I'm in-house counsel at a tech company. We deal with these engagement letters all the time. Here's what we typically negotiate:
Liability cap: We push for 3x fees or $1M, whichever is greater. For audit work specifically, we push higher because audit opinions are relied on by third parties.
Carve-outs: Make sure the cap doesn't apply to gross negligence, willful misconduct, breach of confidentiality, or violation of professional standards.
Arbitration: We often accept it but modify the clause - we want to select one arbitrator jointly rather than each side picking one. We also add that the arbitrator must follow the law (some clauses let them ignore it).
Indemnification: Many engagement letters have the client indemnifying the CPA. Push back hard on this - they should carry their own insurance.
Don't be afraid to redline the engagement letter. Reputable firms expect negotiation from sophisticated clients.