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

# Connect your app

> How your app reaches the sandbox: change the address, not the request.

Once a sandbox is running, how does your app actually send its calls to *us* instead of the real
vendor? The answer is deliberately boring: **you change the base URL.** Nothing intercepts traffic.

## Change the address, not the request

An API call is just an HTTPS request to a base URL baked into the SDK. `stripe.charges.create(...)`
becomes `POST https://api.stripe.com/v1/charges`. That base URL is the *only* thing tying the call to
the vendor's servers — the path, headers, body, and key are just data riding along.

So we don't intercept in the wiretap sense. We point the SDK's base URL at the sandbox, and every
call arrives at us instead, **byte-for-byte unchanged**:

```ts theme={null}
const stripe = new Stripe(PRODBREAK_KEY, {
  host: PRODBREAK_URL,            // http://localhost:8801 locally, or a hosted URL
});
```

Because your app *willingly* connects to our host, **HTTPS just works with a normal certificate** —
no MITM proxy, no custom CA to install. In a test, the fixture does this for you: `sandbox.client()`
returns the SDK already pointed and keyed.

<Note>
  The instance key **authenticates** you to one world — it does not *select* among worlds (there's
  only one per instance). A wrong key is a `401`. See [Worlds](/concepts/worlds).
</Note>

## Why armed faults are the lever

This is the most important consequence of "change the address." Because the request rides in
**unmodified**, any control that has to travel *inside* the request only works when your **test code**
makes the call directly. The headline use case — point your *unmodified app* at the sandbox and run
its real code path — can only be steered from **out of band**.

| Mechanism                              | Test makes the call | Unmodified app makes the call           |
| -------------------------------------- | ------------------- | --------------------------------------- |
| **Armed fault** (`sandbox.faults.arm`) | ✅                   | ✅ — out-of-band, always reachable       |
| Scenario header (`X-Mock-Scenario`)    | ✅                   | ❌ — your app won't add the header       |
| Magic input value                      | ✅                   | ⚠️ — only if the app happens to send it |

**Takeaway:** when you're testing your app's real code path, reach for
[armed faults](/guides/force-an-error). Headers and magic values are for fixture-style scaffolding
from inside test code.

## When base-URL override isn't enough

Almost every modern SDK exposes a base-URL option or env var, so this is the default and covers the
common case. For the rare app that *hardcodes* the vendor host with no override, ProdBreak has a
spectrum of lower-level options — DNS override + local CA, an HTTP(S) proxy, or a Docker-network
gateway for CI.

<Warning>
  These lower layers are **post-MVP**. If your SDK can't be repointed, get in touch — but for every SDK
  we've checked, the one-line base-URL change is all you need.
</Warning>