Pacta sunt servanda
agreements must be kept
Where the phrase comes from
Article 26 of the Vienna Convention on the Law of Treaties (1969) uses this sentence for treaties in force: they bind the parties and must be performed in good faith.
What lawyers are pointing at
The maxim is the starting point of contract and treaty law, and it is not a magic wand. A clause can be void for illegality, cut down by a mandatory statute, or limited by the words the parties actually used. Good faith in performance is part of the Vienna Convention formula. Printing the Latin on a website does not create a contract and does not make an unfair term enforceable.
The same shape in code
A published schema, an OpenAPI file, and a status page are pacta with the callers who built against them. Renaming a JSON field in place breaks the agreement. The honest performance is a new version, a deprecation window, and a changelog that says what changed. Silent drift is a breach of the pact the client relied on.
A concrete picture
Two companies sign a supply schedule. The seller later prefers a higher spot price. Pacta sunt servanda is why the schedule still governs, unless the contract itself allows the change or a statute says that kind of term cannot stand.