markstream-custom-components – Coding skill for AI agents

Markstream Custom Components

Overview

Customize specific Markstream nodes or trusted custom tags without replacing the parser or leaking global renderer state. Read [references/patterns.md](references/patterns.md) first.

When to Use

Use to replace built-ins such as image, link, code_block, mermaid, or inline_code; render trusted tags such as thinking; or scope overrides to one renderer or app. Use parser transforms only when token or AST reshaping is required.

Workflow

Before changing dependencies or source files, inspect the existing package manager and project conventions, preview the intended edits, and obtain explicit user approval.

1. Classify the change as a built-in override, trusted tag, or parser transform.

2. Prefer scoped mappings. Vue, Vue 2, Svelte, and Angular can use setCustomComponents(customId, mapping); Svelte and Angular can also pass renderer-local maps.

3. In React, prefer streamingComponents for parser-backed nodes and htmlComponents for sanitized attributes plus children.

4. Start with leaf nodes before containers that must preserve children.

5. For trusted tag bodies containing Markdown, use a nested renderer with the same allowlist. Do not add a second smooth-streaming loop.

6. Preserve node/loading props, identity keys, scope IDs, theme state, and preview-height estimates for async diagrams.

7. Remove temporary scoped registrations on cleanup and validate repeated and nested tags.

Example

import MarkdownRender, {
  type NodeComponentProps,
  setCustomComponents,
} from 'markstream-react'
import 'markstream-react/index.css'

function ThinkingNode({ node }: NodeComponentProps<any>) {
  return <details><summary>Thinking</summary>{node.content}</details>
}

setCustomComponents('assistant-panel', { thinking: ThinkingNode })

export function Answer({ markdown }: { markdown: string }) {
  return (
    <MarkdownRender
      content={markdown}
      customId="assistant-panel"
      customHtmlTags={['thinking']}
      htmlPolicy="safe"
    />
  )
}

Limitations

• Component overrides cannot reproduce arbitrary remark/rehype transforms.
• Container overrides require careful child rendering and accessibility review.
• Framework registration APIs are not interchangeable.

Security & Safety Notes

Treat custom HTML-like tags as trusted input only. Keep safe HTML enabled and do not pass unsanitized attributes into host components.