Feature request: opt-in literal text mode for custom tag children (`allowedTags`)

[songkeys]

Summary

Please add an opt-in way to render children of selected custom tags as plain text (no markdown parsing).

Motivation

In AI UIs, custom tags are often used for structured entities such as mentions:

<mention user_id="123">@_some_username_</mention>

Today, markdown in tag children is still parsed, so _some_username_ can become italics.

For entity tags (usernames/handles/IDs), this is often unintended.

Current Behavior

With allowedTags + components, child text is parsed as markdown (expected CommonMark/react-markdown behavior), e.g.:

• _some text_ -> emphasis
• @_foo_ -> emphasis in handle-like content

This happens even with parseIncompleteMarkdown={false} (because this is normal markdown parsing, not remend completion).

Why this matters

For mention/entity tags, children are usually data labels, not prose. Requiring model-side escaping (\\_) everywhere is fragile and adds prompt/runtime complexity.

Minimal Repro

import { Streamdown } from 'streamdown';

function Mention(props: any) {
  return <span data-tag="mention" {...props} />;
}

<Streamdown
  allowedTags={{ mention: ['user_id'] }}
  components={{ mention: Mention }}
>
  {`<mention user_id="123">_some text_</mention>`}
</Streamdown>

Actual output renders emphasis inside mention content.
Desired output is literal _some text_.

Requested API (non-breaking, opt-in)

Any equivalent API would help, for example:

<Streamdown
  allowedTags={{ mention: ['user_id'] }}
  literalTagContent={['mention']}
/>

or per-tag config:

allowedTags={{
  mention: { attributes: ['user_id'], literalChildren: true }
}}

Expected semantics for literal mode

1. Children text for specified tags is treated as plain text.
2. Markdown emphasis/link/code parsing is skipped for those children.
3. Default behavior remains unchanged for all other tags and users.

Workaround today

Escaping underscores (\\_) works, but is brittle for AI-generated content and requires extra sanitization logic in app code.

An official opt-in mode would make mention/entity rendering much more robust.