Live mode

By default a check runs once, at the boundary, and hands you an ordinary object. Nothing stops that object from drifting a moment later:

const user: User = await fetch('/api/me').then(r => r.json())
// checked ✓
user.age = 'twenty'   // ✗ not checked again — nothing catches this

Zod, Valibot and the rest work the same way: they check once and return a plain object.

Live mode keeps the type alive for the object’s whole lifetime. Instead of the one-shot check, the plugin emits a guard that validates the same way and then returns a recursive Proxy:

• writing an off-type value to a known property is caught;
• writing a property the type doesn’t declare is caught;
• deleting a required property is caught;
• nested objects, arrays, records and tuples are wrapped lazily on first read (identity is preserved — user.address === user.address).

const user: User = await fetch('/api/me').then(r => r.json())

user.age = 'twenty'   // 💥 ValidationError at "user.age" — caught live

Turning it on

It’s controlled by the plugin’s live option:

// vite.config.ts
t12n({
  mode: 'auto',
  live: true,   // force the guard on for every build
})

Live mode is orthogonal to auto / manual / off: the mode decides where checks go, live decides whether they keep watching afterwards. Set it to true to watch mutations in production too (pair it with a reporting handler — see Errors & configure), or false to never use Proxies.

The cost

There’s a real cost: every property access on a guarded object goes through a trap. That’s why the default keeps Proxies in dev and ships the cheap compiled check to production. Proxies also don’t survive structured-clone boundaries (postMessage, workers) — the receiving side just re-guards at its own boundary.