We study mechanism design under endogenously incomplete commitment as it arises in contracting with escape clauses. An escape clause permits the agent to end a contractual relationship under specified circumstances, after which the principal can offer an ex-post contract. Escape clauses are valuable when the maximal number of initial contracts is smaller than the number of agent types. We identify a sufficient condition for incentive optimality of ex-post contracting. Escape clauses are always incentive optimal under severely constrained contracting. On the margin, the optimal escape clause balances the benefit of a better-adapted contract against an increase in dynamic inefficiency.