[rig-sampler] Improve missing-required-field error messages with richer type descriptions

[github-actions]

Samples run

Five consecutive samples were executed via the stub Copilot SDK runner (npm run sample):

#	Sample	Result	Notes
22	22-config-normalizer.ts	✅ Pass	Output { normalized: s.unknown, warnings: s.array(s.string) } — s.unknown is correctly loose here; no repair needed
23	23-schema-inference.ts	✅ Pass	Nested object array in output; stub synthesised valid output on first turn
24	24-error-message-improver.ts	✅ Pass	Uses s.optional(s.string) for context; optional field correctly excluded from required
25	25-migration-guide.ts	✅ Pass	Deeply nested s.array(s.object({ before, after })) — first-turn success
26	26-design-review.ts	✅ Pass	Mirrors sample 25's migration-guide spec; no repair turns observed

All 5 samples completed in a single turn with no repair loops.

Improvement: richer missing-field error messages

What was observed

Samples 23, 25, and 26 all declare output schemas that include required array, object, and nullable fields. When a model omits one of these fields, the existing validator surfaced:

$.items: missing required field (expected value)

The word "value" is unhelpful — it gives the model no signal about what kind of value to supply during a repair loop.

What changed

skills/rig/rig.ts — extracted a describeSchemaType helper that is called instead of the inline ternary:

• s.array(...) → reports "array"
• s.object(...) / s.record(...) → reports "object"
• s.nullable(s.number) → reports "number | null"
• s.enum("open", "closed") → reports "open" | "closed" (inline enum values)
• Primitive schemas (s.string, s.boolean, etc.) → unchanged, still reports the type name

src/rig.test.ts — added three new cases to the "missing required field error message" describe block covering the array, nullable, and enum improvements.

Why this matters

The repair prompt (defaultRepairPrompt) includes the exact validation error string. A precise message like "expected number | null" or "expected array" is directly actionable for the model in a repair turn, reducing the chance of a second incorrect attempt.

Generated by Daily Rig Sampler · sonnet46 79.7 AIC · ⌖ 8.18 AIC · ⊞ 5.4K · ◷

[github-actions]

🧠 Matt Pocock Skills Reviewer has completed the skills-based review. ✅

[github-actions]

Skills-Based Review 🧠

Applied /tdd — one minor gap found; leaving as COMMENT (no blocking issues).

📋 Key Themes & Highlights

Finding

• Missing test for object branch: describeSchemaType has five branches; only four are tested. The untested properties/additionalProperties branch would silently regress to "value", exactly undoing the intent of this PR.

Positive Highlights

• ✅ Clean extraction of describeSchemaType — single-responsibility, well-named, easy to unit-test independently if needed
• ✅ Recursive nullable handling (describeSchemaType(inner) + " | null") is elegant and correct
• ✅ Enum values are quoted via JSON.stringify, so string values render with quotes (e.g. "open" | "closed") — this is exactly what a model needs to produce valid JSON
• ✅ Three new tests are well-structured and assertive; the pattern is consistent with the existing describe block
• ✅ The fallback return "value" is preserved — safe for unknown/future schema shapes

🧠 Reviewed using Matt Pocock's skills by Matt Pocock Skills Reviewer · sonnet46 27.4 AIC · ⌖ 4.21 AIC · ⊞ 6.3K
_{Comment /matt to run again}

[github-actions]

[/tdd] The "object" branch in describeSchemaType (the properties / additionalProperties check) has no corresponding test. If that branch regresses, the repair prompt silently falls back to "value" — the exact issue this PR was designed to fix.

💡 Suggested test

it("reports a missing required object field with type 'object'", () => {
  const schema = s.object({ meta: s.object({ key: s.string }), name: s.string });
  const result = analyzeResponse(JSON.stringify({ name: "x" }), schema, "test", 1);
  expect(result.ok).toBe(false);
  if (!result.ok) {
    expect(result.error.message).toContain("missing required field");
    expect(result.error.message).toContain("$.meta");
    expect(result.error.message).toContain("object");
  }
});

All four describeSchemaType non-trivial branches (array, nullable, enum, object) should have coverage to match the precision the other three tests establish.