Spec Approach

Take an existing feature spec and add the ## Approach — the HOW that tdd needs to execute. Same file, append only.

Skip spec-approach for

• Bug specs (output of diagnose — already contain Root cause and Fix).
• Trivial changes — single file, obvious extension of an existing pattern.
• Specs that already have ## Approach — unless the user explicitly asks to revise.

If invoked in a skip case, recognise it and tell the user "this looks trivial / already approached — go straight to tdd". Do not write anything.

Before you start

Identify the spec. A path or slug for an existing spec/<slug>.md must be given. If neither was provided, ask the user which spec — do not guess. Never operate without an explicit target.

Validate it. Read the file. It must contain Goal, Behaviors, and Acceptance criteria. If any are missing, abort and tell the user to run interview-to-spec first. Never try to fill in WHAT.

The work

1. Read the spec. Internalise Goal, Behaviors, AC, OOS, Edge cases.
2. Explore the codebase. Find relevant modules, patterns, existing ADRs, similar features. The approach must be grounded in what is actually there — not invented.
3. Propose. State a recommended approach. If multiple paths are viable, show 2–3 with tradeoffs and recommend one.
4. Validate with the user. Ask one question at a time, only when judgment is required (which alternative? what's the priority?). Do not ask what the codebase can answer — read the codebase.
5. Append. Add ## Approach to the same spec file.

How to append (safety rules)

• Use Edit, not Write. Anchor on the last existing line or section of the spec; the new content is the anchor + \n\n## Approach\n....
• Never create a new file.
• Never modify the WHAT sections above (Goal, Behaviors, AC, OOS, Edge cases). Touch only the bottom of the file.
• If the spec ends with a trailing newline, preserve it.

The ## Approach section

## Approach
- **Strategy.** One paragraph describing how the work will be done end-to-end.
- **Modules / files.** Concrete paths to create or modify.
- **Key decisions.** The alternative considered and why this one. One bullet per decision.
- **Schema / API changes.** If any. Otherwise omit this bullet.
- **Risks.** Anything that might surprise us during execution. If none, omit this bullet.

Keep each bullet tight. The reader is tdd — it needs enough to execute, not a design doc.

When ## Approach already exists

Do not overwrite silently. Read the existing approach, then ask the user: "There's already an approach here. Want me to revise it, or is it still good?" Only edit if they confirm. When revising, replace the existing ## Approach block via Edit — do not append a second one.

Where this skill ends

The spec is now self-sufficient. tdd consumes it directly — Behaviors + Acceptance criteria become the test queue; Approach guides the implementation.