diff --git a/src/core/content-sanity.ts b/src/core/content-sanity.ts
index 1d516075a..ca5260162 100644
--- a/src/core/content-sanity.ts
+++ b/src/core/content-sanity.ts
@@ -376,14 +376,18 @@ export function assessContentSanity(opts: {
   // doesn't repeat the lowercase per literal.
   const bodyHead = body.slice(0, SCAN_HEAD_BYTES);
   const bodyHeadLower = bodyHead.toLowerCase();
-  const titleLower = opts.title.toLowerCase();
+  // Defensive coercion (issue #1939): this is a pure exported fn; lint.ts and
+  // import-file both pass `parsed.title`, which a malformed YAML date/number
+  // title could make non-string. Never throw on a bad title.
+  const title = String(opts.title ?? '');
+  const titleLower = title.toLowerCase();
 
   const junk_pattern_matches: string[] = [];
   for (const p of BUILT_IN_JUNK_PATTERNS) {
     const scope = p.applies_to ?? 'both';
     let matched = false;
     if (scope === 'title' || scope === 'both') {
-      if (p.pattern.test(opts.title)) matched = true;
+      if (p.pattern.test(title)) matched = true;
     }
     if (!matched && (scope === 'body' || scope === 'both')) {
       if (p.pattern.test(bodyHead)) matched = true;
diff --git a/src/core/enrichment/completeness.ts b/src/core/enrichment/completeness.ts
index 8b91afd6d..b5e7997aa 100644
--- a/src/core/enrichment/completeness.ts
+++ b/src/core/enrichment/completeness.ts
@@ -112,7 +112,8 @@ function nonRedundancy(page: Page): number {
 }
 
 function hasTitle(page: Page): number {
-  return page.title && page.title.trim().length > 0 ? 1 : 0;
+  // Coerce (issue #1939): a malformed YAML date/number title could be non-string.
+  return String(page.title ?? '').trim().length > 0 ? 1 : 0;
 }
 
 function hasBody(page: Page): number {
diff --git a/src/core/markdown.ts b/src/core/markdown.ts
index 19cf0474f..9a6359b6b 100644
--- a/src/core/markdown.ts
+++ b/src/core/markdown.ts
@@ -49,6 +49,25 @@ export interface ParsedMarkdown {
   errors?: ParseValidationError[];
 }
 
+/**
+ * Coerce a raw YAML frontmatter value into a string.
+ *
+ * js-yaml parses unquoted scalars by type: `title: 2024-06-01` becomes a JS
+ * `Date`, `title: 1458` becomes a `number`. The old `(frontmatter.X as string)`
+ * cast was a compile-time lie — at runtime the value stayed a Date/number, so
+ * any downstream `.toLowerCase()` / `.trim()` threw and (via the importer's
+ * failure gate) could wedge sync indefinitely (issue #1939).
+ *
+ * Dates coerce to their UTC ISO date (`2024-06-01`) — deterministic across
+ * machines and matching the on-disk source token, unlike `String(date)` which
+ * renders a timezone-dependent long form. Everything else uses `String()`.
+ */
+export function coerceFrontmatterString(v: unknown): string {
+  if (v == null) return '';
+  if (v instanceof Date) return v.toISOString().slice(0, 10);
+  return String(v);
+}
+
 /**
  * Parse a markdown file with YAML frontmatter into its components.
  *
@@ -105,12 +124,12 @@ export function parseMarkdown(
 
   const { compiled_truth, timeline } = splitBody(body);
 
-  const type = (frontmatter.type as string) || (
+  const type = coerceFrontmatterString(frontmatter.type) || (
     opts?.activePack ? inferTypeFromPack(filePath, opts.activePack) : inferType(filePath)
   );
-  const title = (frontmatter.title as string) || inferTitle(filePath);
+  const title = coerceFrontmatterString(frontmatter.title).trim() || inferTitle(filePath);
   const tags = extractTags(frontmatter);
-  const slug = (frontmatter.slug as string) || inferSlug(filePath);
+  const slug = coerceFrontmatterString(frontmatter.slug) || inferSlug(filePath);
 
   const cleanFrontmatter = { ...frontmatter };
   delete cleanFrontmatter.type;
diff --git a/test/content-sanity.test.ts b/test/content-sanity.test.ts
index d94bef96f..80e2aaa98 100644
--- a/test/content-sanity.test.ts
+++ b/test/content-sanity.test.ts
@@ -640,3 +640,28 @@ describe('assessContentSanity — confidence split (Q1=A)', () => {
     expect(r.shouldFlag).toBe(false);
   });
 });
+
+// issue #1939 — assessContentSanity is a pure exported fn; lint.ts and
+// import-file both pass `parsed.title`, which a malformed YAML date/number title
+// could make non-string. It must coerce defensively and never throw.
+describe('issue #1939 — non-string title defensive coercion', () => {
+  const base = { compiled_truth: 'some prose body here', timeline: '' };
+
+  test('Date title does not throw', () => {
+    expect(() =>
+      assessContentSanity({ ...base, title: new Date('2024-06-01') as unknown as string }),
+    ).not.toThrow();
+  });
+
+  test('number title does not throw', () => {
+    expect(() =>
+      assessContentSanity({ ...base, title: 1458 as unknown as string }),
+    ).not.toThrow();
+  });
+
+  test('null/undefined title does not throw and yields a normal result', () => {
+    const r = assessContentSanity({ ...base, title: undefined as unknown as string });
+    expect(r).toBeDefined();
+    expect(typeof r.shouldQuarantine).toBe('boolean');
+  });
+});
diff --git a/test/markdown.test.ts b/test/markdown.test.ts
index 2d6f165ba..2e485f521 100644
--- a/test/markdown.test.ts
+++ b/test/markdown.test.ts
@@ -302,3 +302,44 @@ Some content.`;
     expect(parseMarkdown('', 'projects/blog/writing/essay.md').type).toBe('writing');
   });
 });
+
+// issue #1939 — js-yaml parses `title: 2024-06-01` as a Date and `title: 1458`
+// as a number. The old `(frontmatter.title as string)` cast was a compile-time
+// lie; at runtime downstream `.toLowerCase()` threw and wedged sync. Coercion
+// must be non-throwing AND deterministic (UTC ISO for dates, no timezone drift).
+describe('issue #1939 — non-string frontmatter coercion', () => {
+  test('date title coerces to its UTC ISO date string', () => {
+    const parsed = parseMarkdown('---\ntitle: 2024-06-01\n---\nbody\n', 'apple-notes/x.md');
+    expect(parsed.title).toBe('2024-06-01');
+    expect(typeof parsed.title).toBe('string');
+  });
+
+  test('number title coerces to its string form', () => {
+    const parsed = parseMarkdown('---\ntitle: 1458\n---\nbody\n', 'apple-notes/x.md');
+    expect(parsed.title).toBe('1458');
+  });
+
+  test('date title is timezone-independent (UTC) — repro file shape', () => {
+    // sources/apple-notes/YC/Talks YC/2023-04-25 1458.md style page.
+    const parsed = parseMarkdown('---\ntitle: 2023-04-25\n---\nnotes\n', 'apple-notes/2023-04-25 1458.md');
+    expect(parsed.title).toBe('2023-04-25'); // never "Mon Apr 24 2023 ...GMT-0700"
+  });
+
+  test('date/number slug + type coerce without throwing', () => {
+    const parsed = parseMarkdown('---\nslug: 2024-06-01\ntype: 2024\n---\nbody\n', 'x.md');
+    expect(typeof parsed.slug).toBe('string');
+    expect(parsed.slug).toBe('2024-06-01');
+    expect(typeof parsed.type).toBe('string');
+  });
+
+  test('missing/empty title falls back to inferred title (no throw)', () => {
+    const parsed = parseMarkdown('---\ntype: note\n---\nbody\n', 'people/alice-example.md');
+    expect(typeof parsed.title).toBe('string');
+    expect(parsed.title.length).toBeGreaterThan(0);
+  });
+
+  test('string title still passes through unchanged', () => {
+    const parsed = parseMarkdown('---\ntitle: A Normal Title\n---\nbody\n', 'x.md');
+    expect(parsed.title).toBe('A Normal Title');
+  });
+});