I Built a Fully Local Prompt Enhancer Chrome Extension with Gemini Nano

Source: Dev.to

The Problem: We Underspecify Prompts

Most people use ChatGPT (or any LLM) like a search bar: dump a half‑formed thought, hit Enter, and hope for magic. The result is usually OK, but not great.

Common issues

• Vague prompts such as “explain this topic for my exam” with no level, context, or constraints.
• Coding requests lacking input/output format, edge cases, or performance constraints.
• Writing tasks missing target audience, tone, or length.

A well‑designed prompt can:

• Save multiple back‑and‑forth iterations.
• Produce more reliable and testable outputs (especially for code and analysis).
• Make AI tools actually usable in real workflows.

The Idea

What if you could type as you usually do, then hit one button that rewrites your rough text into a high‑quality prompt?

Prompt Enhancer does exactly that on top of ChatGPT, adding a tiny bit of super‑power without changing the usual workflow.

How It Works from the User’s Perspective

1. Install and enable the extension.
2. Open ChatGPT in Chrome.
3. Type any rough idea into the input box.
4. A small “Enhance” button appears beside the textarea.
5. Click Enhance (or use the keyboard shortcut) → wait a moment → the rough text is replaced with a refined, structured prompt.

The enhanced prompt typically:

• Adds missing context (audience, constraints, format).
• Clarifies the task (summarise vs critique vs generate vs explain).
• Specifies outputs (bullet list, table, code with comments, etc.).

You can review the rewritten prompt, tweak it if needed, then hit Enter as usual.

Keyboard Shortcut

A configurable shortcut lets power users keep the flow entirely on the keyboard:

Type → Enhance → Enter

Technical Architecture

Prompt Enhancer is a Manifest V3 Chrome extension composed of three main pieces:

1. manifest.json – declares permissions, content‑script matches, and other metadata.
2. Content script – interacts with the ChatGPT DOM, injects the Enhance button, and handles events.
3. Prompt API integration – calls the built‑in Gemini Nano model locally.

Manifest V3 Basics

{
  "manifest_version": 3,
  "name": "Prompt Enhancer",
  "version": "1.0",
  "description": "Rewrite rough prompts into high‑quality prompts for ChatGPT using Gemini Nano.",
  "permissions": ["activeTab", "scripting"],
  "host_permissions": ["https://chat.openai.com/*"],
  "content_scripts": [
    {
      "matches": ["https://chat.openai.com/*"],
      "js": ["contentScript.js"]
    }
  ]
}

• Minimal permissions keep the extension secure and simplify Chrome Web Store review.
• The content script detects the ChatGPT page, locates the main textarea, injects the floating Enhance button, and listens for click/keyboard events.
• Because ChatGPT’s UI is a frequently‑changing React app, the script uses stable selectors, fallback heuristics (e.g., “largest textarea”), and a MutationObserver to re‑attach the button after re‑renders.

Gemini Nano Integration via Prompt API

// Example of creating a Gemini Nano model instance
if (chrome.ai && chrome.ai.languageModel) {
  const model = await chrome.ai.languageModel.create({
    temperature: 0.2,
    topK: 40,
    // other model options...
  });

  const metaPrompt = `
    You are a prompt‑engineering assistant. Rewrite the user's text as a clear,
    detailed prompt that specifies context, output format, and constraints,
    without changing the core intent.
  `;

  const response = await model.prompt(`${metaPrompt}\nUser: ${roughText}`);
  // `response` now contains the enhanced prompt
}

• Local execution means low, predictable latency and no network round‑trip.
• The extension works even with Wi‑Fi off, as long as Chrome provides Gemini Nano.
• Defensive error handling is required because Gemini Nano is still emerging; a graceful fallback message is shown if the model isn’t available.

What’s Shipped Today (v1)

• Floating “Enhance” button on the ChatGPT prompt textarea.
• On‑device prompt enhancement using Gemini Nano via Chrome’s Prompt API.
• Full local processing with no external APIs or API keys.
• Keyboard shortcut support for power users.
• Minimal, native‑looking UX that feels integrated with ChatGPT.

The extension is live on the Chrome Web Store and can be installed like any other extension.

Lessons Learned

• Permissions & privacy: Clearly explain what data is accessed and why.
• User flow matters more than the model: Keep the typing flow uninterrupted, provide quick visual feedback, and respect the original intent.
• Early adoption trade‑off: Some users need to enable flags or use Dev/Canary builds; robust fallback messaging is essential.

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Prompt Enhancer demonstrates how a simple, well‑integrated UX around a local LLM can dramatically improve day‑to‑day AI usage.