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vercel__next.js/evals/eval.config.json
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Aurora Scharff 393de1ebe0 skill: add Partial Prefetching optimizer (#96471)
## Summary

- add a Partial Prefetching optimizer Skill for selected source Links
and destinations
- keep prefetch stages and Link policy in the public [Optimizing
prefetching
guide](https://nextjs.org/docs/app/guides/optimizing-prefetching), and
rely on the same static-shell documentation as the Cache Components
optimizer for cache placement, Suspense, loading UI, and authorization
- keep the Skill focused on the product decision, production `instant()`
loop, differential verification, and mutation freshness when an
optimization adds a cache around mutable data
- reference the canonical
[`unstable_prefetch()`](https://nextjs.org/docs/app/api-reference/functions/prefetch)
and
[`unstable_navigation()`](https://nextjs.org/docs/app/api-reference/functions/navigation)
docs instead of explaining the APIs again in the Skill
- link cache-writing optimizations to the canonical
[Revalidating](https://nextjs.org/docs/app/getting-started/caching-and-revalidating#revalidating)
documentation and verify a populated-cache mutation in the Skill and
eval
- bundle a self-contained production rig aligned with the related
adoption and optimizer Skills, tuned to record each source-link and
prefetch-stage contract
- add an eval that compares baseline, the bundled guide, and the local
Skill against the same prefetched-UI and cache-freshness goal
- document the optimizer in the AI agents guide

This follows the same guide/Skill split as #97737. Partial Prefetching
adoption preserves existing behavior; this optimizer is the later
decision about what is worth preparing before a click and what should
wait for navigation.

Related: #97712, #97737, #97759, #98122, #98175

## Eval

One run per treatment with Claude Opus 4.8 and Claude Sonnet 4.5, using
a fixed Claude Haiku 4.5 judge. The bundled-guide and Skill treatments
use a local Next.js tarball containing the new guide and API references.

| Model | Treatment | Criteria | Score change | Overall | Duration |
Tokens processed | Total run cost | Cost change |
| --- | --- | ---: | ---: | ---: | ---: | ---: | ---: | ---: |
| Opus 4.8 | Baseline | 10/10 (100%) | — | Pass | 18m 42s | 3.39M |
$3.46 | — |
| Opus 4.8 | Bundled guide | 9/10 (90%) | -10 pp | Fail | 13m 37s |
3.02M | $2.84 | -18.0% |
| Opus 4.8 | Local Skill | 10/10 (100%) | +10 pp | Pass | 17m 22s |
2.80M | $2.54 | -10.6% |
| Sonnet 4.5 | Baseline | 2/10 (20%) | — | Fail | 15m 55s | 1.97M |
$1.06 | — |
| Sonnet 4.5 | Bundled guide | 5/10 (50%) | +30 pp | Fail | 15m 49s |
2.17M | $1.11 | +4.5% |
| Sonnet 4.5 | Local Skill | 8/10 (80%) | +30 pp | Fail | 15m 31s |
1.41M | $0.77 | -30.4% |

Token totals count each unique model message once and include uncached
input, cache creation, cache reads, and output. Cache reads account for
most of the total in every treatment. Total run cost applies the current
AI Gateway base rates to each token category: Opus 4.8 costs $5/M input,
$6.25/M cache writes, $0.50/M cache reads, and $25/M output; Sonnet 4.5
costs $3/M input, $3.75/M cache writes, $0.30/M cache reads, and $15/M
output. The six agent runs cost $11.78 in total.

Compared with baseline, the Opus Skill run maintained a 100% criteria
pass rate while reducing cost by 26.7%. The Sonnet Skill run improved
the criteria pass rate from 20% to 80%, a 60 percentage-point increase,
while reducing cost by 27.3%.

The stronger model could solve the fixture without the Skill, although
it independently searched the bundled Next.js documentation and took
slightly longer than the Skill treatment. The Opus guide-only run
implemented the correct behavior but missed the required
RED-before-GREEN proof.

The Sonnet treatments show a clearer progression. Baseline did not
implement the required caching, navigation stage, `instant()` contract,
or cache invalidation. The bundled guide added the cached summary and
production test, but missed the navigation stage, deferred-state
assertions, tag invalidation, and RED-before-GREEN proof. The Skill
treatment completed the `instant()` RED-to-GREEN loop and final
production behavior, but left `connection()` in the related-sessions
read and used path revalidation instead of tagging the cached read. It
therefore passed 8 of 10 criteria but failed the overall eval.

All six measured runs completed through grading. Chromium and its system
dependencies are provisioned before the agent starts, so missing browser
libraries did not determine these results.

## Verification

- `pnpm --filter=next build`
- `pnpm prettier --check` for the changed guide, prompt, Skill, and eval
fixture
- `pnpm exec tsc -p
evals/evals/agent-051-optimize-partial-prefetching/tsconfig.json
--noEmit`
- `node --check run-evals.js`
- `git diff --check`
- packed a local Next.js tarball and confirmed it contains the optimizer
guide plus the `navigation()` and `prefetch()` references
- verified the `navigation()` contract in the minimal reproduction and
NextBeats on Next.js 16.4.0-canary.22
- merged the latest `canary`, preserving both generic fixture setup and
Playwright provisioning

<!-- NEXT_JS_LLM -->
2026-09-10 22:27:36 +02:00

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{
"agent-046-adopt-partial-prefetching": {
"skills": ["next-partial-prefetching-adoption"],
"timeout": 1800
},
"agent-047-adopt-cache-components": {
"skills": ["next-cache-components-adoption"],
"timeout": 1800
},
"agent-051-optimize-partial-prefetching": {
"skills": ["next-partial-prefetching-optimizer"],
"timeout": 1800
},
"agent-054-cache-components-empty-static-params": {
"skills": ["next-cache-components-adoption"],
"timeout": 1800
},
"agent-055-stale-build-error": {
"skills": ["next-dev-loop"],
"timeout": 900
}
}