In my experience (Australia) you cannot negotiate directly with the potential employer you have no choice but to go through the recruiter.
I have heard of people who got a "signing bonus" at the end of their probation period which was kept a secret from the recruiter.
Personally, I usually asked for a condition in the contract for a review at the end of 6 months. In that, I would negotiate a 10%+ increase. Which is usually easy when they see that you are competent and do good work. In my experience, that approach works in small firms and hardly ever with large firms.
I always got the impression that they want to close the deal as soon as possible. They won’t make much more money if you squabble over 5-15k, so they will usually advocate that you just accept the initial offering by telling you ‘this is a great deal’. It’s much better to let them know ahead of time what your salary range is because they don’t want to deal with the negotiation, I think. They just want to close.
Just make it clear so they don’t waste anyone’s time trying to land you in a gig that doesn’t fit your expectations - all that work for nothing is no good.