MD020 - Missing Space Inside Hashes on Closed ATX Style Headings

Severity: Warning
Category: Headings
Auto-fix: ✓ Available

Rule Description

This rule checks heading-like closed ATX syntax for missing separators between the heading text and its opening or closing hash sequence. A closed ATX heading uses spaces or tabs inside both sets of hashes.

Per CommonMark, a trailing hash sequence is a closing delimiter only when it is preceded by a space or tab. Content-adjacent hashes in headings such as C# and F# are ordinary heading text and do not trigger this rule.

Examples

❌ Incorrect

#Heading 1#
##Heading 2 ##

✅ Correct

# Heading 1 #
## Heading 2 ##

# Open headings are also valid
### C#
### F#

### Heading# is an open ATX heading whose text ends in #. MD020 does not guess that the final hash was intended as a closing delimiter.

Configuration

This rule has no configuration options.

Automatic Fix

The fix adds missing separators where closed-heading intent is unambiguous:

##Heading##  ->  ## Heading ##
##Heading ## ->  ## Heading ##

Content-adjacent hashes are never removed or converted into delimiters.

# Apply fixes
mdbook-lint lint --fix docs/

# Preview fixes
mdbook-lint lint --fix --dry-run docs/

MD003 - Heading style
MD018 - Missing space after hashes on ATX headings
MD019 - Multiple spaces after hashes on ATX headings
MD021 - Multiple spaces inside closed ATX headings

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