CARD: scaffold-c-runnable-contracts — Runnable Contracts

**Discipline v0.2 · BETA**

Classification: layer=E (verification); mechanism=scaffold C.

Intent: Express pre/post-conditions and invariants as EXECUTING assertions or proofs attached to the unit and restated at use sites — so a paged reader gets ground truth without simulating the body.

Also Known As: design by contract; debug_assert; refinement; require/ensure; invariant check; Kani contract.

Applicability / Recognition: Apply when — a function has a non-obvious precondition; a cross-cell invariant is relied upon far from where it is established (R3-009); a behavioral claim is currently only in prose. *Detector seed:* a comment asserting a property ("x is sorted", "non-empty", "already validated") with no adjacent runnable check → recognition fires (a prose claim is adversarial if it lies, R2C-004).

Motivation: A weak agent edits a function relying on "input already deduplicated" stated only in a doc-comment three modules away. It cannot do the whole-program inference to confirm. A `debug_assert!(is_unique(&input))` at the use site either holds or fires — the invariant is local ground truth.

Structure & Participants: *Precondition/postcondition* (debug_assert or contract macro) · *Use-site witness* (assertion where the invariant is relied upon) · *Proof* (Kani requires/ensures, for safety-critical) · *Property test* (behavioral backing).

Collaborations: Defines "equivalent" for Class D oracles; pairs with Class B (types for protocol, contracts for value invariants); failures emit Class F diagnostics.

Goals / Non-Goals: *Goals:* make load-bearing invariants machine-checked at the point of reliance. *Non-Goals:* NOT proving everything (cost) — proofs for safety-critical, asserts elsewhere; NOT a substitute for the spec that JUSTIFIES the invariant.

Consequences: (+) invariants become local and checkable; (+) a paged reader trusts the assertion, not distant prose. (−) restatements can drift — pair with drift detection or keep them assertions (which fail, not mislead); (−) debug_assert is debug-only — use real checks on untrusted boundaries.

Alternatives: types (Class B) when the invariant is a protocol; a comment (rejected — lies silently); full proof when tractable and critical.

Risks & Assumptions: assumes the invariant is expressible as a runnable predicate. *Sunset:* if types or vibe-tcg make the invariant statically guaranteed, the assert retires there.

Evidence & Transfer-strength: DR1-019 (contracts give success criterion without body, theory), R3-009 (use-site restatement, theory), R2C-004 (prose lies harm, benchmark). Class: benchmark + theory. Tag: **[E-mid]**.

trigger: WHEN a prose property claim lacks an adjacent runnable check, OR a cross-cell invariant is used far from its definition THEN apply mode: inline routine: 1. State the invariant as a boolean predicate. 2. Add debug_assert!/contract at the definition AND at each use site relying on it. 3. For safety-critical invariants, add a Kani requires/ensures proof. 4. Back behavioral claims with a property test. 5. Replace the prose claim with the assertion (or label it verified, linking the check). checker: conform T-sem `invariant-witnessed` (flags declared invariants with unwitnessed use sites) + cargo test raid_role: layer=cells; order=after:naming-uniformity; batch=cell budget: active_rules=2; first_signal=cargo test -p <cell> (<60s)