ci: seed DerivedData on the 12 vCPU macOS 26 pool

[teamleaderleo]

Main's DerivedData seed waited 19 to 25 minutes for a runner this morning, so by the time it published, main had moved on. The seed now runs on blacksmith-12vcpu-macos-26, which uses the same image as the pool pull requests run on but isn't contended.

Queue, 2026-09-24 (job created_at to started_at)

Seed run	Job created	Runner started	Wait
35964625403	06:31	06:50	19.5 min
35967425494	07:08	07:33	25.3 min
35971831008	07:50	still queued at 08:34	44+ min

Earlier seeds (05:18, 05:41, 06:08) started within 20 s, before the PR load picked up. Across every macOS job created 07:00 to 08:20: blacksmith-6vcpu-macos-26 had 68 jobs with an 18 min median wait and 20 still queued at 08:34. blacksmith-12vcpu-macos-26 started its 3 jobs in about 15 s, and blacksmith-6vcpu-macos-15 had a 0.3 min median. The 6 vCPU macOS 26 pool is the one MACOS_RUNNER_PR points at.

Why the seed still hits. The seed key is RUNNER_OS, RUNNER_ARCH, xcodebuild -version, and the DerivedData basename. The product key in reuse_app_host_products.py already says a 6 and a 12 vCPU runner name one product. Both pools select /Applications/Xcode_26.6.app through CMUX_CI_XCODE_APP_PR, and the logs of seed run 35967425494 (6 vCPU) and nightly run 35968952872 (12 vCPU) show the same macOS 26.3, rootfs 26-20260916, and Xcode 26.6 (17F113).

Scope. Only MACOS_RUNNER_PR == 'blacksmith-6vcpu-macos-26' maps to the larger runner. Any other value keeps the seed on admission's own pool, as before. The nightly cold seeder gets the same expression because test_seed_derived_data.py requires the two seeders to match. The concurrency group follows the new label, so it stays one group per seed pool. A 12 vCPU runner costs twice as much per minute, but it also compiles faster, and seeds run a few times an hour.

Overlap. #14158 touches the same files. Its new evaluator test sets MACOS_RUNNER_PR to a placeholder or leaves it unset, and this expression passes that test unchanged. The conflict is textual only.

Validation. python3 -m unittest tests/test_seed_derived_data.py passes locally (12 tests), including a new test pinning that the seed pool differs from admission's only in runner size. Fork CI: https://github.com/teamleaderleo/cmux/actions/runs/35976525479. What proves the fix is the next seed after merge: its job should start within a minute and adopt should hit.

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Summary by CodeRabbit

• Improvements
  • Cache refresh and data-seeding workflows now use a larger macOS runner when the 6-vCPU macOS 26 runner is selected, providing additional processing capacity. In other cases, the existing runner selection remains unchanged, including the current fallback options.

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📝 Walkthrough

Walkthrough

The nightly cache-refresh and seed workflows select a 12-vCPU macOS 26 runner when the configured runner is the 6-vCPU macOS 26 runner. A test checks the seed workflow’s runner expression against the admission workflow’s fallback.

Changes

macOS runner selection

Layer / File(s)	Summary
Runner selection and test .github/workflows/nightly.yml, .github/workflows/seed-derived-data.yml, tests/test_seed_derived_data.py	The nightly cache-refresh and seed jobs select blacksmith-12vcpu-macos-26 for the specified runner setting. The seed workflow’s concurrency group uses the same selection. A test checks the seed and admission runner expressions.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~8 minutes

Change: Bug fix

Merge Risk: 🔵 Low · up to a85c8

The workflow still selects the intended larger runner, but its misleading comment could lead maintainers to remove the override and bring back queue delays. Correct the comment; the remaining merge risk is low.

🚥 Pre-merge checks | ✅ 24 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 1 files. (2 skipped: 2…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (24 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Cmux Cloud Persistent Session And Early Input	✅ Passed	PASS. The pull request changes only GitHub Actions runner-selection/concurrency expressions and a wiring test. It does not change Cloud terminal creation, transport, PTY or shell readiness, input owne…
Cmux Swift Actor Isolation	✅ Passed	The pull request changes only two GitHub Actions YAML workflows and a Python test. The authoritative diff contains no Swift, Objective-C, or Objective-C++ files. Therefore, it introduces no Swift acto…
Cmux Swift Blocking Runtime	✅ Passed	The pull request changes only two workflow YAML files and one Python test file. It introduces no production Swift code and no blocking or timing synchronization primitive. The check is therefore not a…
Cmux Browser Automation Off-Main	✅ Passed	PASS: The pull request changes only .github/workflows/nightly.yml, .github/workflows/seed-derived-data.yml, and tests/test_seed_derived_data.py. It does not change `Sources/TerminalController.sw…
Cmux Expensive Synchronous Load	✅ Passed	PASS: The pull request changes only two GitHub Actions YAML workflows and one Python test. The diff contains no Swift production changes and no agent-history load, main-actor load, or interactive-path…
Cmux Cache Substitution Correctness	✅ Passed	PASS: The pull request changes only GitHub Actions workflow YAML and a Python test. It does not change production Swift, TypeScript, or JavaScript code, and it does not substitute a cached value for a…
Cmux No Hacky Sleeps	✅ Passed	The pull request changes only GitHub Actions YAML runner expressions and a Python test. The rule explicitly excludes workflow YAML, and the test adds deterministic wiring assertions only. The diff add…
Cmux Algorithmic Complexity	✅ Passed	PASS: The PR changes only two GitHub Actions YAML workflows and one Python test. It adds runner-selection expressions and a test assertion; it does not add or modify production Swift, TypeScript, Java…
Cmux Swift Concurrency	✅ Passed	The pull request changes only two GitHub Actions workflow files and one Python test. The reviewed diff contains no Swift files or changed Swift concurrency code, so it does not introduce or expand any…
Cmux Swift @Concurrent	✅ Passed	The review-scoped diff changes only two GitHub Actions workflows and one Python test. It contains no Swift files, Swift declarations, or Swift call-site changes. The Swift @concurrent check is therefo…
Cmux Swift Package Boundaries	✅ Passed	The pull request changes only two GitHub Actions workflow files and one Python test. The authoritative diff contains no Swift files and no production Swift changes, so the Swift package-boundaries rul…
Cmux Swiftpm Lockfiles	✅ Passed	The PR changes only two workflow runner expressions and one test. No Package.swift, Package.resolved, .gitignore, or Xcode project files changed, and the diff introduces no dependency or package…
Cmux Swift Logging	✅ Passed	The pull request changes only two GitHub Actions YAML workflows and one Python test. The diff adds or changes runner-selection expressions, a workflow comment, and test assertions. It introduces no Sw…
Cmux User-Facing Error Privacy	✅ Passed	PASS. The PR changes only GitHub Actions runner selection, concurrency grouping, and a developer test/comment. The diff adds no user-facing errors, alerts, command output, API bodies, or recovery copy…
Cmux Full Internationalization	✅ Passed	PASS: The pull request changes only GitHub Actions workflow configuration, CI comments, and a wiring test. It adds no user-facing Swift text, string catalog or Info.plist entries, web UI/metadata/API/…
Cmux Swiftui State Layout	✅ Passed	PASS: The pull request changes only GitHub Actions YAML files and a Python test. The authoritative diff contains no Swift files or SwiftUI state/layout code, so the SwiftUI-specific failure conditions…
Cmux Architecture Rethink	✅ Passed	The pull request changes only GitHub Actions workflow expressions and a Python wiring test. It introduces no Swift code or Swift lifecycle, state, synchronization, observer, bridge, or entrypoint chan…
Cmux Swift Auxiliary Window Close Shortcuts	✅ Passed	The pull request changes only two GitHub Actions workflow files and one Python test. The authoritative diff contains no Swift changes and no NSWindow, NSPanel, NSWindowController, SwiftUI Window, or W…
Cmux Source Artifacts	✅ Passed	The pull request changes only two hand-written workflow configurations and one Python test: .github/workflows/nightly.yml, .github/workflows/seed-derived-data.yml, and `tests/test_seed_derived_dat…
Cmux No Test Or Debug Seam In Production Source	✅ Passed	PASS: The pull request changes only two workflow YAML files and tests/test_seed_derived_data.py. The authoritative diff contains no Swift file under a production Sources/ path, so it cannot introd…
Title check	✅ Passed	The title clearly identifies the main change: moving DerivedData seeding to the 12 vCPU macOS 26 pool.
Description check	✅ Passed	The description explains the queueing problem, resulting runner behavior, scope, cache compatibility, overlap, and validation. It includes a detailed Testing section with the executed test command and…

Full details: Docstring Coverage

Explanation

Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 1 files. (2 skipped: 2 unsupported.)

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In @.github/workflows/nightly.yml:
- Line 564: Update the comment near the `runs-on` expression to describe that
the `blacksmith-6vcpu-macos-26` setting is intentionally overridden with the
same-image, same-Xcode `blacksmith-12vcpu-macos-26` runner to avoid queueing;
clarify that this is a size-only override for the pull-request pool.

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ℹ️ Review info

⚙️ Run configuration

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

Review profile: ASSERTIVE

Plan: Advanced

Run ID: 63e2dff3-0454-4535-96fe-8bf4be08fd75

📥 Commits

Reviewing files that changed from the base of the PR and between 2837f22 and a85c86e.

📒 Files selected for processing (3)

• .github/workflows/nightly.yml
• .github/workflows/seed-derived-data.yml
• tests/test_seed_derived_data.py

Included review availability: Your plan provides up to 10 included reviews per hour; 0 remain after this review.

[teamleaderleo]

Measured after merge (read-only; seed-derived-data.yml seed jobs).

	Before: 6 vCPU, runs created 06:00Z to 09:01Z (n=6)	After: 12 vCPU, runs created 09:12Z to 14:00Z (n=16)
Seed job queue wait	median 1,342 s	16 s
Push to seed saved	2,628 s	740 s
Build step	734 s	256 s

• Before, queue waits: 1,173, 1,520, 2,179, 837 and 1,512 s (runs 35964625403, 35967425494, 35971831008, 35976771125, 35978671030). The first run on 12 vCPU, 35979832359, waited 14 s.
• Caveats:
  • Push to saved also reflects the faster build, and ci: order R2 cache pointers by run ID, so seeder saves take effect #14174 and ci: adopt the seed nearest the commit a PR merges onto #14190.
  • After 14:00Z the 12 vCPU pool began queueing too: 621 s on run 36011739150 and 663 s on run 36022616301. PR admissions now prefer that pool (ci: pick one macOS pool per pull request run by preference and live queue depth #14205).

🤖 Generated with Claude Code