# Contract Testing

Fixtures cover HTTP responses, authenticated WebSocket events, errors, permissions,
version negotiation, reconnects, and server-owned state transitions. Fixtures are
deterministic, synthetic, and secret-free. Every new contract must include at
least one valid fixture and a negative case where validation rules are important.

The current realtime envelope is deliberately transport-neutral: the server may
deliver it over an authenticated WebSocket or a future compatible transport, but
the event identity and payload rules remain the same. `event_id` is the stable
deduplication key for at-least-once delivery.

Browser clients authenticate the WebSocket with the first text frame described
by `realtime-auth-frame.json`. Session tokens are sent in that frame rather
than in a URL query parameter. The server closes the connection when the frame
is missing, malformed, or invalid, and sends `connection.ready` only after
successful validation.

The restored-session contract has Draft 2020-12 validation for both credential
replacement states, missing fields, wrong types, and unexpected properties:

```sh
uv run --with jsonschema==4.26.0 python tests/session_contract.py
```

This isolated validation dependency is tooling only, not an application dependency.