perf(shared): scan PATH once per command before spawning, not on every spawn

[SkiTee3000]

Part of #11221 and #12498: this is the uncached PATH scan the triage on #12498 points at ("The 30 s CommandResolutionCache is only on resolveCommandPath / isCommandAvailable, not on the spawn path"). Picking Git's cmd\git.exe launcher over the real binary, also described in #11221, is not touched here.

What Changed

resolveSpawnCommand now remembers the result of its executable lookup per (platform, PATH, PATHEXT, command) in the existing CommandResolutionCache, with the same 30 second lifetime and the same rules as resolveCommandPath: a change to the search environment is a different key, and explicit paths are never cached.

Why

On Windows every child process the server starts goes through resolveSpawnExecutableWithNode, which walks every PATH entry times every PATHEXT extension with statSync. Nothing was memoized, and the walk is synchronous on the event loop. On the machine in the issue that is about 22 ms per spawn: 2345 shell.resolveSpawnCommand spans in a 76-minute trace, roughly 580 s of blocked event loop over 13 hours.

resolveCommandPath, a few lines above, already caches the same scan for the same reason. The spawn path did not use it.

Unlike that cache, a miss is never remembered. A failed spawn of the bare name is how providers report "not installed" (ClaudeProvider, providerMaintenanceRunner), so it has to clear the moment the binary appears. What can be stale for up to 30 seconds is only which of several matching executables on PATH wins.

Tests

One new test: repeat lookups of a found command scan once, a missing command scans every time, a different PATH scans again, explicit paths always scan, and the entry expires after the TTL. The existing spawn tests now get their own cache instead of sharing the process-wide one.

Related

Other pull requests for #12498:

• feat(server): make the pull request lookup interval a background activity setting #12601 makes the pull request lookup interval a setting
• perf(server): answer cheap git metadata from repository files instead of spawning git #12602 reads git metadata from repository files instead of spawning git

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes (no UI change)
• I included a video for animation/interaction changes (none)

Model: Claude Fable 5.1. Harness: Claude Code, running inside T3 Code.

Summary by CodeRabbit

• Performance
  • Improved Windows command resolution by reusing recent executable lookups for up to 30 seconds.
  • Repeated commands with unchanged environment settings now resolve faster.
• Reliability
  • Explicit executable paths bypass cached lookups.
  • Unresolved commands are rechecked so newly available programs can be detected promptly.
  • Changes to PATH trigger fresh executable resolution.
• Tests
  • Added coverage for caching, expiration, environment changes, explicit paths, and unresolved commands.

[macroscopeapp]

Approvability

Verdict: Approved at c33b0ad

Macroscope's review found this PR approvable — This is a focused Windows performance optimization that caches successful executable lookups while preserving uncached behavior for explicit paths and misses. The production impact is localized and covered by tests for cache reuse, environment changes, and expiry.

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📝 Walkthrough

Walkthrough

Windows command resolution now caches successful PATH lookups for 30 seconds. Cache entries include resolver identity. Explicit paths and failed lookups bypass the cache. Tests validate cache reuse, resolver isolation, rescans, environment changes, and expiry.

Changes

Windows command resolution

Layer / File(s)	Summary
Resolution cache integration packages/shared/src/shell.ts	resolveSpawnCommand caches successful Windows executable resolutions by resolver identity, platform, PATH, PATHEXT, and command. Explicit paths and misses are not cached.
Cache behavior validation packages/shared/src/shell.test.ts	Tests use isolated caches and verify cache reuse, resolver isolation, failed-resolution rescans, explicit-path probes, PATH changes, and 30-second expiry.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Refactor

Suggested reviewers: utkarshusername

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: caching PATH scans before spawning.
Description check	✅ Passed	The description explains what changed, why the change is needed, test coverage, related work, and the absence of UI changes. The required sections are present. The UI checklist items remain unchecked,…
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.

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Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
In `@packages/shared/src/shell.ts`:
- Around line 652-658: Update the cache-key logic in the spawn resolution flow
around SpawnExecutableResolution and CommandResolutionCache so injected resolver
functions cannot reuse entries created by a different resolver for the same
environment and command. Include resolver identity in the cache partition, or
bypass caching when a resolver is injected, while preserving caching for the
default resolver.

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