diff --git a/lib/__tests__/placeholder-check.test.ts b/lib/__tests__/placeholder-check.test.ts
new file mode 100644
index 000000000..897b5f921
--- /dev/null
+++ b/lib/__tests__/placeholder-check.test.ts
@@ -0,0 +1,84 @@
+import { describe, it, expect } from 'vitest';
+import { findUnfilledPlaceholders, stripEmbeddedSignatureHtml } from '../placeholder-check';
+
+describe('findUnfilledPlaceholders', () => {
+  it('finds template placeholders in the subject and the body', () => {
+    expect(findUnfilledPlaceholders('Follow-up: {{subject}}', 'Hi {{ recipient_name }},')).toEqual([
+      '{{subject}}',
+      '{{ recipient_name }}',
+    ]);
+  });
+
+  it('finds upper-case fill-ins in brackets', () => {
+    expect(findUnfilledPlaceholders('', 'See you at [TIME]. It costs [PRICE] per [UNIT_NAME].'))
+      .toEqual(['[TIME]', '[PRICE]', '[UNIT_NAME]']);
+    expect(findUnfilledPlaceholders('', 'Best regards, [YOUR NAME]')).toEqual(['[YOUR NAME]']);
+    expect(findUnfilledPlaceholders('', 'Treffpunkt: [ÜBERGABEORT]')).toEqual(['[ÜBERGABEORT]']);
+  });
+
+  it('finds fill-ins that ask the sender to write something', () => {
+    expect(findUnfilledPlaceholders('', 'Best regards,\n[Your Name]\n[Insert date]')).toEqual([
+      '[Your Name]',
+      '[Insert date]',
+    ]);
+    expect(findUnfilledPlaceholders('', 'Viele Grüße\n[Ihr Name], [Datum einfügen]')).toEqual([
+      '[Ihr Name]',
+      '[Datum einfügen]',
+    ]);
+  });
+
+  it('finds unfinished-text markers', () => {
+    expect(findUnfilledPlaceholders('', 'The venue is TBD. TODO: add the agenda. Room XXX')).toEqual([
+      'TBD',
+      'TODO',
+      'XXX',
+    ]);
+  });
+
+  it('reports a marker in brackets once', () => {
+    expect(findUnfilledPlaceholders('', 'Agenda: [TODO]')).toEqual(['[TODO]']);
+  });
+
+  it('lists each placeholder once', () => {
+    expect(findUnfilledPlaceholders('{{name}}', 'Hi {{name}}, at [TIME] or [TIME]?')).toEqual([
+      '{{name}}',
+      '[TIME]',
+    ]);
+  });
+
+  it('leaves ordinary text alone', () => {
+    const text = [
+      'See footnote [1] and tick [x].',
+      'Option [A] is fine, so is [3D] and [2026].',
+      'I still have a todo list and the date is tba.',
+      'Read [the docs](https://example.com) and [Your account](https://example.com/me).',
+      'The mastodon handle is TODOS and the size is XXXL.',
+      'Signed, [Yours truly].',
+      'A {single} brace and {{}} are not placeholders.',
+    ].join('\n');
+    expect(findUnfilledPlaceholders('', text)).toEqual([]);
+  });
+
+  it('ignores bracket tags and markers in the subject', () => {
+    expect(findUnfilledPlaceholders('Re: [EXTERNAL] Venue TBD', 'Thanks!')).toEqual([]);
+  });
+});
+
+describe('stripEmbeddedSignatureHtml', () => {
+  it('drops the bracketed signature range and keeps the rest', () => {
+    const html = '<p>Hello</p><p data-signature-block="start"></p><p>[COMPANY] TODO</p><p data-signature-block="end"></p><p>After</p>';
+    const stripped = stripEmbeddedSignatureHtml(html);
+    expect(stripped).toContain('Hello');
+    expect(stripped).toContain('After');
+    expect(stripped).not.toContain('[COMPANY]');
+  });
+
+  it('drops a signature atom without range markers', () => {
+    const html = '<p>Hello [TIME]</p><div data-signature-block-node="">[COMPANY]</div>';
+    expect(stripEmbeddedSignatureHtml(html)).toBe('<p>Hello [TIME]</p>');
+  });
+
+  it('returns a body without a signature unchanged', () => {
+    expect(stripEmbeddedSignatureHtml('<p>Hello</p>')).toBe('<p>Hello</p>');
+  });
+});
diff --git a/lib/placeholder-check.ts b/lib/placeholder-check.ts
new file mode 100644
index 000000000..a02666092
--- /dev/null
+++ b/lib/placeholder-check.ts
@@ -0,0 +1,83 @@
+// Finds text the sender meant to replace before sending: template
+// placeholders, bracketed fill-ins that AI drafts and copied templates leave
+// behind, and markers like TODO.
+
+// Template syntax, as in template-utils. Spaces inside the braces are allowed
+// because templates pasted from other tools often have them.
+const TEMPLATE_PLACEHOLDER = /\{\{\s*[\w.-]+\s*\}\}/g;
+
+// [TIME], [PRICE], [YOUR NAME]. Not followed by "(", which would make it a
+// Markdown link.
+const CAPS_BRACKET = /\[[\p{Lu}\d][\p{Lu}\d _./-]{0,28}[\p{Lu}\d]\](?!\()/gu;
+
+// [Your Name], [Insert date], [Ihr Name], [Datum einfügen].
+const PROMPT_BRACKET = /\[(?:(?:your|insert|enter|add|ihr|ihre|ihren|dein|deine|deinen)\s[^[\]\n]{1,40}|[^[\]\n]{1,40}\seinfügen)\](?!\()/giu;
+
+// Upper case only: "todo" or "tba" in a sentence is usually meant.
+const UNFINISHED_WORD = /(?<![\p{L}\d_])(?:TODO|TBD|TBA|TBC|FIXME|XXX)(?![\p{L}\d_])/gu;
+
+const SIGNATURE_START_SELECTOR = '[data-signature-block="separator"], [data-signature-block="start"]';
+const SIGNATURE_END_SELECTOR = '[data-signature-block="end"]';
+
+interface Match {
+  start: number;
+  end: number;
+  text: string;
+}
+
+function collect(text: string, patterns: RegExp[]): Match[] {
+  const matches: Match[] = [];
+  for (const pattern of patterns) {
+    for (const m of text.matchAll(pattern)) {
+      if (pattern === CAPS_BRACKET && (m[0].match(/\p{Lu}/gu)?.length ?? 0) < 2) continue;
+      matches.push({ start: m.index, end: m.index + m[0].length, text: m[0] });
+    }
+  }
+  // "[TODO]" is a bracket and a word; keep the outer match only.
+  matches.sort((a, b) => a.start - b.start || b.end - a.end);
+  const kept: Match[] = [];
+  for (const match of matches) {
+    const last = kept[kept.length - 1];
+    if (last && match.start < last.end) continue;
+    kept.push(match);
+  }
+  return kept;
+}
+
+/**
+ * The unfilled placeholders in a message, each once, subject first. The
+ * subject is only checked for template placeholders: it often carries tags
+ * like [EXTERNAL] or words from the original message, which the sender did
+ * not write. `bodyText` should be the text the sender wrote, without quoted
+ * replies and the signature.
+ */
+export function findUnfilledPlaceholders(subject: string, bodyText: string): string[] {
+  const found = [
+    ...collect(subject, [TEMPLATE_PLACEHOLDER]),
+    ...collect(bodyText, [TEMPLATE_PLACEHOLDER, CAPS_BRACKET, PROMPT_BRACKET, UNFINISHED_WORD]),
+  ].map((m) => m.text);
+  return Array.from(new Set(found));
+}
+
+/**
+ * A compose body without its embedded signature. A signature is the same in
+ * every message, so a placeholder in it would stop every send.
+ */
+export function stripEmbeddedSignatureHtml(html: string): string {
+  if (!html.includes('data-signature-block')) return html;
+  const doc = new DOMParser().parseFromString(`<body>${html}</body>`, 'text/html');
+  const startEl = doc.querySelector(SIGNATURE_START_SELECTOR);
+  if (startEl) {
+    const endEl = doc.querySelector(SIGNATURE_END_SELECTOR);
+    let cursor: Node | null = startEl;
+    while (cursor) {
+      const next: Node | null = cursor.nextSibling;
+      const isEnd = cursor === endEl;
+      cursor.parentNode?.removeChild(cursor);
+      if (isEnd) break;
+      cursor = next;
+    }
+  }
+  doc.querySelectorAll('[data-signature-block-node]').forEach((el) => el.remove());
+  return doc.body.innerHTML;
+}