What must always happen is enforced in code

The principle

The prompt asks, the code guarantees. A rule that must hold every time (an order of execution, a mandatory step, a condition before writing) is checked in code, not only in the instructions.

Why it matters

A model follows its instructions most of the time. A rule reduces a behavior without eliminating it, and a long prompt is followed less well than a short one. For an action that writes, pays or commits, “most of the time” isn’t enough.

The mechanism

1. List the invariants: what must always happen, what must never happen.
2. A code mechanism for each: sequential execution, a forced final step, a field check, a conditional write.
3. Pin the request before the action exists, and compare the action to that pin before acting.
4. Split the roles: judgment to the model, execution and verification to the code.
5. Test that the guarantee holds even when the model gets it wrong.

In the field

• AWS Scholars, chapter 1: a permissive rule let through a ticket filled with nothing. In production, field validation would go in the tool, not in the prompt.
• AWS Scholars, chapter 3: “one tool at a time” was in the prompt, and the framework ran the tools in parallel. Fixes in code: a sequential executor, a forced final step, the customer check inside the tools.
• Dev3Pack, Final: the model chooses the paragraphs, the code copies them, and citationsCitationThe ID of the document that supports an answer. A correct citation doesn't prove the sentence comes from the document.See in the glossary →Close can no longer be invented.
• Dev3Pack, Gecko: the request is pinned before the transaction, and nothing is signed until every field matches.

The question to ask

If the model ignored this instruction once in a hundred times, what would happen?