Poka-Yoke Guardrails

Design-time devices protect the code you are writing now. Guardrails protect the code everyone writes later, including the version of you who is in a hurry. They are Shingo's successive check: the next station refuses to accept bad work.

The reason this mode exists as its own thing: the most common failure in software quality is agreeing on a rule and then writing it down. A rule in a wiki has a half-life of about one onboarding. The same rule wired into a gate applies itself and costs nothing to remember.

Building, not reviewing

Most of the time this mode is reached while someone is building the thing, not afterwards. That changes the deliverable. They asked for the config, so produce the config, working, complete, in their stack. Do not hand back a severity table when the person is mid-feature; a list of findings about code they have not written yet is not useful to them.

Then add a short closing note, three or four lines, covering:

• which misuses the shape you chose makes impossible, and at which rung,
• what you left possible on purpose, and why that tradeoff is the right one here.

That closing note is what stops the device being undone in six months by someone who cannot see why it is there. It is also the difference between mistake-proofing and a code generator: the reasoning travels with the code.

When the code already exists and they are asking what is wrong with it, switch to the audit voice, ranked findings with the mistake, the consequence, and the device. Match the mode to where they are in the work, not to this file's default.

Pick the earliest gate that can hold the rule

The same rule can live at several points in the lifecycle. Earlier is better, feedback is faster, cheaper, and lands while the author still has the context in their head. But earlier is also easier to bypass. The resolution is to place the device early and back it with a gate that cannot be skipped.

Gate	Feedback speed	Bypassable?	Best for
Type system / compiler	instant	no	anything the types can express, always first choice
Editor + lint	seconds	yes (ignore comment)	style, banned APIs, unsafe patterns
Pre-commit hook	seconds	yes (--no-verify)	fast checks: secrets, formatting, obvious footguns
Pre-push hook	~a minute	yes	medium checks you don't want to wait for on every commit
CI required check	minutes	no, with branch protection	the real enforcement, everything that must not merge
Database constraint	instant, at write	no	data invariants, across every service and every script
Runtime assertion	at execution	no	invariants no earlier gate can see

Never rely on a pre-commit hook alone for anything that matters. --no-verify exists, and people under deadline use it. Use the hook for speed and the CI check for authority; run the same script in both so they cannot drift.

The devices worth installing

Ready-to-adapt templates live in ../../assets/devices/. Read the relevant one, adapt it to the repo's actual stack, and show the user the file before writing it.

• ../../assets/devices/pre-commit/, .pre-commit-config.yaml covering secrets, large files, merge conflict markers, formatting, and a hook for repo-specific rules
• ../../assets/devices/github-actions/: a required-check workflow, plus a migration-safety gate
• ../../assets/devices/lint/: ESLint and Ruff rule sets chosen specifically for mistake-prevention rather than style
• ../../assets/devices/claude-hooks/: Claude Code hooks (see agent-guardrails)

The rules that pay for themselves in nearly every repo, roughly in order of value:

1. Secret scanning at commit time. A leaked key is irreversible; rotation is the only remedy. This is the highest blast-radius mistake a hook can prevent.
2. Type checking as a required check, tsc --noEmit, mypy --strict, go vet. This is what makes every design-time device in design actually load-bearing. A branded type with no type check in CI is decoration.
3. The specific lint rules that catch silent failure: floating promises, unhandled rejections, unchecked errors, bare except, empty catch blocks, non-exhaustive switches. Ordinary style rules are not poka-yoke; these are.
4. Migration safety: block destructive DDL, or require an explicit acknowledgment for it. Dropping a column in a deploy is a classic irreversible mistake with a trivial device.
5. Test integrity: fail CI on it.only, fdescribe, @pytest.mark.skip left behind. A skipped test is a detection device that has been switched off, usually by accident.
6. Branch protection with required checks. Without it, none of the above is enforcement.

Install carefully: a guardrail people hate gets removed

This is the mode where a well-intentioned change most easily backfires. A gate that fires constantly on pre-existing code teaches everyone to bypass gates, which is strictly worse than not adding it. Three rules:

Baseline first, then ratchet. Turning on a strict rule in a large repo yields hundreds of failures and the rule gets reverted by Friday. Instead: enforce on changed files only, or generate a baseline of existing violations and fail only on new ones. The violation count can only go down. This is how strictness actually lands.

Be fast or be asynchronous. A pre-commit hook over about five seconds gets bypassed. Keep commit-time checks to changed files, push the slow work to CI.

Make the failure message teach. A gate that says error: rule violated produces a confused engineer and a workaround. Say what was done, why it is dangerous, and the exact command or edit that fixes it. This is the one place prose belongs in a poka-yoke: at the moment of failure, when someone is guaranteed to read it.

Also check what already exists before adding anything. Repos frequently have a lint config or CI workflow that already covers the rule but isn't wired into branch protection, or is set to warn instead of error. Flipping an existing warning to an error is a better change than a new tool.

Verify the device actually fires

An untested guardrail is a guardrail you believe in, which is worse than none. It creates confidence without protection. Before you call it done, demonstrate it:

1. Write the mistake it is supposed to catch, deliberately.
2. Run the gate. Confirm it fails, and that the message is the one you wrote.
3. Remove the mistake. Confirm it passes.
4. Show the user both outcomes.

Then leave a poka-yoke: marker comment on the rule naming the mistake it prevents, see the recording section in audit. A device whose purpose nobody remembers is a device that gets deleted during the next cleanup.

Propose first

Show the config files and what they will reject before writing them. Guardrails change how everyone on the team works, and that is not a change to make on someone's behalf without their explicit sign-off, especially the branch-protection and required-check pieces, which you generally cannot apply yourself anyway. For those, hand over the exact settings to click or the gh api command to run.