MD070 - Nested Code Fence Collision

March 7, 2026 · View on GitHub

Aliases: nested-code-fence

Enabled by default: No (opt-in)

Fixable: Yes

To enable this rule, add it to your .rumdl.toml:

[global]
extend-enable = ["MD070"]

Description

Detects when a fenced code block contains fence markers that would cause premature closure. This commonly occurs when documenting markdown examples that themselves contain code blocks.

Why This Matters

When a code block's content contains fence markers (``` or ~~~) of the same type and equal or greater length, CommonMark parsers treat the inner fence as the closing fence, causing:

Premature block closure
Content appearing outside the intended block
Rendering issues in documentation

Examples

Incorrect

The following markdown has a nested fence collision:

```markdown
1. First item

   ```python
   code_in_list()
   ```

1. Second item

```

The inner ``` on line 4 closes the outer block prematurely.

Correct

Use longer fences for the outer block:

````markdown
1. First item

   ```python
   code_in_list()
   ```

1. Second item

````

Automatic Fixes

This rule automatically:

Increases the outer fence length to be longer than any inner fence
Updates both the opening and closing fences

For example, if content contains triple backticks (```), the fix changes both fences to quadruple backticks (````).

When This Rule Applies

This rule only triggers for:

Fenced code blocks (``` or ~~~)
Markdown-related blocks: empty language, markdown, or md
When the content contains fence-like patterns that would close the outer block

Programming language blocks (Python, Rust, JavaScript, etc.) are not checked since backticks in those languages don't represent markdown fences.

Different fence types don't collide

Tildes (~~~) and backticks (```) don't conflict:

~~~markdown
```python
code()
```
~~~

This is valid because the inner backtick fence doesn't close the outer tilde fence.

MD031 - Fenced code blocks should be surrounded by blank lines
MD040 - Fenced code blocks should have a language specified
MD046 - Code block style (fenced vs indented)
MD048 - Code fence style (backticks vs tildes)