opencode-harper

An OpenCode plugin that silently spell-checks your prompts using Harper before they reach the LLM. The model never knows spell checking happened; it just the fixed text.

The spell checking is quite aggressive. It can often make mistakes, but I haven't seen any way to make the decisions dynamic/user triggered.

Installation

Add the plugin path to your opencode.json:

{
  "plugin": ["/path/to/opencode-harper/index.ts"]
}

Or place index.ts directly in your global plugins directory (~/.config/opencode/plugins/) or project plugins directory (.opencode/plugins/).

How it works

1. Intercepts user prompts via chat.message hook (when a new user message is created, before it's saved)
2. Skips messages from non-primary agents - only processes messages where input.agent is "build" or "plan". Subagent messages ("explore", "general", etc.) and internal agents ("compaction", "title", "summary") are ignored.
3. Filters to original user text only - skips parts marked synthetic (system-injected content like file reads, tool output summaries) and ignored parts
4. Runs harper-cli lint --format json on each user text part via Bun.spawn()
5. Applies all Harper corrections (spelling, grammar, style, typos)
6. Shows a toast notification with changes made

Note: Only user's chat input is spell checked: model output is untouched.

Protected regions

Corrections are skipped inside:

• Fenced code blocks (```...```)
• Inline code (`...`)
• Double-quoted strings ("...", including smart quotes)
• Single-quoted strings ('...', including smart quotes) — contractions like it's, don't are preserved via lookbehind/lookahead regex
• File paths (absolute /home/user/..., home-relative ~/..., relative ./... and ../...)
• Filenames with common extensions (*.ts, *.py, *.json, etc.)

Requirements

• OpenCode
• Harper CLI installed and available in PATH

Testing

bun test.ts

License

MIT