Marc Ramsey wrote:

But doesn't TCAS gives you resolution without the need to spot the
threat?
I was under the impression that it does.

Only if the threat is also equipped with TCAS...

Marc

A TCAS equipped aircraft with be given a resolution advisory for a
threat aircraft that has a mode C or Mode S transponder. The threat
aircraft does not need to have TCAS, and does not need to be
interrogated by an ATC radar site.

When both involved aircraft have TCAS the avoidance maneuver is agreed
between them. If only one has TCAS it decides what to do on it's own.

Andy