## TL;DR
Selenium can only set a cookie for the domain the browser is currently on; calling `add_cookie` before navigating (or while on a different domain) throws the domain mismatch error. The fix is to `get()` any page on the target domain first, then add the cookie, then navigate to the page that needs it. Strip leading dots from the domain field and make sure the cookie domain matches the current host exactly, subdomains included.

## The query
```text
selenium cookie domain mismatch: cannot set cookie
```

## Use this when
- `add_cookie` throws an invalid cookie domain error
- Seeding auth/session cookies before a test
- Tests pass locally but fail in an environment with a different host

## Not for
- Reading or asserting cookies after login
- HttpOnly cookie limitations (JS cannot touch those either)
- Choosing cookie vs token auth architectures

## Steps
1. Navigate the driver to any URL on the target domain (even the bare domain root) before touching cookies. Expected output: the browser's current domain matches the cookie's domain.
2. Call `add_cookie` with the domain matching the current host (no leading-dot tricks, correct subdomain). Expected output: no error; the cookie is stored.
3. Navigate to the page under test (or refresh) so the cookie is sent with requests. Expected output: the app sees the session/auth state the cookie provides.
4. Verify with `get_cookies()` that the cookie exists with the expected domain. Expected output: the cookie list contains your cookie on the right domain.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_B1LKYkK3ToGYHyDD3z64JA