/switch-model: change the implementer's or the reviewer's model

Each role runs its OpenCode chain, then Claude: today, GLM-5.3 Flash on Z.AI implements, then MiMo V2.6 Flash on Go (then Sonnet), and GPT-5.6 Luna on the direct OpenAI route reviews (then Opus). The user decided this on 2026-10-02 (L27) and on 2026-10-04 (L42, L51). A switch changes one role's model; it is the user's decision, so you propose it and the user says yes.

1. Why. Name the reason: a measured failure (the run logs, the measurement comments), a cheaper or better candidate, or the user's request. A model that failed once on a strange task is not yet a reason (L14).

2. What runs now. node tools/harness/switch-model.mjs --show.

3. Candidates, live (L25), never from memory:

   • implementers on the Go subscription: opencode models opencode-go;
   • the direct routes, with their own logins: opencode models openai, and so on.

   Leave out a model of the other role's family (the reviewer is never the implementer's family), and models already ruled out in docs/lessons.md (L27: Go's own GPT-6 Luna returns Bad Request in long loops). --show lists the models on watch (L35): registered candidates, each with a watch note in harness.json on what to look for, which the runner prints when it runs one; their measured runs are the best reason to switch.

4. Dry run with a probe: node tools/harness/switch-model.mjs --role implementer|reviewer --model PROVIDER/ID --probe --dry-run.

   • It checks the id against OpenCode's list in the scripts' data directory. On exit 3, give the user the login command it prints; the user runs it.
   • The effort, unless --variant gives one: high for a light model; for a heavy one (Sol, GLM-5.3, MiMo V2.6 Pro, Opus) low, else medium, else the lowest OpenCode offers (L54). max is refused.
   • It shows the family and refuses the other role's. --force is only for when the user is switching both roles.
   • The probe is one billed one-word call at that effort.

5. Ask. Show the user the before and after lines and the probe's time, with your recommendation. Switch only on their yes.

6. Switch. Run the same command without --dry-run (and without --probe, which already passed). Then commit harness.json on main with the reason and the probe in the message (L6, L25). Keep the old model's entry: an explicit --model can still use it. A model on watch keeps its note through the switch; only the user's decision removes it.

7. Watch the first run on the new model. That means the log's tail and, for a reviewer, the posted review. Afterwards, add a line to the task's measurement comment (process.md §9). Switch back the same way if it fails.

Exits: 0 switched (or shown, or a dry run); 1 refused (the family rule, or a taken name); 2 usage; 3 OpenCode, the login or the probe failed, and nothing was written.