ci: stop restoring the test compilation cache in compile admission

[teamleaderleo]

Compile admission downloaded the nightly Debug compilation cache (932 MB, 13–43 s) on every pull request, but the app target never hit it. Sampled admission logs from 2026-09-24 show cmux at 581 misses / 0 hits and 566 / 0, with cmuxUITests also all misses. The key, canonical paths, runner pool and Xcode (26.6, 17F113) match the nightly seeder exactly: fingerprint bac5277611926ed6b40291f1640fed47 on both sides.

The cause is structural, not a key mismatch. Any change to the one-module app target invalidated every one of its cached compile jobs in these samples, and every PR changes it. Only modules unchanged since the six-hourly cache save hit (C targets, untouched packages), and the adopted DerivedData seed already leaves those built. Cold admission builds with the cache (816–957 s, n=4) were no faster than the nightly's cold builds with an empty cache (774–1153 s, n=5).

This removes the restore step from compile admission. The fingerprint step stays because it keys the DerivedData seed. nightly.yml still writes the cache, since the same job writes the cold DerivedData seed and SPM cache that admission reads; nothing on main reads the compilation cache now. #14015 proposes E2E as a reader.

The first commit changes tests/test_ci_test_compilation_cache_seed.sh to require that admission does not restore the cache and still carries the fingerprint (red). The second commit removes the step (green).

Validation: tests/test_ci_test_compilation_cache_seed.sh, tests/test_ci_pull_request_caches_are_read_only.py, all 46 tests that parse ci-macos.yml/nightly.yml, and actionlint.

Not verified: hit counts are 5,000-line window samples (the job-log API returns only the tail of finished jobs), not per-build totals, and timings are small-sample. Expected saving: 13–43 s and one 932 MB download per admission run.

🤖 Generated with Claude Code

https://claude.ai/code/session_012pAcDGiHaibDaMU4CPvXAP

---

Generated by Claude Code

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}

---

Summary by cubic

Stops compile admission from restoring the nightly Debug compilation cache, which the app target never hits.

Swift keys every compile job on its whole module, so any change to cmux or cmuxUITests misses all cached jobs: sampled admission logs show 581/581 and 566/566 misses even with the key, paths, and Xcode matching the nightly seed exactly. Cold admission builds with the cache (816–957 s) were no faster than the nightly's cold builds with an empty cache (774–1153 s).

• Removes the restore step from compile admission; the fingerprint step stays because it keys the DerivedData seed.
• nightly.yml still writes the cache, but nothing on main reads it now.
• Expected saving per admission run: 13–43 s and one 932 MB download.

^{Written for commit cc57adf. Summary will update on new commits.}

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[coderabbitai]

Navigate logical layers of code changes, visualize relationships, and explore their blast radius.

Note

Currently processing new changes in this PR. This may take a few minutes, please wait...

⚙️ Run configuration

Configuration used: Repository: manaflow-ai/cmux/.coderabbit.yaml

Review profile: ASSERTIVE

Plan: Advanced

Run ID: 4594c28f-519c-40b8-bdce-cd2875ff189f

📥 Commits

Reviewing files that changed from the base of the PR and between 5da27d6 and cc57adf.

📒 Files selected for processing (3)

• .github/workflows/ci-macos.yml
• .github/workflows/nightly.yml
• tests/test_ci_test_compilation_cache_seed.sh

 ___________________________________________________________
< This loop is doing cardio. Your users are doing timeouts. >
 -----------------------------------------------------------
  \
   \   (\__/)
       (•ㅅ•)
       / 　 づ

✨ Finishing Touches

📝 Generate docstrings

• Commit to this branch
• Create a new PR

🧪 Generate unit tests (beta)

• Commit to this branch
• Create a new PR

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}