Pre-Publish Validation: Enforce Excerpt Requirements via `addFilter`

[kjroelke]

Summary

Introduce a pre-publish validation system that prevents publishing when editorial rules are not met. The first rule will enforce excerpt length requirements across all post types that support excerpts.

Requirements

Excerpt Validation

A post must not be published unless its excerpt:

• Is not empty
• Is at least 120 characters
• Is no more than 160 characters

This applies only to post types that support excerpts.

Allowed Statuses

Validation must not run when transitioning to:

• draft
• pending_review

Only publishing-related transitions (such as publish, future) should be blocked.

User Feedback

If the excerpt fails validation:

• Publishing is blocked
• A clear warning is displayed in the editor (using core/notices)

Extensibility

The implementation must support additional pre-publish rules. Example follow-up validations might require:

• At least one category that isn’t Uncategorized
• A featured image
• Required custom fields

The system should expose a registration API so new rules can be added without modifying core logic.

Technical Approach

• Use an addFilter hook to intercept publish attempts.
• Read post data from the editor store (core/editor).
• Skip validation when the post type does not support excerpts.
• Provide a reusable rule registry, e.g.:

  registerPrePublishRule( 'excerptLength', ( post ) => {
      // return { valid: boolean, message: string }
  } );

• Evaluate all registered rules before allowing the publish.
• If any rule fails:
  • Block publish
  • Add a notice via core/notices

Acceptance Criteria

• Publishing is blocked when excerpt length is invalid.
• Draft and pending review operations are unaffected.
• Unsupported post types are ignored.
• Multiple rules can be added through a stable registration API.
• Editor notices appear consistently and clearly.

Notes

• This is the first step of a general pre-publish validation framework. Additional checks will be added in later issues.