Poka-Yoke: Make the Mistake Unsayable

People will always make mistakes. That is not the problem worth solving. The problem is letting a mistake become a defect.

Shigeo Shingo, a Japanese industrial engineer, worked this out on a switch assembly line in 1961. Workers kept forgetting a spring. The fix was not a reminder: the job was split so the worker first laid both springs in a dish, then fitted them from the dish. A spring left over was the error announcing itself, before the unit could move on.

The dish is a device. "Please remember the spring" is not.

The line that does most of the work

A comment, a docstring, a wiki page, a review checklist, or a line in an instructions file saying "don't do X" is not a poka-yoke. It is training, and training degrades. A device does not. If your fix relies on someone remembering something, keep going.

This applies to your own instructions too. A rule written into a config file competes for attention with every other rule there and loses a little more as the file grows. A check that fails the build does not.

What this changes about the output

Given a design, models will readily list what to fix. They rarely state what the fix makes impossible, and that is the difference between advice you agree with and a constraint you can rely on. That habit is most of what this skill is for.

The other half is refusing to accept a non-device as a fix. "Add validation", "be careful with this function", "document the invariant" are all rung zero. Each has a real device behind it, and naming that device is the work.

Axis 1: what happens when the mistake occurs

Rank every finding on this ladder, and say which rung the current code sits on and which rung your fix reaches.

Rung	Name	Meaning
1	Control	The wrong action cannot be performed. Type error, database constraint, missing permission.
2	Warning	It is possible, but announces itself as it happens. A linter, a runtime assertion, a confirmation you cannot skip.
3	Detection	It happens, and you find out afterwards. Tests, logging, monitoring, code review.
0	rung zero	Telling people to be careful. Docs, comments, "please remember to".

Detection is not failure; sometimes it is all that is available. But a plan that stops at Detection should say so, rather than presenting it as prevention.

Axis 2: how the device notices

Shingo's three inspection lenses. They are a checklist for finding hazards, not decoration:

• Contact — can the wrong thing physically fit? Two adjacent parameters of the same type can be swapped silently. A string that should be one of four values. Money as a float.
• Fixed-value — is the set complete? A switch with no exhaustiveness check. A config where a missing key silently means "off". An enum handled in three of five places.
• Motion-step — is the order right, and did every step happen? A two-phase write with no transaction. A retry with no idempotency key. A resource acquired on one path and released on another.

Inspect at the source

The cheapest place to catch a mistake is where it is made, not where it surfaces. A validation that runs three layers below the input has already let the bad value travel, and the stack trace will point at the wrong module. Push the check to the boundary the value crosses.

Designing something new

Mistake-proofing is cheapest before the code has callers. Once it has them, every device is a migration; before it has them, a device is free.

Work from the call site. A signature that reads fine in isolation often reads terribly where it is used:

# the mistake is expressible: nothing stops refunding an order that was never paid
def refund(order: dict) -> Refund:
    return payments.refund(order["payment_id"])

# the mistake is no longer expressible
def refund(order: PaidOrder) -> Refund: ...

The moves, roughly in order of how often they apply:

Make invalid states unrepresentable. A bag of optional fields where only certain combinations are legal becomes a discriminated union where the illegal ones cannot be constructed.

Parse, don't validate. Convert unstructured input into a type that carries proof at the boundary, once, rather than re-checking the same string in nine places.

Distinguish concepts that share a primitive. transfer(from: str, to: str) accepts its arguments transposed. Distinct types for the two concepts, or keyword-only parameters, make the transposition a compile error.

Encode the order. When calls must happen in sequence, let each step return the type the next step requires, so the wrong order does not typecheck.

Make the destructive path narrower than the safe one. A required, non-defaulting argument for the scope of a delete. A default that means "nothing" rather than "everything".

Close by naming what the design now makes impossible, and, just as importantly, what you deliberately left possible and why. A design whose limits are unstated will be trusted past them.

Auditing code that already exists

You are not looking for bugs. A bug is a mistake that already happened. You are looking for mistakes that are available: places where doing the wrong thing is easy, silent, and looks correct.

Run the bundled scanner first for the textually detectable shapes, then read for the ones no scanner can see:

python3 scripts/detect_hazards.py --paths .          # whole tree
python3 scripts/detect_hazards.py --staged           # pre-commit
python3 scripts/detect_hazards.py --diff --json      # CI, exits non-zero on findings
python3 scripts/detect_hazards.py --severity high

No dependencies, so it runs in CI and in a pre-commit hook without an install step. It reports what it scanned: a scan of zero files exits non-zero rather than reporting a clean bill of health, because an all-clear you got by typo is worse than no check.

Rank findings by blast radius times ease of the mistake. An unchecked value reaching a write, a delete, a payment or an auth decision outranks one that can only produce a clean crash. For each finding, state: where it is, what the mistake is, what the consequence is, what device exists today, what device would close it, and which rung that reaches.

references/hazard-catalog.md is the taxonomy of shapes with their IDs and devices. Language-specific patterns are in references/lang-python.md, references/lang-typescript.md and references/lang-rust-go.md.

After an incident

Separate three things that get conflated, because the fix belongs to the third:

• Defect — what the user experienced.
• Mistake — the specific wrong action someone took.
• Hazard — the property of the system that made that mistake available.

Fixing the mistake fixes one case. Fixing the hazard fixes the class. Then sweep: the same shape almost certainly exists elsewhere, and finding the second and third instance is the difference between a patch and a lesson.

Attribute cause to the system rather than to a person. Not primarily for kindness: "they made a mistake" is a complete-sounding explanation that predicts nothing and prevents nothing, and it ends the investigation early.

What good output looks like

• Anchored to lines. orders.py:142, apply_discount is reviewable; "the discount logic" is not.
• Ranked, with the ranking visible, so a reader who stops halfway has still covered the ones that matter.
• A named device per finding, not "add validation".
• The rung stated, before and after.
• The limits stated. What the fix does not cover is the part readers most need and most often do not get.
• Sized honestly. Three findings that matter beat eleven padded to a round number.

What to avoid

Accepting rung zero as a fix. If the proposal is a comment, a doc, or a convention, the work is not finished.

Devices nobody can bypass being confused with devices nobody does bypass. A pre-commit hook is skippable with --no-verify; it needs CI behind it to be a real gate. Say which one you are proposing.

Over-fitting to one incident. Machinery that prevents one specific failure must itself be understood and maintained. Ask whether the shape is common enough to justify it.

Treating monitoring as prevention. Detection lowers the cost of a failure; it does not lower the likelihood. Both are worth having, and conflating them means the likelihood never gets addressed.

Evidence, and its limits

This method was benchmarked at 591 blind-graded runs across six model families, scored against assertions written before the runs by a grader that never saw which configuration produced a response. The behaviour it most reliably changes is stating what a design forecloses: 45% of responses did that unprompted, 80% with the method applied, across 132 graded verdicts.

That average conceals where the effect lives. Asked squarely to design an interface, models already do it 77% of the time; the skills add eleven points. The large gains are in tasks where nobody asked for a design review — writing an endpoint goes 14% to 79%, shipping an agent feature 33% to 83%, building a form 29% to 64%.

Stated honestly, because the limits matter: every run was the first turn of a fresh session, so this measures the ceiling rather than what survives a long working session. The comparison was against no methodology at all, not against a different one, so it does not establish that this method is what produced the gain. And the method costs something measurable: responses became somewhat worse at spotting the specific defect already on the page while becoming better at changing the shape that allowed it. If you want the bug in front of you found, use a reviewer. If you want that class of bug to stop being expressible, use this.

Raw runs, the harness and the assertion checklists are at https://github.com/rainmanjam/poka-yoke.