[FEATURE]: TUI plugin API to render fenced code blocks in session content #54126

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Let TUI plugins register a renderer for a fenced code block language, so assistant replies can show rich content (diagrams, charts, math, 3D) in the conversation instead of raw source.

Proposal

api.markdown.codeBlock.register(lang, (block, ctx) => Renderable | undefined): () => void

block = { lang, text, complete, key, sessionID, messageID }
// complete: closing fence received (false while streaming)
// key: stable per block (message part + position) so plugins can keep state across streaming updates
// returning undefined falls back to the normal code block

It builds on what's already there: OpenTUI's Markdown.renderNode and createMarkdownCodeBlockRenderer. Changes are small and follow existing patterns:

• @opencode-ai/plugin/tui: add markdown to TuiPluginApi
• tui/plugin/runtime.tsx: a code-block registry, same shape as the plugin routes registry
• opencode/plugin/tui/runtime.ts: wrap registrations with scope.track() so they're removed when the plugin is disabled or unloaded
• routes/session/index.tsx (TextPart): build renderNode from the registry

Why implement this

• V2 TUI: add native Mermaid and LaTeX rendering #40508 asks for "the minimum plugin API needed for internal plugins to recognize and render custom session content" (Mermaid, LaTeX). This would be that extension point, public, so built-in and third-party plugins use the same mechanism.
• Coding agents are used more and more to explain code and systems. Diagrams are far better than ASCII art, and today the TUI can only show the source.
• It keeps renderers out of core: d2, Mermaid, LaTeX, charts and so on can live in plugins with their own dependencies.

Prototype

I built a quick prototype on a fork to validate the idea: https://github.com/koladev32/opencode/tree/inline-diagrams (work in progress, rendering code still in core; it would move to a plugin on top of this API).

• d2 blocks render as themed diagrams while the reply streams, with step-by-step animation
• scene3d blocks (YAML parts, never code) render as 3D exploded views (three.js + WebGPU, headless) that you can rotate
• Display uses Kitty graphics Unicode placeholders (works in Ghostty, and through tmux)

Here is a link to a demo I've done: https://x.com/koladev32/status/2108224214628991334

Design considerations

Security (renderers run on model output, i.e. untrusted input):

• core catches renderer errors and falls back to the code block (OpenTUI's Markdown doesn't guard custom renderers)
• copy still copies the original source; the existing "Disable code concealment" toggle disables renderers and shows the raw source
• optionally reserve shell languages (sh, bash, …) so a plugin can't display something different from the command a user might copy and run
• conflicts: last registration wins (like routes), with a warning

Performance:

• no registered renderers means no renderNode at all, so it's identical to today
• O(1) lookup by language; the code-block-only flag is kept so other Markdown elements skip the hook
• Markdown only re-invokes a renderer when that block's text changed; complete and key let plugins avoid work while streaming

Terminal support:

• The API is rendering-agnostic: a plugin returns an OpenTUI Renderable, so it decides how to draw based on the terminal's capabilities (api.renderer.capabilities).
• Graphics degrade through OpenTUI's ImageRenderable: Kitty graphics → Sixel → Unicode half-block characters (any truecolor terminal), so images still show in terminals without a graphics protocol, at lower fidelity.
• Plugins can also render text only (e.g. box-drawing characters) or return undefined to keep the normal code block; that is also the behavior on any renderer error.
• My prototype uses Kitty Unicode placeholders where available (flicker-free animation, works through tmux) and falls back to ImageRenderable elsewhere.

Happy to open a PR if the direction looks right.