The rendering engine, CLI, MCP server, VS Code extension, and GitHub Action are free and open source. Run compatibility checks locally, in CI, or inside Claude and Cursor, then optionally plug into the hosted Emailens app when you need screenshots and share links.
same engine as the SaaS, no lock-in
Start here
The hosted app at emailens.dev is separate (not open source today). These paths are MIT-licensed repos on GitHub.
Lint HTML in pre-commit hooks or GitHub Actions.
CLI docsAnalyze and fix emails from Claude, Cursor, or Windsurf.
MCP setupLive preview and scores while you edit MJML or React Email.
VS Code docsUse @emailens/engine as a library in Node.js services.
Engine APICopy & paste
# analyze a template locally (no account)
npx @emailens/cli analyze ./emails/welcome.html --format json# add to Claude Code in one line
claude mcp add emailens -- npx -y @emailens/mcp- uses: emailens/action@v1
with:
pattern: "emails/**/*.html"
threshold: "70"When OSS is not enough
Open source covers analysis and CI. Sign up when collaborators need links, screenshots, or API keys, still powered by the same @emailens/engine.
Generate shareable preview URLs so PMs and designers review scores and warnings in the browser without installing anything.
Try shareable previewsSimulated transforms catch most CSS bugs; hosted capture renders light and dark mode PNGs across 21 clients when you need pixel proof.
Try screenshot previewsFree tier includes 30 hosted previews per day. Upgrade when you need more keys, CI screenshot gates, or dashboard history.
Create free accountHonest comparison
Only MIT repos on github.com/emailens. Cloud API rows need an Emailens account; the web app itself is not open source.
| Capability | Engine | CLI | MCP | VS Code | Action |
|---|---|---|---|---|---|
| CSS/HTML analysis | Local | Local | Local | Local | Local |
| CI lint exit codes | Local | Local | — | — | Local |
| Dark mode simulation | Local | Local | Local | Local | Local |
| Deliverability checks | Local | Local | Local | — | — |
| Browser screenshots (cloud) | — | — | Cloud API | — | — |
| Share preview links (cloud) | — | — | Cloud API | — | — |
MCP can call hosted screenshots/shares with an API key. The proprietary app adds dashboard, team usage, and Figma/Builder integrations. API access
Repositories
Each repo is maintained on github.com/emailens . Published npm packages ship from @emailens/engine, @emailens/cli, and @emailens/mcp.
Core compatibility engine
Parses HTML email, evaluates CSS against 21 client support matrices, transforms markup per client, scores compatibility, simulates dark mode, and emits framework-native fix hints. The same engine powers the CLI, MCP server, VS Code extension, and hosted preview API.
Terminal lint and CI reports
Preview and analyze emails from the terminal. Pipe HTML in, get structured scores and warnings out. The lint command exits with codes your CI pipeline can gate on.
Email QA inside AI assistants
Model Context Protocol server with nine tools: analyze, preview, audit, fix, diff, deliverability, client list, and optional hosted screenshots and share links when you add an API key.
Live preview in your editor
Side-by-side email preview and compatibility scores while you edit HTML, MJML, or React Email. Uses the open-source engine locally so your source never leaves the machine for analysis.
GitHub Action CI gate
Fail pull requests when email HTML drops below your compatibility threshold. Runs @emailens/cli in GitHub Actions with cached installs and structured annotations.
Architecture
Free tier includes 30 previews per day. See pricing or try the web app .
FAQ
The email compatibility engine and developer tooling (@emailens/engine, @emailens/cli, @emailens/mcp, the VS Code extension, and the GitHub Action) are open source under the MIT license on github.com/emailens. The hosted web app, screenshot rendering farm, and team features are the commercial SaaS product.
No. Local preview, analysis, linting, and MCP tools that run on @emailens/engine work without signup. Optional hosted features (browser screenshots, shareable preview links, REST API quotas) use the Emailens cloud and may require a free or paid API key.
The SaaS at emailens.dev runs the same @emailens/engine logic server-side, adds authenticated storage, share links, screenshot capture across real browser profiles, billing, and team workflows. You can use only the open-source stack, only the cloud, or both.
Install @emailens/cli for terminal workflows, add @emailens/mcp if you use Claude or Cursor, or read the @emailens/engine docs if you want to embed scoring in your own product. The documentation at docs.emailens.dev covers each path.
Stay on open source for local linting, CI gates, and editor workflows. Create a free Emailens account when you need shareable preview links for teammates, browser screenshots across clients, or API keys for MCP screenshot and share tools. The free plan includes 30 hosted previews per day.
Yes. Stars on github.com/emailens help other developers discover the engine and tools, signal maintenance to employers evaluating the stack, and notify you of releases. The product roadmap for open-source packages is public in each repo.
Most developers start with the CLI or MCP locally. Create a free account when you need share links, screenshots, or team API keys.
Want to contribute rules or fixes? Contributing guide