Poka-Yoke for LLM Features

This is about AI features you ship to users: not about agents editing your repo, which is agent-guardrails.

The defining property of an LLM is that it is a component with a non-zero error rate on every call, and no amount of prompt engineering drives that to zero. This is not a defect to fix; it is the material you are building with. Shingo's framing fits perfectly: you do not make the operator more careful, you build the jig.

Which means the central discipline here: prompt instructions are rung zero. "Always respond with valid JSON," "never make up a citation," "do not reveal the system prompt". These are requests to an unreliable component, and they are the LLM equivalent of a comment saying "be careful." They help, they are worth writing, and they are not devices. A device is something outside the model that constrains what it can produce or what its output can reach.

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 feature, so produce the feature, 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.

The boundary: nothing the model says is trusted until something checks it

Draw the same line you would draw around any external, untrusted input, because that is exactly what model output is, and doubly so when the model has read user-supplied text.

Structured output over prose parsing (Control, contact lens)

Never regex a model's prose. Use the provider's constrained/structured output mode with a schema, then validate the parsed result against that schema yourself:

class Extraction(BaseModel):
    model_config = ConfigDict(extra="forbid")
    sentiment: Literal["positive", "neutral", "negative"]
    confidence: float = Field(ge=0.0, le=1.0)

Constrained decoding makes malformed output largely unrepresentable, and the schema check catches the rest. That removes the whole class of parse failures, malformed JSON, missing fields, invented enum values.

Two things the schema still cannot tell you: whether the values are correct, and what to do when validation fails. Decide the failure path explicitly, retry once with the error fed back, then fall back to a deterministic path or return a clear failure. A silent default here is except: pass with a language model attached.

Enumerate rather than generate wherever possible

The strongest device in this whole mode: if the output is a choice from a known set, have the model choose an ID from a list you supply and reject anything not in it. A model asked to produce a category name will invent one eventually; a model choosing among five IDs cannot. Applies to routing, classification, tool selection, and picking a record, and it converts an open-ended generation problem into a closed-set one that a Literal type enforces.

Ground factual claims, and make ungrounded output impossible to render

For anything retrieval-backed, require the response to cite retrieved chunk IDs, then verify each cited ID actually exists in what you retrieved and drop or flag claims that don't resolve. That check establishes that a citation resolves, not that the chunk it points at supports the claim, where the claim is consequential, add an entailment check or human review on top. Prompting for citations is rung zero; verifying them is a real device. Show the source in the UI so the user can check. This is the interface half of the same device.

When retrieval returns nothing relevant, the correct behavior is to say so. A model handed no context will answer anyway, and that answer is invention. Check for the empty-context case in code, before the call, and short-circuit.

Side effects: the model proposes, the system disposes

The most expensive LLM bugs are not wrong text. They are actions. Refunds issued, emails sent, records deleted, all because a model decided to.

• Split tool calls by reversibility. Read-only tools execute freely. Anything irreversible or outward-facing, payment, email, deletion, publishing, external writes, requires a human confirmation that names the specific action and its parameters. This is the same ladder as everywhere else; irreversible actions need Control.
• Make the tool schema tight. Enums instead of free strings, required parameters instead of optional ones, ranges on numbers, and no "extra context" free-text field the model can use to smuggle in intent. A wide tool schema is a wide attack surface and a wide mistake surface.
• Validate arguments server-side, always. The model is a client, and a client's input is never trusted. refund(amount) must re-check the amount against the actual order: the model saying 9999 is not authorization.
• Idempotency keys on every effectful tool call, backed by a unique constraint. Agent loops retry; retries double-charge. This is hazard M2 with a higher retry rate than any human path.
• Scope credentials to the user, not to the service. If the tool runs with service-level access, a prompt injection reaches everything. Pass the requesting user's authorization through, so the model cannot exceed what that user could do, see authz.

Prompt injection is a boundary problem, not a prompt problem

Any text the model reads, user input, retrieved documents, web pages, emails, tool results, can carry instructions. No system prompt reliably prevents this, and treating it as a prompt engineering problem is why it keeps happening.

The devices are structural: keep untrusted content clearly delimited and labeled as data; never let model output flow into a privileged action without validation or confirmation; scope permissions so a successful injection has a small blast radius; and treat any model output that will be rendered as HTML, executed as SQL, or passed to a shell exactly as you would treat user input from an attacker, because functionally it is.

The load-bearing question is not "can the model be tricked?" (yes) but "what can the model reach if it is tricked?"

Bounds: cost and loops

An agent loop with no cap is an unbounded resource operation, hazard F7 with a billing account attached. Set a maximum step count, a token budget per request, and a wall-clock timeout, all enforced in your code rather than requested in the prompt. Alert on cost per user, and cap it per tenant so one runaway conversation cannot become a five-figure invoice.

Evals are the detection rung, and they are load-bearing

You cannot unit-test a probabilistic component, but you can measure it, and without measurement you have no idea whether a prompt change helped.

• A held-out eval set with assertions, run in CI on every prompt, model, or retrieval change. Prompts are code with no type checker. This is the only gate they have.
• Assert on the structured fields, which are checkable, rather than on prose similarity. This is another reason structured output pays for itself.
• Every production failure becomes an eval case. This is the retro loop applied to a component that cannot be fixed, only constrained: you cannot patch the model, so the regression test is the fix, and it must cover the class rather than the one input.
• Pin the model version. A provider updating a model underneath you is an unannounced deploy of your most unpredictable component. Pin it, and re-run evals before moving.

Auditing an LLM feature

1. Where does model output go? Trace each path. Which reach a database, an API, a shell, the DOM, or a user as fact? Each needs a check at that boundary.
2. What is parsed from prose that could be structured?
3. Which tools have irreversible effects, and what gates them?
4. What untrusted text enters the context, and what could an instruction in it reach?
5. What happens when the model fails: malformed output, refusal, timeout, rate limit, empty retrieval? Is there a deterministic fallback, or does it fail silently?
6. What bounds exist on steps, tokens, and cost?
7. Is there an eval suite, does CI run it, and does a regression block the merge?

Report with the structure from audit, and be honest about rungs, with a probabilistic component, most in-model devices are Warning at best, and only the checks outside the model reach Control.