ADR-0127 — C004 exempts SOL INTERFACE blocks (contract 0.12.0 → 0.13.0)

Context

C004 (one-strength-word, at-least-one since ADR-0126) fired on every parsed requirement, including SOL INTERFACE (IF-) blocks. But an INTERFACE is a signature declaration, not an obligation: its grammar (docs/reference/structured-requirements.md) is RETURNS / ACCEPTS / ERRORS / OWNED BY — there is no strength-word slot. A conformant INTERFACE therefore always counts zero strength words and always draws the C004 warning "add the one word (MUST/SHOULD/…) it binds on" — a warning with no valid fix, since there is no MUST to add to a RETURNS. Measured in the round-5 SOL stress test (suspec-works #96): only INTERFACE mis-fired; CONSTRAINT (MUST NOT) and INVARIANT (MUST) in the same spec did not. It trains users to ignore C004 or deters them from the documented, encouraged way to declare a boundary.

Decision

C004 applies to obligation blocks only. SOL INTERFACE (IF-) is exempt — it carries no strength word by grammar, so "binds on nothing" is inapplicable, not a defect. QUESTION (Q-) is already excluded upstream (it never parses as a requirement). C004 continues to apply to REQ (AC-), CONSTRAINT (C-), and INVARIANT (I-), which do bear an obligation.

INTERFACE ids remain parsed, scopeable, and coverage-tracked (unchanged) — the exemption is C004's applicability only, not the block's status as a requirement. Contract version bumps 0.12.0 → 0.13.0; C004's id, name, and severity are unchanged (the applicability rule is the row's comment in checks.yaml).

Consequences

• suspec-cli pins 0.13.0 and skips IF- ids in check_one_strength_word; the drift guard reconciles the version.
• The checks.yaml C004 row and the canon checks.md row note the obligation-blocks-only applicability.
• An INTERFACE contract with no MUST no longer draws an un-actionable warning; the honest signal (a REQ/CONSTRAINT/INVARIANT that binds on nothing) is unchanged.

Status

Accepted (2026-07-04).