I do my best work when I'm trusted to think, argue, and own the outcome. The clients I work well with want a partner who'll engage with the actual problem, push on assumptions, and take responsibility for the answer. A few things that come from that:
I'll tell you not to build it. If the thing isn't worth building, or you're solving the wrong problem, I'll say so.
I push back, and I expect you to push back on me. I have opinions and I back them. The arrangement only works if both sides argue honestly and the better idea wins.
I look for the risks and gaps you haven't named yet. The most useful thing I do early in a conversation is usually pointing at something you suspected but couldn't articulate, or surfacing a risk you hadn't seen yet. Most of what "first principles thinking" actually amounts to in practice is just being willing to look at what's really there.
I take responsibility, and I expect the authority that goes with it. Power and responsibility belong together. If you want me to own the technical outcome, I need room to make decisions. If you want to make the decisions yourself, I can execute against them, but that's a different kind of engagement.
I'm not the right fit for execution-only work. If you want someone to execute against tickets you've already defined, there are excellent people for that. I select for relationships that can become long-term, which means I select for clients who want a partner.