rubber-duck

Explaining a problem aloud finds the flaw. Explaining it to a different model family finds the flaw your own family is biased to miss — that is the whole delta over talking to yourself, and the reason the listener must not be the model already driving the session.

One pass, advisory only. The duck reads and reasons; it never edits, builds, tests, or lands. You decide what to do with what comes back.

Host

Copilot CLI ships this as a built-in agent, so use that and stop here. The rows below exist because other hosts have no built-in, and the skills in this library call for a rubber-duck pass regardless of where they run.

Model identifiers move between generations. Resolve the current full, non-mini, non-codex model at session start rather than trusting an example. dual-review carries the same transport table; when both run in one session, resolve the models once.

Read-only is not a formality here. The duck's value is an outside view of work it did not write, and a listener that starts editing the tree stops being a listener. Where the transport offers a read-only sandbox, use it, so the boundary holds mechanically rather than by instruction.

What to send

Send the smallest packet that lets an outsider judge the thing:

• the decision, plan, trace, or diff in question;
• what you are trying to achieve, and what is explicitly out of scope;
• what you already tried and what it did;
• the specific question you want answered.

A duck given the whole repository returns generalities. A duck given one ambiguous decision and its constraints returns an opinion you can act on.

Ask for blind spots, design flaws, and substantive issues. Ask it to say plainly when it would choose differently and why — a duck that only validates is a duck you did not need.

What comes back

Feedback is evidence, not instruction. Take the parts that survive your own check of the code, and say so when you set a point aside; an unexamined "the duck said so" is how a second opinion becomes an unreviewed edit.

One pass is the norm. Reaching for a second round on the same question usually means the packet was unclear rather than the answer, so sharpen the question before spending another pass.

Boundary with dual-review

Reach for the duck to make a decision; reach for dual-review to land a change.

The duck is one advisory pass on a question still open. dual-review is two reviewers, adjudication, and a bounded round budget over a change that already works, and it owns the landing gate. A duck pass never satisfies that gate, and a change that only needs a second opinion never needs the full protocol.

Skills that offer both name their own threshold — guardrails sets one for amending an invariant. Follow the calling skill's rule rather than a copy of it kept here.