Design twice (or three times)

After Ousterhout's "Design It Twice" and Pocock's design-an-interface. The most expensive bug is a wrong design baked in early; the cheapest fix is a second alternative drafted before the first ships.

Procedure

Step 1 — Frame the design problem

Read upstream:

• specs/<slug>/research.md
• specs/<slug>/requirements.md
• docs/CONTEXT.md, docs/glossary/*.md (per ADR-0010; legacy docs/UBIQUITOUS_LANGUAGE.md if present)

Restate the design problem in one sentence. Confirm with the user.

Step 2 — Pick three divergent constraints

Pick three constraints that will pull the design in genuinely different directions. Defaults that work for most module-shaping problems:

• A — Minimize surface. Smallest possible public interface. Bias toward fewer methods, fewer parameters, more depth.
• B — Maximize flexibility. Design for extension. Bias toward composition, plug points, orthogonal primitives.
• C — Optimize the common case. The most frequent caller writes the least code. Bias toward affordances over generality.

For UI/UX problems, swap to: A — minimize click depth, B — maximize discoverability, C — minimize cognitive load. For architecture, swap to: A — minimize moving parts, B — maximize testability, C — minimize blast radius on failure.

If the user has stated a constraint that suggests a fourth axis, add it as D.

Step 3 — Dispatch three parallel subagents

Issue the Task calls in the same turn so they run concurrently. Each subagent gets:

• The same upstream context references.
• Its single divergent constraint.
• Output destination: specs/<slug>/design-alt-<A|B|C>.md.
• Return contract: 5–10 line summary, not full content.

Each subagent should produce: Module shape, Public interface (signatures only), Data flow, Trade-offs explicitly accepted.

Step 4 — Synthesize

When all three return, you (in the main thread) write specs/<slug>/design-comparison.md with sections:

• Compared alternatives — one paragraph per option summarizing shape and trade-offs.
• Recommendation — which one and why (cite specific upstream constraints).
• Hybrid moves — borrow X from B, drop Y from A.
• Rejected alternatives — what we lost by not picking each.

Then ask the user via AskUserQuestion: Adopt recommendation (Recommended) / Adopt alternative <X> / Synthesize a hybrid (you describe) / Run another design pass with a new constraint.

Step 5 — Hand off to /spec:design

The chosen alternative becomes input to /spec:design. The architect inside /spec:design reads design-comparison.md and writes the canonical design.md. Do not let design-twice pretend to be the design artifact itself — it is exploration, not commitment.

Rules

• Constraints must be genuinely divergent, not three flavors of the same idea.
• Subagents must not see each other's drafts during their pass.
• Synthesis is your job in the main thread. Don't dispatch a fourth subagent to "pick the winner" — that defers your judgment.
• Keep alternatives at interface/shape level. No implementation, no tests.
• File ADRs for any irreversible decisions the synthesis bakes in (use the record-decision skill).