diff --git a/packages/repl-sdk/src/compilers/markdown/heading-id.js b/packages/repl-sdk/src/compilers/markdown/heading-id.js
index 4f2647d9e..f679ba029 100644
--- a/packages/repl-sdk/src/compilers/markdown/heading-id.js
+++ b/packages/repl-sdk/src/compilers/markdown/heading-id.js
@@ -38,7 +38,7 @@ function extractText(children) {
  * @param {string} value
  */
 function formatDefaultId(value) {
-  return kebabCase(value.replaceAll(/\\s+/g, ' ').trim());
+  return kebabCase(value.replaceAll(/\s+/g, ' ').trim());
 }
 
 /**
diff --git a/packages/repl-sdk/src/compilers/markdown/parse.test.ts b/packages/repl-sdk/src/compilers/markdown/parse.test.ts
index 213679e87..ae952b93a 100644
--- a/packages/repl-sdk/src/compilers/markdown/parse.test.ts
+++ b/packages/repl-sdk/src/compilers/markdown/parse.test.ts
@@ -237,6 +237,24 @@ describe('default features', () => {
   });
 });
 
+describe('heading ids', () => {
+  it('collapses runs of whitespace in the heading text', async () => {
+    const result = await parseMarkdown(`##   Hello    World  `, { ...defaults });
+
+    // Only the id is normalized; the rendered text keeps the author's spacing.
+    expect(result.text).toContain('id="hello-world"');
+  });
+
+  it('treats a literal \\s in a heading as text, not as whitespace', async () => {
+    // The regex in formatDefaultId used to be /\\s+/g -- a literal backslash
+    // followed by `s`, rather than whitespace -- so `\s` here was replaced with
+    // a space and the `s` vanished from the id.
+    const result = await parseMarkdown(`## a \\s b`, { ...defaults });
+
+    expect(result.text).toContain('id="a-s-b"');
+  });
+});
+
 describe('options', () => {
   describe('remarkPlugins', () => {
     it('works', async () => {