Bently 905373a712 fix(frontend): use singleton Shiki highlighter for code syntax highlighting (#12144) ...

## Summary
Addresses SENTRY-1051: Shiki warning about multiple highlighter
instances.

## Problem
The `@streamdown/code` package creates a **new Shiki highlighter for
each language** encountered. When users view AI chat responses with code
blocks in multiple languages (JavaScript, Python, JSON, YAML, etc.),
this creates 10+ highlighter instances, triggering Shiki's warning:

> "10 instances have been created. Shiki is supposed to be used as a
singleton, consider refactoring your code to cache your highlighter
instance"

This causes memory bloat and performance degradation.

## Solution
Introduced a custom code highlighting plugin that properly implements
the singleton pattern:

### New files:
- `src/lib/shiki-highlighter.ts` - Singleton highlighter management
- `src/lib/streamdown-code-plugin.ts` - Drop-in replacement for
`@streamdown/code`

### Key features:
- **Single shared highlighter** - One instance serves all code blocks
- **Preloaded common languages** - JS, TS, Python, JSON, Bash, YAML,
etc.
- **Lazy loading** - Additional languages loaded on demand
- **Result caching** - Avoids re-highlighting identical code blocks

### Changes:
- Added `shiki` as direct dependency
- Updated `message.tsx` to use the new plugin

## Testing
- [ ] Verify code blocks render correctly in AI chat
- [ ] Confirm no Shiki singleton warnings in console
- [ ] Test with multiple languages in same conversation

## Related
- Linear: SENTRY-1051
- Sentry: Multiple Shiki instances warning

<!-- greptile_comment -->

<details><summary><h3>Greptile Summary</h3></summary>

Replaced `@streamdown/code` with a custom singleton-based Shiki
highlighter implementation to resolve memory bloat from creating
multiple highlighter instances per language. The new implementation
creates a single shared highlighter with preloaded common languages (JS,
TS, Python, JSON, etc.) and lazy-loads additional languages on demand.
Results are cached to avoid re-highlighting identical code blocks.

**Key changes:**
- Added `shiki` v3.21.0 as a direct dependency
- Created `shiki-highlighter.ts` with singleton pattern and language
management utilities
- Created `streamdown-code-plugin.ts` as a drop-in replacement for
`@streamdown/code`
- Updated `message.tsx` to import from the new plugin instead of
`@streamdown/code`

The implementation follows React best practices with async highlighting
and callback-based notifications. The cache key uses code length +
prefix/suffix for efficient lookups on large code blocks.
</details>


<details><summary><h3>Confidence Score: 4/5</h3></summary>

- Safe to merge with minor considerations for edge cases
- The implementation is solid with proper singleton pattern, caching,
and async handling. The code is well-structured and addresses the stated
problem. However, there's a subtle potential race condition in the
callback handling where multiple concurrent requests for the same cache
key could trigger duplicate highlight operations before the first
completes. The cache key generation using prefix/suffix could
theoretically cause false cache hits for large files with identical
prefixes and suffixes. Despite these edge cases, the implementation
should work correctly for the vast majority of use cases.
- No files require special attention
</details>


<details><summary><h3>Sequence Diagram</h3></summary>

```mermaid
sequenceDiagram
    participant UI as Streamdown Component
    participant Plugin as Custom Code Plugin
    participant Cache as Token Cache
    participant Singleton as Shiki Highlighter (Singleton)
    participant Callbacks as Pending Callbacks

    UI->>Plugin: highlight(code, lang)
    Plugin->>Cache: Check cache key
    
    alt Cache hit
        Cache-->>Plugin: Return cached result
        Plugin-->>UI: Return highlighted tokens
    else Cache miss
        Plugin->>Callbacks: Register callback
        Plugin->>Singleton: Get highlighter instance
        
        alt First call
            Singleton->>Singleton: Create highlighter with preloaded languages
        end
        
        Singleton-->>Plugin: Return highlighter
        
        alt Language not loaded
            Plugin->>Singleton: Load language dynamically
        end
        
        Plugin->>Singleton: codeToTokens(code, lang, themes)
        Singleton-->>Plugin: Return tokens
        Plugin->>Cache: Store result
        Plugin->>Callbacks: Notify all waiting callbacks
        Callbacks-->>UI: Async callback with result
    end
```
</details>


<sub>Last reviewed commit: 96c793b</sub>

<!-- greptile_other_comments_section -->

<!-- /greptile_comment -->

2026-02-17 12:15:53 +00:00

6.3 KiB

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