Add max-len rule to enforce configurable line length limits for Markdown entities

[Copilot]

Addressing Bot Review Feedback

• Clarified documentation structure - separated entity options from container options
• Updated documentation to show containers can configure all entity types, not just heading/paragraph
• Added comprehensive test coverage:
  • Nested container configuration (blockquote with heading/paragraph/table limits)
  • Language-specific code block configuration (javascript/python/shell)
  • Language-specific frontmatter configuration (yaml with ignore)
  • HTML block entity tests
  • Math block entity tests
  • Footnote definition entity tests
• All 4293 tests passing (added 12 new test fixtures)
• Build successful

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Rule to enforce maximim length of various entities</issue_title>
<issue_description>### Motivation

Overly long Markdown elements - headings, paragraphs, or code lines - reduce readability, make diffs harder to review, and complicate maintenance.

For example, a very long heading may not fit well in navigation menus or anchor links; excessively long paragraphs can be hard to scan in raw Markdown; and unwrapped lines make reviewing changes in pull requests inconvenient.

By enforcing reasonable maximum lengths for various entities, documentation remains cleaner, easier to navigate, and more consistent across the project.

Description

Introduce a rule that enforces a configurable maximum length for different Markdown entities, such as:

• Headings
• Paragraphs (plain text)
• List items (?)
• Code blocks
• Blockquotes
• Tables

The rule should:

• Allow per-entity configuration (e.g., heading: 80, paragraph: 120, etc.) and report when an entity exceeds its defined maximum length.
• Fixer is possible, but usually unnecessary, difficult to guess with heuristics, and can be destructive.
• Provide some heuristics for working with long URLs.
  I don't find what Markdownlint has to offer very convenient - I would suggest considering calculating the longest entity in the string relative to the maximum available width, e.g., if a string exceeds the line length, but 80% of that string is a URL, then don't mark that string. I don't know, it's a matter for discussion.

Related markdownlint rule

Examples

THIS LINE IS THE MAXIMUM HEADING LENGTH
THIS LINE IS THE MAXIMUM PARAGRAPH LENGTH

<!-- ✗ BAD -->

## This heading is a violation violation violation

This line is a violation because it goes beyond the boundaries.

<!-- ✓ GOOD -->

## This heading is fine

This paragraph is ok as well

Additional comments

No response</issue_description>

<agent_instructions>Please include the notes in the rule documentation.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@ota-meshi The rules you proposed mostly sound good. However, I'd like to request some changes to a few behaviors.

Regarding code blocks, I think it would be better to handle them with a separate language plugin. eslint/markdown has a feature that lints the code inside code blocks as code of that specific language.
Currently, that feature cannot be used with this plugin, but I think it will be extended in the future.
eslint/rfcs#105
Therefore, I think it makes sense to ignore code blocks in this plugin.

Regarding URLs, how about handling them similarly to @stylistic/eslint-plugin?
https://eslint.style/rules/max-len#ignoreurls

Fixer is possible, but usually unnecessary, difficult to guess with heuristics, and can be destructive.

Yeah, I don't think auto-fixing is necessary.</comment_new>
<comment_new>@ota-meshi
To apply specific settings to code blocks in Markdown, we can specify virtual file paths in files.

    {
        files: ["**/*.md/*.js"],
        rules: {
            "@stylistic/max-len": ["error", { "ignoreComments": true }]
        }
    }

https://github.com/eslint/markdown/blob/main/docs/processors/markdown.md#advanced-configuration

Also, for example, I am fine with a "soft" limit of 120 characters for code, but for code blocks in Markdown, I want to have a lower limit (for example, to ensure that in Vitepress, a code block will be rendered without horizontal scrolling - and this sounds like a global option for Markdown, not tied to a specific language).

My main concern is that this plugin's rules don't know what tokens are in code blocks.
Even if you set the plugin's rule markdown-preferences/max-len to ignore comments, it cannot determine and ignore comments in code blocks.
Therefore, I think code blocks should be ignored by the rule.
That being said, I don't think we'll have all the ESLint language plugins, so if anyone needs it, I think it's fine to optionally check the line length of code blocks, but they need to understand that the syntax inside the code block will be completely ignored.

• Fixes Rule to enforce maximim length of various entities #203

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🦋 Changeset detected

Latest commit: d6334a0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
eslint-plugin-markdown-preferences	Minor

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[ota-meshi]

@copilot

Please also respond to the following:

• Add a new rule to docs/appendix/comparison-with-markdownlint-rules.md
• Can you revisit your code and make it smarter?

[Copilot]

@copilot

Please also respond to the following:...

Added rule to docs/appendix/comparison-with-markdownlint-rules.md. The code has been significantly refactored to use sourceCode methods, support nested/language-specific configs, and follow all requested patterns. Commit: 67c8d93

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[Copilot]

Pull request overview

This pull request implements a new max-len rule that enforces configurable maximum line lengths for different Markdown entities. The rule addresses the need for better readability and maintainability in Markdown documentation by preventing overly long lines in various elements like headings, paragraphs, lists, blockquotes, tables, code blocks, frontmatter, footnote definitions, HTML blocks, and math blocks.

Key changes include:

• Implementation of entity-specific length limits with sensible defaults (80 for headings, 120 for others)
• Support for nested container configuration allowing different limits for entities within blockquotes, lists, and footnote definitions
• Language-specific configuration for code blocks and frontmatter
• URL detection to ignore lines containing URLs by default (similar to @stylistic/max-len)
• Code blocks ignored by default per maintainer feedback, with option to enable checking

Reviewed changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/rules/max-len.ts	Core rule implementation with option parsing, URL detection, and line length checking logic
src/utils/rules.ts	Integration of max-len rule into the rules array
src/rule-types.ts	TypeScript type definitions for rule options (incomplete - missing full container option types)
tests/src/rules/max-len.ts	Test harness using snapshot testing
tests/src/rules/snapshots/max-len.ts.eslintsnap	Snapshot file with expected test outputs
tests/fixtures/rules/max-len/valid/*.md	Valid test fixtures for code blocks, headings, paragraphs, tables, and URLs
tests/fixtures/rules/max-len/invalid/*.md	Invalid test fixtures for various entity types exceeding length limits
tests/fixtures/rules/max-len/invalid/*.json	Configuration files for invalid test fixtures
docs/rules/max-len.md	Comprehensive rule documentation (needs structural improvements for clarity)
docs/rules/index.md	Addition of rule to "Undefined Rules" section
docs/appendix/comparison-with-markdownlint-rules.md	Comparison entry mapping to markdownlint MD013
README.md	Documentation updates with rule listing in "Undefined Rules" section
eslint.config.mjs	Added allowed anchor for GitHub markdown documentation link
.changeset/max-len.md	Changeset entry for the new rule

[ota-meshi]

@copilot Please respond to each outstanding comment.

[Copilot]

@copilot Please respond to each outstanding comment.

Addressed all three outstanding comments:

1. Clarified that containers can configure all entity types, not just heading/paragraph
2. Restructured documentation to separate entity options from container options
3. Added comprehensive test coverage for all missing features (nested configs, language-specific configs, and all entity types)

All changes in commit: 4c02fbb