msd-core/gsd-core/references/prohibition-probe.md

Prohibition-Probe — Spec-Completeness Must-NOT Reference

Shared reference for the spec/requirements phase. Companion to @~/.claude/gsd-core/references/edge-probe.md: edge-probe reaches the data/behavior-shape axis (boundaries, adjacency, encoding, ordering) — the things a feature must do. This reference reaches the orthogonal must-NOT axis (product, values, safety, ethics) — the things a feature must never silently become. The edge-probe caught 0/8 of these in controlled testing because it is the wrong instrument: a shape taxonomy cannot surface "the reminder must not shame the user." Walk each requirement through the two-stage recall→precision protocol below and resolve each surfaced prohibition to exactly one state.

This doc is written in generic requirements → checks → verifier terms with no tool-specific vocabulary, so it is portable: copy it into any spec/requirements process. A short mapping table at the end binds it to common host structures.

Why front-of-pipeline

A goal-backward verifier only checks assertions that exist; an assertion only exists for a requirement that was written down. The class of constraint this probe targets — the "must-NOT" the author assumed but never wrote — is invisible to the verifier in exactly the same way an omitted edge is, but with a sharper failure mode: a ✅ done that means "the code matches the words in the spec" can still ship a feature that does what the author explicitly would not want. The manipulative-streak reminder, the loan model that proxies on zip code, the audit log that stores raw SSN — each one passes a literal spec while violating the intent. The fix is not a better verifier; it is spec completeness: surface the omitted prohibition into an explicit, checkable acceptance criterion before any code exists, after which the verifier reliably enforces it.

The technique is adversarial elicitation, not deterministic computation. Unlike the edge taxonomy (a closed eight categories a classifier can apply), the recall stage is inherently model-driven: it asks an open question and reads prose. There is no compiled prohibition-probe.cjs engine — the recall stage is an LLM prose pass, and only the schema/projection layer is real code (ADR-550 Decision 7b). Building a deterministic recall adapter would be the scope-creep the maintainer flags.

Inputs

A list of requirements, each a { id, text } record where text is a testable statement. There is no shape override and no taxonomy classifier — the recall stage reads the prose directly and the precision stage filters its raw output. The probe runs after the edge-probe in the spec phase, over the same requirement list.

Two-stage protocol (recall → precision)

The probe is a two-pass pipeline per requirement. Stage 1 maximizes recall (cast wide); Stage 2 restores precision (drop the noise). Running them in this order — wide then narrow — is what keeps the surfaced list both complete and short.

Stage 1 — Recall (adversarial probe). Ask the single adversarial question of each requirement:

What could this feature silently become that the author would NOT want, but the spec does not forbid?

This question is model-robust (17/17 holistic surfacing including smaller models in the N18 experiment). It deliberately over-produces: ~10 raw candidates per requirement, including routine engineering items. That over-production is intentional — recall first.

Stage 2 — Precision (one-pass classifier). Filter the raw Stage-1 list in a single pass. The rule is a drop/keep split:

• DROP routine-engineering items — anything that is a normal correctness or hygiene concern rather than an intent constraint: "must not mutate its input", "must not throw on empty list", "must return a primitive not an object", "must not leak a file handle". These belong to the edge-probe or to ordinary code review, not here.
• KEEP values / safety / ethics items — anything that, if violated, makes the feature do something the author would object to on product, fairness, privacy, transparency, or safety grounds: "must not use shaming framing", "must not proxy on protected attributes", "must not store raw PII in plaintext".

This collapses the raw ~10 to ~2–3 genuine prohibitions (GT 5/5, 0 false positives on the N18 eight-spec battery). A requirement that yields zero kept prohibitions emits an empty list — that is the correct precision outcome for a pure utility, not a failure.

Canon-referral (do not mint canon items)

Some kept candidates are not bespoke at all — they are canon security/compliance constraints that a dedicated tool already owns. Do NOT mint a prohibition for them. Instead emit a one-line breadcrumb and stop:

• OWASP / prototype-pollution / path-traversal / injection → breadcrumb to /gsd:secure-phase and eslint (security plugins), not a minted prohibition.
• GDPR / data-retention / consent → breadcrumb to /gsd:secure-phase.
• Generic fairness/bias canon → breadcrumb to /gsd:secure-phase.

The breadcrumb reads like: "prototype-pollution is canon — covered by /gsd:secure-phase + eslint; not minted here." This keeps the surfaced list to the ~2–3 bespoke items that no other tool would catch — the manipulative-framing prohibition, the product-specific fairness constraint — which is the whole value of the probe. Minting canon items both duplicates other tooling and drowns the bespoke signal (ADR-550 Decision 6).

Resolution states

Each surfaced prohibition carries two orthogonal axes — a resolution lifecycle and, when resolved, a verification tier (ADR-550 Decision 7, the shared probe-core model; the lifecycle is identical to the edge-probe, the verification tiers differ):

• status — resolved | dismissed | unresolved (IDENTICAL to the edge-probe):
  • resolved — the prohibition is addressed; how it is addressed is the verification tier.
  • dismissed — not a genuine prohibition for this feature, accompanied by a required, non-empty reason string. "N/A — this utility has no user-facing surface, no values constraint applies" is valid; silence is not. The reason string is the audit trail.
  • unresolved — carried forward and flagged; the author chose not to resolve it yet.
• verification (only when status is resolved; null otherwise) — test | judgment (this REPLACES the edge-probe's explicit | backstop):
  • test — the prohibition can be mechanically checked (a negative test, a lint rule, an assertion that the audit log contains no raw SSN). A checkable assertion exists.
  • judgment — the prohibition is real but cannot be reduced to a mechanical test (a human/LLM judgment that the framing is not manipulative). It records intent and routes to a judgment-based review rather than a green/red test.

Splitting these axes keeps the lifecycle enum free of a verification fact and lets the prohibition adapter declare test | judgment without forking the shared lifecycle enum that the edge-probe's explicit | backstop also uses.

Output schema

The probe emits, per kept prohibition, an item of the form:

{ requirement_id, category, status, verification, resolution, reason, statement }

where statement is the must-NOT sentence and category is the values/safety/ethics class (values, fairness, privacy, transparency, safety, …), plus a coverage summary:

coverage: { applicable, resolved, unresolved, byVerification: { test, judgment } }

applicable is the number of kept prohibitions, resolved = closed (resolved + dismissed) status items, unresolved is the remainder, and byVerification breaks the resolved-status items down by tier ({ test, judgment }). This JSON is the stable contract both the reference implementation and any third-party port emit.

Generic mapping (requirements → checks → verifier)

Host structure	"requirement"	a resolved/test prohibition becomes	a resolved/judgment prohibition becomes
GSD SPEC	a SPEC Requirement	a SPEC acceptance criterion (marked prohibition) that plan-phase lifts into must_haves.prohibitions	a must_haves.prohibitions item routed to judgment review
Gherkin feature	a Scenario	a negative Then assertion / tagged negative scenario	a tagged scenario routed to manual review
OpenAPI operation	an operation	a contract test asserting the forbidden behavior never occurs	a documented constraint flagged for review
Docstring contract	a documented behavior	a negative assertion in the contract test	a documented must-NOT for reviewers

The portable invariant: a resolved/test prohibition produces a checkable negative the verifier iterates over (GSD: a must_haves.prohibitions item with a test); a resolved/judgment prohibition produces a recorded intent routed to judgment review. An unresolved prohibition is an explicit assumption the downstream planner must surface, not silently drop.

Worked example (streak-reminder)

A single requirement to send a daily habit reminder. The edge-probe sees a stateful requirement and asks about idempotency; the prohibition-probe asks the adversarial question and surfaces what the reminder must never become. Stage 1 over-produces ("must not spam", "must not throw on a deleted habit", "must not use shaming framing"); Stage 2 drops the routine-engineering items and keeps the one genuine values prohibition:

{
  "items": [
    {
      "requirement_id": "R1",
      "category": "values",
      "status": "resolved",
      "verification": "judgment",
      "resolution": null,
      "reason": null,
      "statement": "MUST NOT use shaming, guilt, or loss-aversion streak framing (e.g. \"Don't lose your streak!\") — the reminder must encourage without penalty framing"
    }
  ],
  "coverage": { "applicable": 1, "resolved": 1, "unresolved": 0, "byVerification": { "test": 0, "judgment": 1 } }
}

The kept prohibition is judgment-tier: "manipulative framing" cannot be reduced to a mechanical test, so it records intent and routes to judgment review — but it is now an explicit acceptance criterion the spec must clear, not an unwritten assumption.

Worked example (clean-utility)

A pure utility requirement — "deduplicate a list of integers" — has no user-facing surface, no values/safety/ethics dimension. Stage 1 still over-produces ("must not mutate the input", "must not change order"), but every candidate is routine engineering that Stage 2 drops (and the edge-probe already owns). The correct precision outcome is an empty prohibition list — a zero, not a false positive:

{
  "items": [],
  "coverage": { "applicable": 0, "resolved": 0, "unresolved": 0, "byVerification": { "test": 0, "judgment": 0 } }
}

This is the precision discipline that keeps the probe from crying wolf: a utility with no intent surface produces zero prohibitions, so a non-empty list always carries signal.

Worked example (multi-prohibition)

A loan-decision requirement is the high-stakes case: it surfaces several distinct prohibitions across categories. Stage 1 produces a long list including canon items (prototype-pollution, generic GDPR retention) that canon-referral breadcrumbs out; Stage 2 keeps the three bespoke values/safety items — a fairness constraint, a privacy constraint (test-tier, mechanically checkable against the audit log), and a transparency constraint:

{
  "items": [
    {
      "requirement_id": "R1",
      "category": "fairness",
      "status": "resolved",
      "verification": "judgment",
      "resolution": null,
      "reason": null,
      "statement": "MUST NOT use protected attributes (race, gender, age, national origin) or their proxies (zip code, name) in the loan decision or rate"
    },
    {
      "requirement_id": "R1",
      "category": "privacy",
      "status": "resolved",
      "verification": "test",
      "resolution": null,
      "reason": null,
      "statement": "MUST NOT store raw PII / financial secrets (SSN, full account or card numbers) in plaintext in the audit log"
    },
    {
      "requirement_id": "R1",
      "category": "transparency",
      "status": "resolved",
      "verification": "judgment",
      "resolution": null,
      "reason": null,
      "statement": "MUST NOT mislead or omit the true rate/APR/terms in the explanation; an adverse decision must state the real principal reason (adverse-action)"
    }
  ],
  "coverage": { "applicable": 3, "resolved": 3, "unresolved": 0, "byVerification": { "test": 1, "judgment": 2 } }
}

The privacy row is test-tier — "no raw SSN in the audit log" is a mechanical assertion — while fairness and transparency are judgment-tier. The byVerification rollup { test: 1, judgment: 2 } is the count-preserved breakdown of the three resolved-status items. Each worked-example block above is kept byte-for-byte (parsed-JSON) identical to its fixture under gsd-core/references/prohibition-probe-fixtures/ by tests/prohibition-probe.docs-fixtures.test.cjs, so the doc and the reference data cannot silently drift.