PaperCast

Documentation

Content Locking

Prevent accidental textual modifications on specific nodes.

PaperCast provides a granular Content Locking mechanism designed to prevent accidental modifications to the core data (text, URLs) of a node while still permitting layout and stylistic adjustments.

This is extremely useful when building templates (e.g., Invoices or Legal Contracts) where the host application or an AI agent binds specific data to a node, and the end-user should only be allowed to style it, not change the actual data.

Enabling Content Lock

You can lock any node by setting the lockContent flag inside its config object:

{
  "id": "node_123",
  "type": "text",
  "config": {
    "lockContent": true
  },
  "props": {
    "literal": "Invoice #1024"
  }
}

How It Works

Content Locking is enforced at two distinct layers to ensure both a smooth UX and strict structural integrity.

1. Client-Side UX: ContentLockExtension (ProseMirror)

For rich text editing, PaperCast uses TipTap (built on ProseMirror). When a node is locked, the InlineRichTextEditor component dynamically injects the ContentLockExtension.

This custom ProseMirror plugin intercepts user input events at the editor level. It blocks keyboard input and pasting, effectively making the text editor read-only, while still allowing the user to select text and apply inline styles (like bold, italic, or color changes).

2. Structural Guardrail: AST Diffing

Because users can edit the JSON directly (or an AI agent might attempt an illegal edit via MCP), client-side UX blocks are not enough.

PaperCast runs a strict AST-diffing guardrail (contentLockGuardrail.ts) every time the document state is updated.

  1. Extraction: The guardrail traverses the incoming AST. When it finds a node with lockContent: true, it extracts the "textual content" using a discriminated union:
    • text / richText: Compares props.literal / props.htmlLiteral
    • image: Compares props.srcLiteral
    • Custom Nodes: Falls back to comparing JSON.stringify(node.props) to ensure no custom properties are maliciously altered.
  2. Comparison: It compares the extracted content against the previous, known-good state of the document.
  3. Enforcement: If the content or the bind.path has been altered, the state update is immediately rejected, and an error is thrown: Node [...] is content-locked. Textual content cannot be modified.

Summary

By combining proactive client-side UX extensions with reactive AST-diffing guardrails, PaperCast guarantees that your protected document data remains pristine, no matter how the user or AI interacts with the builder.

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