diff --git a/src/checks/observability/markdown-content-parity.ts b/src/checks/observability/markdown-content-parity.ts index 475bab2..43ab104 100644 --- a/src/checks/observability/markdown-content-parity.ts +++ b/src/checks/observability/markdown-content-parity.ts @@ -407,8 +407,13 @@ function extractMarkdownText(markdown: string): string { // Replace entire fenced blocks (``` ... ```) with placeholders so // heading/link/emphasis/blockquote regexes don't modify literal content // that the HTML side preserves as-is inside 
 tags.
+  //
+  // Per CommonMark §4.5, a fence opens with N>=3 backticks and closes only
+  // on a run of >=N. Capture the opener so the close-side backreference
+  // matches; otherwise nested example fences (4-backtick outer, 3-backtick
+  // inner) get mis-paired and inner markers leak out as text.
   const codeBlocks: string[] = [];
-  text = text.replace(/^```[^`\n]*\n([\s\S]*?)^```\s*$/gm, (_match, content) => {
+  text = text.replace(/^(`{3,})[^`\n]*\n([\s\S]*?)^\1`*\s*$/gm, (_match, _opener, content) => {
     const idx = codeBlocks.length;
     codeBlocks.push(content);
     return `\x00BLOCK${idx}\x00`;
diff --git a/test/unit/checks/markdown-content-parity.test.ts b/test/unit/checks/markdown-content-parity.test.ts
index 2a690bc..177af73 100644
--- a/test/unit/checks/markdown-content-parity.test.ts
+++ b/test/unit/checks/markdown-content-parity.test.ts
@@ -2330,4 +2330,211 @@ All requests are authenticated automatically using the configured API credential
     const pageResults = result.details?.pageResults as Array<{ error?: string }>;
     expect(pageResults[0].error).toContain('Invalid CSS selector');
   });
+
+  it('honors fence length: an N-backtick fence contains shorter runs as content', async () => {
+    // 4-backtick outer fence wrapping 3-backtick inner fences. The inner
+    // markers are content per CommonMark §4.5 and must not be consumed as
+    // delimiters when extracting markdown text.
+    const html = `

+      
The following demonstrates how an outer fenced block can contain shorter fences.

+      
Readers should see the inner backtick lines exactly as written, with no rendering applied.

+      
\`\`\`alpha filename="first.txt"
+first inner body line that is long enough to count
+\`\`\`
+
+\`\`\`beta filename="second.txt"
+second inner body line that is long enough to count
+\`\`\`
+
+\`\`\`gamma filename="third.txt"
+third inner body line that is long enough to count
+\`\`\`

+      
This pattern lets prose explain how to author fenced blocks without rendering them.

+      
The same shape appears in any documentation that teaches markdown or MDX syntax.

+      
It is also used when one tool's output happens to include backtick-delimited regions.

+      
Comparing the two formats requires both sides to treat the inner lines as plain text.

+      
Otherwise the markdown extractor will consume the inner markers as real delimiters.

+      
That asymmetry produces missing segments even though the two sources carry equal text.

+      
Extracting the markdown side must respect the opener length and look for a matching close.

+      
A four-backtick opener can only be terminated by a four-or-more-backtick closing line.

+    
`;
+
+    const markdown = `The following demonstrates how an outer fenced block can contain shorter fences.
+
+Readers should see the inner backtick lines exactly as written, with no rendering applied.
+
+\`\`\`\`outer filename="example.mdx"
+\`\`\`alpha filename="first.txt"
+first inner body line that is long enough to count
+\`\`\`
+
+\`\`\`beta filename="second.txt"
+second inner body line that is long enough to count
+\`\`\`
+
+\`\`\`gamma filename="third.txt"
+third inner body line that is long enough to count
+\`\`\`
+\`\`\`\`
+
+This pattern lets prose explain how to author fenced blocks without rendering them.
+
+The same shape appears in any documentation that teaches markdown or MDX syntax.
+
+It is also used when one tool's output happens to include backtick-delimited regions.
+
+Comparing the two formats requires both sides to treat the inner lines as plain text.
+
+Otherwise the markdown extractor will consume the inner markers as real delimiters.
+
+That asymmetry produces missing segments even though the two sources carry equal text.
+
+Extracting the markdown side must respect the opener length and look for a matching close.
+
+A four-backtick opener can only be terminated by a four-or-more-backtick closing line.`;
+
+    const url = 'http://mcp-nested-fence.local/docs/page';
+    server.use(
+      http.get(
+        url,
+        () =>
+          new HttpResponse(html, {
+            status: 200,
+            headers: { 'Content-Type': 'text/html' },
+          }),
+      ),
+    );
+
+    const ctx = makeCtx([{ url, markdown, htmlBody: html }], 'mcp-nested-fence.local');
+    const result = await check.run(ctx);
+    expect(result.status).toBe('pass');
+    const pageResults = result.details?.pageResults as Array<{ missingSegments: number }>;
+    expect(pageResults[0].missingSegments).toBe(0);
+  });
+
+  it('allows a 3-backtick fence to be closed by a longer backtick run', async () => {
+    // CommonMark allows the closer to be longer than the opener. Markdown
+    // syntax inside the block must still be shielded from later stripping.
+    const html = `

+      
The block below is a regular three-backtick fence with a longer closing run.

+      
Both formats should preserve every line inside it as literal text content.

+      
# Heading inside a code block stays as literal text
+- list item inside a code block stays as literal text
+> blockquote inside a code block stays as literal text

+      
Heading-like content inside the fenced block must not be stripped on either side.

+      
The same applies to list bullets and blockquote markers that look like markdown syntax.

+      
This shape appears whenever an author chose a longer run for visual symmetry reasons.

+      
The closer can have any number of backticks above the opener length, with optional spaces.

+      
Trailing whitespace on the closer line is also tolerated by the underlying CommonMark spec.

+      
Authors sometimes pad closers with extra backticks to make blocks visually distinct.

+      
The parity check should treat all of those variations as equivalent fenced regions.

+    
`;
+
+    const markdown = `The block below is a regular three-backtick fence with a longer closing run.
+
+Both formats should preserve every line inside it as literal text content.
+
+\`\`\`
+# Heading inside a code block stays as literal text
+- list item inside a code block stays as literal text
+> blockquote inside a code block stays as literal text
+\`\`\`\`
+
+Heading-like content inside the fenced block must not be stripped on either side.
+
+The same applies to list bullets and blockquote markers that look like markdown syntax.
+
+This shape appears whenever an author chose a longer run for visual symmetry reasons.
+
+The closer can have any number of backticks above the opener length, with optional spaces.
+
+Trailing whitespace on the closer line is also tolerated by the underlying CommonMark spec.
+
+Authors sometimes pad closers with extra backticks to make blocks visually distinct.
+
+The parity check should treat all of those variations as equivalent fenced regions.`;
+
+    const url = 'http://mcp-longer-closer.local/docs/page';
+    server.use(
+      http.get(
+        url,
+        () =>
+          new HttpResponse(html, {
+            status: 200,
+            headers: { 'Content-Type': 'text/html' },
+          }),
+      ),
+    );
+
+    const ctx = makeCtx([{ url, markdown, htmlBody: html }], 'mcp-longer-closer.local');
+    const result = await check.run(ctx);
+    expect(result.status).toBe('pass');
+    const pageResults = result.details?.pageResults as Array<{ missingSegments: number }>;
+    expect(pageResults[0].missingSegments).toBe(0);
+  });
+
+  it('handles deeply nested fences with strictly decreasing backtick runs', async () => {
+    // 5/4/3 nesting. Each level's closer must match its own opener length;
+    // guards against regressions that special-case any single run length.
+    const html = `

+      
The following demonstrates a three-level nest with five, four, and three backticks.

+      
All inner backtick lines must reach the reader as literal text on both sides.

+      
\`\`\`\`level-two filename="middle.md"
+\`\`\`level-three filename="inner.txt"
+innermost body line that is long enough to count
+\`\`\`
+\`\`\`\`

+      
Authors use this pattern when the example itself is meta and contains other examples.

+      
Each layer must independently survive segmentation and comparison without leaks.

+      
The outermost fence opens and closes with five backticks to dominate every inner run.

+      
The middle fence uses four backticks because its content includes triple-backtick lines.

+      
The innermost three-backtick fence is just a normal code block at the bottom of the nest.

+      
Removing any layer would change the literal text and is therefore not equivalent content.

+      
This test guards against regressions that special-case any single backtick run length.

+    
`;
+
+    const markdown = `The following demonstrates a three-level nest with five, four, and three backticks.
+
+All inner backtick lines must reach the reader as literal text on both sides.
+
+\`\`\`\`\`level-one filename="outer.mdx"
+\`\`\`\`level-two filename="middle.md"
+\`\`\`level-three filename="inner.txt"
+innermost body line that is long enough to count
+\`\`\`
+\`\`\`\`
+\`\`\`\`\`
+
+Authors use this pattern when the example itself is meta and contains other examples.
+
+Each layer must independently survive segmentation and comparison without leaks.
+
+The outermost fence opens and closes with five backticks to dominate every inner run.
+
+The middle fence uses four backticks because its content includes triple-backtick lines.
+
+The innermost three-backtick fence is just a normal code block at the bottom of the nest.
+
+Removing any layer would change the literal text and is therefore not equivalent content.
+
+This test guards against regressions that special-case any single backtick run length.`;
+
+    const url = 'http://mcp-deep-nest.local/docs/page';
+    server.use(
+      http.get(
+        url,
+        () =>
+          new HttpResponse(html, {
+            status: 200,
+            headers: { 'Content-Type': 'text/html' },
+          }),
+      ),
+    );
+
+    const ctx = makeCtx([{ url, markdown, htmlBody: html }], 'mcp-deep-nest.local');
+    const result = await check.run(ctx);
+    expect(result.status).toBe('pass');
+    const pageResults = result.details?.pageResults as Array<{ missingSegments: number }>;
+    expect(pageResults[0].missingSegments).toBe(0);
+  });
 });