delegating: which model, and why cheapest alone is wrong

haiku for mechanical, sonnet by default, opus only when the work still
holds a decision — which usually means the brief needs finishing rather
than a bigger model writing it.

and the part that matters: cheapness is a property of the pair, not the
model. a cheap model on a vague brief is the most expensive option
available — it returns something plausible, you reject it, you write the
brief you should have written, and you pay for the round twice.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Officer-Session: 656f5c85-e49a-4f1f-b9f3-042a04b4d043

skills/delegating/SKILL.md

@@ -44,6 +44,28 @@ An agent that has to ask a question has already cost more than it saved. So the
 Run it in the background and carry on. A brief good enough to run unattended is the same brief that makes
 the result reviewable.
 
+# Choosing the model
+
+**The cheapest one that will get it right.** Not the cheapest one; the cheapest one that will get it
+right, which is a different question and has to be asked per task.
+
+- **Haiku** — mechanical and high-volume with no judgement in it. A rename across forty call sites, a
+  JSON transform, moving a block of files to a new shape.
+- **Sonnet** — the default, and what most of this work is. A feature whose design is settled and whose
+  brief names the files: translations, a form, tests for a rule already agreed, a migration over a list.
+- **Opus** — when the work still contains a decision. Which usually means the brief is incomplete, so
+  the first move is to finish the brief rather than to spend more per token on writing it.
+
+## Cheapness is a property of the pair, not the model
+
+A cheap model on a vague brief is the most expensive option available: it returns something plausible,
+you review it, you reject it, you write the brief you should have written, and you pay for the whole
+round twice. The saving comes from the brief being good enough that a smaller model cannot go wrong —
+so if you find yourself reaching for a bigger model to compensate for a thin brief, fix the brief.
+
+The other direction is real too, and cheaper to correct: a task that comes back wrong from Haiku goes to
+Sonnet and costs one wasted run. Start low.
+
 # Reviewing what comes back
 
 **Read the diff, not the summary.** The summary is written by the thing being reviewed. `git diff` is not.