Party A and Party B can negotiate amicably, supplement the missing items and re-sign the contract. If the two sides can't reach an understanding, there is no need to stamp, and the contract can't take effect.

2. If it is a valid contract:

1. The party that needs to add the project can negotiate with the other party to add the project. If the other party agrees, after reaching an agreement, one party can go to the relevant department to "modify the contract" and the other party can sign a "supplementary agreement".

2. If the other party does not agree to add, it depends on whether the ellipsis is important. If it is conventional, established and historical, it can be ignored, because once there is a problem, it will be taken to court, and the court will identify the incomplete contract according to relevant laws and regulations. If there is no law, then follow the actual situation, that is to say, you should pay special attention to the omissions in the actual work and keep the evidence that is beneficial to you everywhere, just in case.