[BUG] Fenced code blocks in assistant messages render without syntax highlighting (VS Code extension)

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

Fenced code blocks in **assistant message text** render as flat monospace with a
per-line gray background and no syntax highlighting, even when the fence carries a
correct language tag (`` ```ts ``, `` ```js ``, `` ```html ``, `` ```tsx ``).

The gray background hugs each line's text width rather than spanning the full block
width, which reads like inline-`` styling being applied to a `

` element.


By contrast, code shown in **Edit / Write tool results** in the same panel renders

with full syntax highlighting and diff coloring. So a highlighter is clearly present

in the webview — it just doesn't appear to be applied to markdown code fences in

message text.

Screenshot:



### What Should Happen?

Fenced code blocks in assistant messages get the same syntax highlighting as code in

tool results.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

1. Open the Claude Code panel in VS Code.

2. Ask any question whose answer contains a multi-line fenced code block with a

   language tag — e.g. "explain this PR with code examples", or simply

   "show me a TypeScript function in a code block".

3. In the same conversation, have Claude edit a file so an Edit tool result appears.

4. Compare the two renderings: the tool-result code is highlighted; the code block in

   the message text is not.

### Claude Model

Opus

### Is this a regression?

No, this never worked

### Last Working Version

_No response_

### Claude Code Version

2.1.219 

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

## Steps to reproduce

1. Open the Claude Code panel in VS Code.

2. Ask any question whose answer contains a multi-line fenced code block with a

   language tag — e.g. "explain this PR with code examples", or simply

   "show me a TypeScript function in a code block".

3. In the same conversation, have Claude edit a file so an Edit tool result appears.

4. Compare the two renderings: the tool-result code is highlighted; the code block in

   the message text is not.

## Impact

Long code-heavy explanations (PR walkthroughs, before/after diffs, design writeups)

are hard to read in the panel. The workaround is to have Claude write the explanation

to a `.md` file and open it in the editor, which defeats the point of explaining

in-chat.

## Notes

- The webview-vs-native-renderer split above is inferred from the behavior difference,

  not from reading the extension source — treat it as a hypothesis, not a diagnosis.

- If the flat style is deliberate rather than a bug, please read this as a feature

  request for opt-in highlighting.

Reproduce the issue in the VS Code panel using a language-tagged fenced block, then compare assistant message rendering with the highlighted Edit or Write tool result. Start by locating the extension entry point that renders assistant markdown; done means fenced blocks in messages receive syntax highlighting and full block-width styling like tool-result code.

Written by the indexing model from the issue text.