ADR-0164 - One spec format; the format selector retires

Context

ADR-0058 gave the spec two surfaces: plain structured markdown (the default) and SOL, a stricter flush-left obligation grammar selected per file by frontmatter format: sol. Both parsed to one requirement record, so format: never unlocked a distinct check — it only chose a surface. SOL's one contribution over the markdown form was the block-type distinction (REQ/CONSTRAINT/ INVARIANT/INTERFACE).

Two facts decided it. SOL had zero adoption — including in this project; its only instances were paired parser-conformance fixtures. And format: is a second discriminator layered on type:, which ADR-0059 already made the single one. Every other artifact has exactly one structured shape implied by its type; the spec was the lone exception carrying a format flag and a second surface.

Decision

1. A spec has one form: structured markdown. Each requirement is a ### AC-NNN heading carrying one obligation, one strength word (MUST/MUST NOT/SHOULD/SHOULD NOT/MAY), observable behavior, and one Verify with: line. Structure is inherent to type: spec, not selected.
2. The format: frontmatter key retires. It is not read, validated, or recognized. The spec frontmatter is type, id, title, status, owner, sources[].
3. The SOL surface retires with it. The flush-left REQ/CONSTRAINT/INVARIANT/INTERFACE openers, the QUESTION Q-NNN [blocking]: header, and the VERIFY BY keyword are no longer parsed; ### AC-NNN + Verify with: is the only requirement grammar. The block-type distinction is not carried; a spec states behaviors. A machine-precise interface contract, if it ever earns its cost, returns through evidence, not a standing second surface.
4. C004 loses its SOL conditioning. It counts strength words over the whole requirement statement; the SOL trigger/response narrowing and the IF- interface exemption retire.
5. The contract advances to 0.22.0. This is a reduction — a surface, a selector, and a parser branch leave. No check ID is added, removed, renamed, or reused.

Superseded and narrowed decisions

• ADR-0058 is superseded. Decision 1 (the markdown spec form) stands as the sole form; Decisions 2-3 (the SOL surface, the two-surface record, the anti-fork fixture pairs) retire — with one surface there is nothing to fork.
• ADR-0027 is narrowed: its obligation semantics survive as the markdown form's uppercase-modal convention; SOL as a distinct notation and surface retires.
• ADR-0059 is upheld and extended: type: is the sole discriminator; the redundant format: axis is removed.

Alternatives considered

Consequences

• The spec is written one way; there is no format to choose or flag.
• The checker parses one requirement grammar; the SOL branch, the format field, and C004's SOL conditioning leave the reference implementation.
• Existing specs are unaffected — all real specs already use the markdown form.
• A spec that still carries format: sol loses its flush-left requirements silently (they no longer parse as ### AC-NNN) and fails on absent requirements; there are none in practice.

Status

Accepted (2026-07-18). Supersedes ADR-0058; narrows ADR-0027; upholds ADR-0059. Contract 0.22.0.

Propagation

• checks/checks.yaml — version 0.22.0; spec_form drops the selector, format scalar, and format_enum; C004 drops the SOL IF- exemption note; the VERIFY BY verify alternative retires.
• corpus-cli — parse_spec_record drops the SOL branch and the format field; checksContract drops the SOL trigger/response narrowing, the IF- exemption, and VERIFY BY; CONTRACT_VERSION 0.22.0; SOL tests retire.
• corpus-mcp — SUPPORTED_CONTRACT_VERSION 0.22.0; fixtures refreshed.
• Canon: docs/reference/structured-requirements.md retires; docs/reference/checks.md, artifact-formats.md, glossary.md, docs/04-writing-specs.md, checks/README.md, and the domain fixtures drop the SOL surface and the spec.sol.md pairs.
• corpus-skills: sus-spec/sus-task already teach only the markdown form (no format flag).
• corpus-website: canon mirror repins; no site copy names format: sol.