> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sendykit.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# The Exoskeleton Model

> How SendyKit wraps around Sendy without modifying it

## Why "Exoskeleton"?

Sendy is a solid, proven email marketing tool — 50K+ licenses sold, reliable SES integration, simple interface. But its API is 7 legacy POST endpoints. No webhooks. No analytics API. No CLI. No AI. No health monitoring.

**SendyKit doesn't replace Sendy. It wraps around it.**

Like an exoskeleton augments a human body — same body, dramatically expanded capabilities:

| Sendy (the body)         | SendyKit (the exoskeleton)            |
| ------------------------ | ------------------------------------- |
| 7 POST endpoints         | 125 REST endpoints                    |
| No CLI                   | 240+ commands                         |
| No webhooks              | HMAC-signed webhook delivery          |
| No analytics API         | Full intelligence engine              |
| No health checks         | 68 automated diagnostics              |
| No audit trail           | Automatic mutation logging            |
| SES only                 | Any SMTP provider                     |
| No AI                    | Subject lines, email gen, insights    |
| No visual editor         | GrapeJS drag-and-drop                 |
| PayPal only (bypassable) | Stripe + cryptographic payment tokens |

## How It Works Technically

### Database Layer

SendyKit reads Sendy's existing tables (`campaigns`, `subscribers`, `lists`, `links`, `login`, `apps`) via **read-only queries**. It never writes to Sendy's core tables.

SendyKit creates its own tables with the `sendykit_` prefix:

* `sendykit_migrations` — migration tracking
* `sendykit_feature_flags` — runtime feature toggles
* `sendykit_webhooks` — webhook endpoint configuration
* `sendykit_proxy_settings` — proxy and transport settings
* `sendykit_analytics_events` — enriched event tracking
* `sendykit_doctor_reports` — health check results
* `sendykit_audit_log` — mutation audit trail
* `sendykit_tenants` — multi-tenant SaaS
* `sendykit_payments` — payment verification

### Proxy Layer

The Sendy admin UI is served through SendyKit's reverse proxy. This allows:

* CSS injection (dark mode, mobile responsive, design system)
* Sidebar navigation (SendyKit pages appear in Sendy's nav)
* Payment interception (Stripe replaces PayPal)
* Brand switching (unified dropdown)

**Sendy's PHP code is never modified.** The proxy intercepts HTTP responses and injects CSS/JS before serving to the browser.

### Safe Migrations

Every database migration is:

1. **Versioned** — `v001_foundation.sql` through `v007_billing.sql`
2. **Non-destructive** — Never ALTERs Sendy tables. Only CREATEs `sendykit_*` tables.
3. **Tracked** — `sendykit_migrations` table records which migrations have run
4. **Idempotent** — Safe to run multiple times
5. **Reversible** — Can DROP all `sendykit_*` tables to fully uninstall

### Fail-Safe Design

If SendyKit stops running:

* Sendy continues working exactly as before
* All campaigns continue sending
* All subscriber management continues
* The only things lost are SendyKit-specific features (API, CLI, admin pages)

This is the key advantage of the exoskeleton model: **zero risk to the existing system.**