Spec driven implementation

Contract

Field	Bound contract
Trigger	Starting a significant feature or when specs are checked in.
Authority	Reversible local: writes only checked-in spec files and implementation; rollback is version control (if commit is not yet pushed). No remote mutation.
Side effect	Creates or updates checked-in spec files within the project.
Done	Specs exist, are behavioral, and stay current with what ships.

Inputs

• Feature brief (required): natural-language description of the feature to implement.
• Existing specs (optional): any spec files already present in the project. If absent, the skill treats the feature as greenfield.
• Scope anchor (required): the module, package, or directory boundary the feature affects.

Refusal

• Spec drift: implementation adds behavior not covered by any spec. Remove the undocumented behavior or add a covering spec before proceeding.
• Scope overrun: the feature brief names more surface than the scope anchor. Stop and report the overrun; do not widen scope without explicit human confirmation.
• Non-behavioral spec: a proposed spec line describes a solution rather than behavior. Reject it; replace with an observable criterion.
• Unreachable rollback: VCS commit already pushed. Authority is exhausted; surface the conflict and stop. Do not overwrite remote history.

Procedure

1. Bound the scope. From the feature brief and scope anchor, enumerate every file that will be created or modified. If the scope is ambiguous, stop and state the ambiguity rather than inferring a larger surface. Done when: the file set is enumerated or ambiguity is reported.
2. Audit existing specs. For each spec file already present in the affected scope, verify it is behavioral: each statement describes observable behavior or acceptance criteria, not implementation design. Discard or narrow any spec line that prescribes a solution path. Done when: every existing spec is verified behavioral.
3. Identify spec gaps. Map every feature-brief item to at least one behavioral statement. Mark any item that cannot be expressed as observable behavior as out of scope. Done when: every brief item maps to a behavioral statement or is marked out of scope.
4. Write or update spec files. For each gap, add a new spec section with a unique label, the behavioral statement, any preconditions and expected outcomes, and the pass/fail criterion that a test could verify. Place each spec in the file that most directly covers the affected surface. Done when: every gap has a spec section.
5. Verify spec-implementation alignment. Confirm that no spec file contradicts an existing implementation. If a conflict exists, surface it before writing. Done when: no spec contradicts existing implementation.
6. Write or confirm the implementation. Implement the feature to satisfy the spec. Do not add behavior not described by a spec. Done when: the implementation satisfies every spec.
7. Commit with a spec-referencing message. VCS-commit the spec and implementation files with a message that names the spec label(s) satisfied. Done when: one commit names the satisfied spec labels.

Output

Checked-in spec files covering all behavioral requirements of the feature, aligned with the implementation, and commit messages naming the spec labels satisfied.