diff --git a/components/mail/mail-app.tsx b/components/mail/mail-app.tsx
index c28c99888..11ba738eb 100644
--- a/components/mail/mail-app.tsx
+++ b/components/mail/mail-app.tsx
@@ -94,7 +94,7 @@ import { plainTextToComposerBody, getQuoteBodies } from "@/lib/email-composer-ut
 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 { buildComposeTabTitle, buildReplySubject } from "@/lib/subject-prefix";
 import { buildForwardAsAttachmentPayload } from "@/lib/forward-as-attachment";
 import { getEffectiveLocale } from '@/i18n/detect-locale';
 import {
@@ -913,17 +913,14 @@ export function MailApp({ linkSegments }: MailAppProps = {}) {
     } : undefined);
 
     const effectiveMode = pendingDraft?.mode ?? composerMode;
-    const baseSubject = (pendingDraft?.subject?.trim() || selectedEmail?.subject?.trim()) ?? '';
-    let title = t('email_composer.new_message');
-    if (baseSubject) {
-      if (effectiveMode === 'reply' || effectiveMode === 'replyAll') {
-        title = buildReplySubject(baseSubject, t('email_composer.prefix.reply'));
-      } else if (effectiveMode === 'forward') {
-        title = buildForwardSubject(baseSubject, t('email_composer.prefix.forward'));
-      } else {
-        title = baseSubject;
-      }
-    }
+    const title = buildComposeTabTitle({
+      mode: effectiveMode,
+      draftSubject: pendingDraft?.subject,
+      sourceSubject: selectedEmail?.subject,
+      replyPrefix: t('email_composer.prefix.reply'),
+      forwardPrefix: t('email_composer.prefix.forward'),
+      fallback: t('email_composer.new_message'),
+    });
 
     useProTabStore.getState().openComposeTab({
       sessionId: composerSessionId + 1,
diff --git a/lib/__tests__/subject-prefix.test.ts b/lib/__tests__/subject-prefix.test.ts
index bb514754b..85cc61d26 100644
--- a/lib/__tests__/subject-prefix.test.ts
+++ b/lib/__tests__/subject-prefix.test.ts
@@ -3,6 +3,7 @@ import {
   stripSubjectPrefixes,
   buildReplySubject,
   buildForwardSubject,
+  buildComposeTabTitle,
 } from '@/lib/subject-prefix';
 
 describe('stripSubjectPrefixes', () => {
@@ -54,3 +55,33 @@ describe('buildReplySubject / buildForwardSubject', () => {
     expect(buildReplySubject('', 'AW:')).toBe('AW:');
   });
 });
+
+describe('buildComposeTabTitle', () => {
+  const prefixes = { replyPrefix: 'Re:', forwardPrefix: 'Fwd:', fallback: 'New message' };
+
+  it('never inherits the selected email subject for a fresh compose (#856)', () => {
+    expect(buildComposeTabTitle({ mode: 'compose', sourceSubject: 'Invoice August 2026', ...prefixes }))
+      .toBe('New message');
+  });
+
+  it('uses the draft subject for a fresh compose when one exists (mailto, draft edit)', () => {
+    expect(buildComposeTabTitle({ mode: 'compose', draftSubject: 'Lunch?', sourceSubject: 'Invoice August 2026', ...prefixes }))
+      .toBe('Lunch?');
+  });
+
+  it('derives reply and forward titles from the source subject', () => {
+    expect(buildComposeTabTitle({ mode: 'reply', sourceSubject: 'Invoice August 2026', ...prefixes }))
+      .toBe('Re: Invoice August 2026');
+    expect(buildComposeTabTitle({ mode: 'replyAll', sourceSubject: 'AW: Invoice', ...prefixes }))
+      .toBe('Re: Invoice');
+    expect(buildComposeTabTitle({ mode: 'forward', sourceSubject: 'Invoice', ...prefixes }))
+      .toBe('Fwd: Invoice');
+  });
+
+  it('prefers a resumed draft subject over the source and falls back when both are blank', () => {
+    expect(buildComposeTabTitle({ mode: 'reply', draftSubject: 'Re: edited', sourceSubject: 'Original', ...prefixes }))
+      .toBe('Re: edited');
+    expect(buildComposeTabTitle({ mode: 'reply', sourceSubject: '   ', ...prefixes }))
+      .toBe('New message');
+  });
+});
diff --git a/lib/subject-prefix.ts b/lib/subject-prefix.ts
index 766155a7d..e61e9f256 100644
--- a/lib/subject-prefix.ts
+++ b/lib/subject-prefix.ts
@@ -119,3 +119,26 @@ export function buildForwardSubject(subject: string | undefined | null, prefix:
   const stripped = stripSubjectPrefixes(subject);
   return stripped ? `${prefix} ${stripped}` : prefix;
 }
+
+/**
+ * Working title for a compose tab. Reply and forward derive it from the
+ * message they act on; a fresh compose must NOT inherit the subject of
+ * whatever email happens to be selected in the list (#856) - it starts on
+ * the fallback ("New message") until a draft subject exists.
+ */
+export function buildComposeTabTitle(opts: {
+  mode: 'compose' | 'reply' | 'replyAll' | 'forward';
+  draftSubject?: string | null;
+  sourceSubject?: string | null;
+  replyPrefix: string;
+  forwardPrefix: string;
+  fallback: string;
+}): string {
+  const draft = opts.draftSubject?.trim() ?? '';
+  const source = opts.mode === 'compose' ? '' : (opts.sourceSubject?.trim() ?? '');
+  const base = draft || source;
+  if (!base) return opts.fallback;
+  if (opts.mode === 'reply' || opts.mode === 'replyAll') return buildReplySubject(base, opts.replyPrefix);
+  if (opts.mode === 'forward') return buildForwardSubject(base, opts.forwardPrefix);
+  return base;
+}