Skill: Mechanism

"The model decides. Code enforces. Never the reverse."

The Standard

• Model decides, code enforces: if a model's output is the enforcement, there is no enforcement.
• Never let a model report its own oracle result: run the command in code, read the exit code in code.
• One answer means code: a single correct answer is computable. A space of acceptable answers is judgment.
• Replay means code: billing, audit, security, migrations. A model is not reproducible across versions, and versions change underneath you.

The Check

Ask yourself:

• One correct answer, or a space of acceptable ones?
• Must this replay identically?
• Is the input space enumerable? A model choosing between four known branches is an expensive, non-deterministic switch statement.
• If this is wrong, will anyone notice?

Silent wrongness with a wide blast radius is code, or a model behind a gate.

The Surface

Frequency × ambiguity. Not a binary.

	Low ambiguity	High ambiguity
High frequency	Code. The economics compound.	Model — and promote aggressively.
Low frequency	Either. Don't over-engineer.	Model. Encoding never pays back.

The Migration

The line moves. Both ways.

• Promote judgment → code when it recurs identically. It gets cheaper forever.
• Demote code → judgment when rules accrete special cases faster than they generalize. A rule set growing one clause per incident is a classifier fighting to be born.

The Sandwich

The practical shape:

code    deterministic preparation, constrained input
  model   the judgment, and only the judgment
code    deterministic validation of the output

Widen the middle only where The Check says judgment is genuinely required.

Before Delegation

Mechanism decides the executor kind. ground-delegation decides the executor context.

Ask this one first. If the answer is code, there is nobody to delegate to.

The Anti-Patterns

Don't	Do	Why
Model doing arithmetic or format	Code	Slower, costlier, sometimes wrong
Model routing over known branches	A switch	Non-deterministic control flow
Model reporting its own test result	Harness reads the exit code	Voids the gate entirely
Regex over natural language	Model	Open sets do not enumerate
Rule engine encoding taste	Model	Expert systems died of this

Real-World Examples

See examples.md.