+/**
+ * Build a reply subject with the given locale-aware prefix. Strips any
+ * pre-existing prefixes (in any language) first so chains don't accumulate.
+ *
+ *   buildReplySubject("AW: WG: foo", "Re:")   === "Re: foo"
+ *   buildReplySubject("foo", "AW:")           === "AW: foo"
+ *   buildReplySubject("", "AW:")              === "AW:"
+ */
+export function buildReplySubject(subject: string | undefined | null, prefix: string): string {
+  const stripped = stripSubjectPrefixes(subject);
+  return stripped ? `${prefix} ${stripped}` : prefix;
+}
+
+/**
+ * Build a forward subject. Same logic as buildReplySubject but conceptually
+ * separate for clarity at the call site.
+ */
+export function buildForwardSubject(subject: string | undefined | null, prefix: string): string {
+  const stripped = stripSubjectPrefixes(subject);
+  return stripped ? `${prefix} ${stripped}` : prefix;
+}

locales/de/common.json

@@ -578,8 +578,8 @@
     "subject_label": "Betreff:",
     "file_size_kb": "KB",
     "prefix": {
-      "forward": "Fwd:",
+      "forward": "WG:",
-      "reply": "Re:"
+      "reply": "AW:"
     },
     "no_subject": "(Kein Betreff)",
     "unknown_sender": "Unbekannt",