Thoth — Local Grammar Checker & AI Writing Assistant (Chrome Extension)

Open-source Grammarly alternative: grammar and spell checking, tone rewrite, paraphrasing, and AI text generation — local-first on your Mac or Windows PC. Works in Gmail, Google Docs, LinkedIn, Slack, and most text fields on the web.

Privacy-first writing assistant — drafts stay on your machine by default (Ollama + LanguageTool). Optional Groq/OpenAI for faster rewrite and generate.

Chrome extension (public): Install Thoth from the Chrome Web Store

Find this repo: search GitHub for grammar checker chrome extension, local grammarly alternative, ollama writing assistant, text humanizer open source, or offline AI writing tool.

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Download & install

Follow the guide for your computer. Each guide is written as simple numbered steps.

Platform	Full step-by-step guide	Download
Chrome extension	Chrome Web Store	Add to Chrome
Windows	Install on Windows	Start Thoth.bat tray app or Releases
macOS	Install on Mac	Thoth.app

Mac users: download Thoth-macOS.zip from the latest release, drag Thoth.app into Applications, and open it once. No Terminal required.

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Windows (step by step)

Step 1: Download these apps

• Chrome
• Python 3.10+ (check Add python.exe to PATH)
• Ollama (open it once after install)
• Java 11+

Step 2: Download Thoth

1. Open https://github.com/Eshan-khan1/Thoth
2. Click Code, then Download ZIP
3. Unzip the folder (example: C:\Users\YourName\Thoth)

Step 3: Paste this into Command Prompt

Open Command Prompt, go into the Thoth folder, then paste:

scripts\install.bat

If needed (Ollama is open):

scripts\setup_models.bat

Step 4: Start the server

Double-click Start Thoth.bat, or paste:

start_server.bat

Keep that window open.

Step 5: Install the Chrome extension

Recommended: Install from the Chrome Web Store (click Add to Chrome).

Or double-click Start Thoth.bat for the tray app (like Mac Thoth.app), then install the extension from the link above.

Or load unpacked (developers):

1. Go to chrome://extensions
2. Turn on Developer mode
3. Click Load unpacked
4. Select the extension folder

More detail: docs/INSTALL_WINDOWS.md

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Mac (step by step)

Step 1: Download these apps

• Chrome
• Python 3.10+ (or brew install python)
• Ollama (open it once after install)
• Java 11+ (Adoptium or brew install openjdk@17)

Step 2: Download Thoth.app

Open the latest GitHub release and download the Mac app:

Download Thoth for Mac

1. Download Thoth-macOS.zip from Assets
2. Unzip the file

Step 3: Install the app (no Terminal)

1. Drag Thoth.app into Applications
2. Open it once (if macOS blocks it: right-click → Open → Open)
3. Look for the Thoth diamond icon in the menu bar (top-right) — there is no Dock icon
4. It quietly starts the server and sets itself to reopen after login or restart

Step 4: Install the Chrome extension

Install Thoth from the Chrome Web Store (click Add to Chrome). Keep Thoth.app running so the local server stays online.

Developers only — load unpacked:

1. In Thoth, use Connect Chrome Extension… (or follow the one-time setup sheet)
2. Open Chrome → chrome://extensions
3. Turn on Developer mode
4. Click Load unpacked
5. Select the folder the app opened/copied (usually
   ~/Library/Application Support/Thoth/ChromeExtension)

More detail: docs/INSTALL_MAC.md

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Check that the server is running

Open http://127.0.0.1:8000/health
You should see "ok": true.

Try it

• Type in any text box to see underlines on mistakes
• Select text to use Rewrite or Generate
• Click the Thoth toolbar icon for Settings

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What is Thoth?

Thoth is an open-source Grammarly alternative — a local grammar checker and AI writing assistant with a Chrome extension for real-time spell check, punctuation fixes, tone rewrite, paraphrasing, and content generation.

It runs a small API server on your computer (localhost) and pairs it with a browser extension that works on Gmail, Google Docs, search bars, and most editable fields on the web.

Feature	What it does
Grammar & spelling	Pink underlines on mistakes; click to accept a fix
Auto-fix	Optionally correct all issues as you type
Rewrite	Select text → change tone (friendly, formal, simpler, etc.)
Generate	Turn short notes into emails, messages, or essays
Humanize	Make AI-generated text sound more natural (local Ollama pipeline)

Everything can run fully offline with local Ollama models. Cloud AI (Groq / OpenAI) is optional for faster rewrite and generate.

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Why Thoth? (open-source Grammarly alternative)

	Thoth	Typical cloud writing tools
Where text is processed	Your machine (127.0.0.1)	Vendor cloud servers
Offline grammar check	Yes (LanguageTool + Java)	Usually requires internet
Local LLM rewrite	Ollama (thoth-writing)	Subscription cloud models
Chrome extension	Gmail, Docs, LinkedIn, Slack, etc.	Similar coverage
Account required	No	Often yes
Open source	Yes — inspect and fork	Closed source

Good fit if you search for: grammar checker chrome extension, spell checker browser extension, local AI writing assistant, Ollama grammar tool, privacy-first Grammarly alternative, text humanizer offline, or open-source paraphrasing tool.

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How it works

Thoth is two parts that talk over localhost:

┌─────────────────────────────────────────────────────────────────┐
│  Chrome (any website)                                             │
│  ┌───────────────────────────────────────────────────────────┐  │
│  │  extension/content.js                                      │  │
│  │  • Mirrors text fields, draws grammar underlines           │  │
│  │  • Rewrite / Generate UI on text selection               │  │
│  └───────────────────────────┬───────────────────────────────┘  │
│                              │ chrome.runtime.sendMessage        │
│  ┌───────────────────────────▼───────────────────────────────┐  │
│  │  extension/background.js  →  fetch(127.0.0.1:8000)        │  │
│  └───────────────────────────────────────────────────────────┘  │
└──────────────────────────────┬──────────────────────────────────┘
                               │ HTTP (localhost only)
┌──────────────────────────────▼──────────────────────────────────┐
│  server.py  (FastAPI + uvicorn, port 8000)                        │
│  ┌─────────────┐  ┌──────────────┐  ┌─────────────────────────┐ │
│  │ LanguageTool│  │ Ollama       │  │ writing_agent.py        │ │
│  │ (Java)      │  │ :11434       │  │ prompts + hard filters  │ │
│  │ grammar     │  │ local LLMs   │  │ rewrite / generate      │ │
│  └─────────────┘  └──────────────┘  └─────────────────────────┘ │
│  Optional: cloud_ai.py → Groq / OpenAI (keys from extension only) │
└───────────────────────────────────────────────────────────────────┘

Grammar flow

1. You type in a text field (Gmail compose, Docs, etc.).
2. content.js debounces input and sends text to POST /grammar.
3. The server runs LanguageTool (and optionally a local grammar model).
4. Matches return as offsets; the extension draws underlines on mistakes.
5. Click an underline → suggestion card → accept replaces the text in-place.

Rewrite flow

1. Select text → floating ↗ button → pick Rewrite.
2. Enter a tone (e.g. “more formal”) → POST /rewrite.
3. writing_agent.py builds a prompt, calls Ollama or cloud AI, applies hard filters (no extra greetings, same length, etc.).
4. Selection is replaced with the rewritten text.

Generate flow

1. Select seed text → Generate icon
2. Chooses format (email / essay), optional one-time note
3. Settings from popup: length, tone, complexity, profile (auto-applied every time)
4. POST /generate expands the note using saved defaults from the extension popup

Full Generate rules: docs/GENERATE_RULES.md

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Project structure

Thoth/
├── extension/                 # Chrome extension (Manifest V3)
├── macos/                     # Thoth.app launcher + menu-bar helpers
├── server.py                  # FastAPI app — all HTTP endpoints
├── writing_agent.py           # Rewrite/Generate prompts & filters
├── thoth.py                   # Standalone humanization library
├── ui.json                    # Active Uber-dark UI tokens
├── Start Thoth.command/.bat   # Double-click launchers
├── scripts/
│   ├── build_macos_app.sh     # Build dist/Thoth.app
│   ├── setup_models.sh/.bat   # Register thoth-grammar / thoth-writing
│   └── create_release.sh      # Publish Mac app to GitHub Releases
└── docs/
    ├── INSTALL_MAC.md
    ├── INSTALL_WINDOWS.md
    └── PROJECT.md

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API reference (local server)

Base URL: http://127.0.0.1:8000

Method	Path	Purpose
GET	/health	Server + dependency check
POST	/grammar	Full grammar check
POST	/grammar/quick	Fast path for typing
POST	/humanize	Legacy humanize endpoint
POST	/rewrite	Tone/style rewrite
POST	/generate	Expand notes to email/essay/etc.

Optional header when auth is enabled:

Authorization: Bearer <THOTH_API_TOKEN>

Rewrite/Generate bodies may include optional cloud AI:

{
  "text": "...",
  "tone": "friendly",
  "ai": { "provider": "groq", "apiKey": "..." }
}

Keys are sent from the extension to your local server only; they are not stored server-side.

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Configuration

Extension settings (popup)

• Check writing while I type — grammar on/off
• Fix all mistakes automatically — auto-apply corrections
• Rewrite in search bars — enable rewrite on search inputs
• AI & API keys — Local Ollama, Groq, or OpenAI
• Generate — profile fields, length, tone, complexity, permanent note

Server environment variables

Variable	Effect
THOTH_REQUIRE_AUTH=1	Require Bearer token on all requests
THOTH_API_TOKEN	Token value (auto-generated if unset)
THOTH_DEBUG=1	Verbose server errors

Example:

THOTH_REQUIRE_AUTH=1 ./start_server.sh

Paste the printed token into the extension → Settings → AI & API keys → Local server token.

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Development

Run in dev mode

./scripts/install.sh          # once
./start_server.sh             # terminal 1
# Chrome extension: https://chromewebstore.google.com/detail/begfbbjincimcjcimpfpkoilbhjphppn?utm_source=item-share-cb
# Developers: load unpacked from extension/
# Optional: ./scripts/dev_chrome.sh  (macOS test profile)

Publish a GitHub Release (Mac app only)

./scripts/create_release.sh

Run benchmarks

With the server running:

# benchmark_tests.json defines rewrite/generate cases
python scripts/benchmark_rewrite.py   # if present

Fine-tuning (advanced)

• scripts/finetune_grammar_lora.py — MLX LoRA for grammar model
• prepare_data.py — training pair preparation
• requirements-finetune.txt — extra training deps

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Models

Thoth expects these Ollama models (created by scripts/setup_models.sh):

Model	Used for
thoth-grammar	Optional grammar assistance
thoth-writing	Rewrite & Generate (local)

If local Modelfiles exist under models/, setup uses them. Otherwise setup pulls small Qwen2.5 bases from Ollama Hub.

Model weights are not committed to git (too large). Each user downloads via Ollama.

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Design system

Active UI tokens are in ui.json (Uber-inspired dark theme used by the extension and Mac settings).

• Near-black surfaces (#000 / #141414) with white primary actions
• Green / gold accents for status and highlights
• Legacy Claude-style palette kept in Thoth ui theme.json for reference

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Privacy

See docs/PRIVACY.md and docs/POLICY.md. In Thoth.app: Settings → Privacy & Policy.

• Default: text is processed on your machine via localhost.
• Grammar uses LanguageTool locally (Java).
• Rewrite/Generate use local Ollama unless you opt into Groq/OpenAI in settings.
• API keys and profile fields live in Chrome local storage on your device and are only sent to 127.0.0.1:8000.
• No Thoth account, telemetry, or central server.
• The popup asks for consent before the extension reads text in page fields.

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Troubleshooting

Problem	Solution
Server offline in popup	Run start_server.bat (Windows) or ./start_server.sh (Mac); check /health
No underlines	Enable “Check writing while I type”; ensure Java is installed
Rewrite/Generate error	Start Ollama; run scripts\setup_models.bat or ./scripts/setup_models.sh
thoth-grammar missing	ollama list then setup_models script for your OS
Extension not updating	chrome://extensions → Reload
Port 8000 in use	Starter scripts free the port; or close the other process
Windows: python not found	Reinstall Python with Add to PATH, open a new terminal

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Contributing

Contributions are welcome.

1. Fork the repo
2. Create a branch (git checkout -b feature/my-change)
3. Commit with a clear message
4. Open a Pull Request on GitHub

Ideas for contributors:

• More language support
• Additional Generate formats
• Benchmark coverage

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License

Open source — use, study, and modify on your own machine. See the repository for license details.

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Links

• Repository: https://github.com/Eshan-khan1/Thoth
• Chrome Web Store (extension): https://chromewebstore.google.com/detail/begfbbjincimcjcimpfpkoilbhjphppn?utm_source=item-share-cb
• Download Mac app: https://github.com/Eshan-khan1/Thoth/releases/latest
• Releases: https://github.com/Eshan-khan1/Thoth/releases
• Issues: https://github.com/Eshan-khan1/Thoth/issues

Related topics on GitHub

grammar-checker · chrome-extension · writing-assistant · grammarly-alternative · ollama · local-ai · humanizer · spell-checker · language-tool · fastapi · open-source · privacy · ai-writing · text-rewriter · offline-ai