Windows: Antigravity runtime install fails with EPERM when publishing to versions directory #29273

Summary

On Windows, installing the managed Antigravity runtime fails at the final step. The download, SHA-256 check, extraction and validation all succeed, but the staging → versions rename fails with EPERM. T3 then shows "Could not publish the Antigravity runtime. The previous release is unchanged. Try again." and the provider keeps reporting "Antigravity is not installed."

Environment

• T3 Code (Alpha) 0.0.45, Windows 11 (10.0.26200), win32-x64
• Pinned release: agy_acp_server_1.1.1-windows-x86_64.zip (sha256 47cb50ee…c3b8)

Error (from server.trace.ndjson, span AntigravityInstallation.install)

AntigravityInstallationError: Could not publish the Antigravity runtime. The previous release is unchanged. Try again.
  [cause]: PlatformError: Unknown: FileSystem.rename (C:\Users\<user>\.t3\tools\antigravity-acp\win32-x64\versions\.install-JnRDGt\runtime)
    [cause]: Error: EPERM: operation not permitted, rename
      'C:\Users\<user>\.t3\tools\antigravity-acp\win32-x64\versions\.install-JnRDGt\runtime' ->
      'C:\Users\<user>\.t3\tools\antigravity-acp\win32-x64\versions\47cb50eef14f0a4655d78cfcfda869bcea7aaee5f9787e936bc2935ea612c3b8'

The follow-up completedRelease lookup then fails with NotFound on .install-complete.json, so it doesn't recover.

Likely cause

On Windows a directory can't be renamed while any file inside it has an open handle. Just before the rename, AntigravityInstallation.install runs validate(...) on the exe inside pairDirectory, which spawns agy_acp_server.exe for the ACP initialize handshake. Two candidates:

1. The validation child (or the localharness_external.exe it starts) hasn't fully exited or released its image handle when fs.rename(pairDirectory, destination) runs.
2. Windows Defender / real-time AV is still scanning the freshly written 430 MB executable.

I haven't confirmed which one it is.

Suggested fix

• Retry the rename on EPERM / EBUSY / EACCES with short backoff (the graceful-fs approach, e.g. up to ~10 s total) before failing, and/or
• Make sure the validation process tree has fully exited before publishing, or validate after the rename.

Workaround

Download the same pinned zip manually, check its SHA-256, extract it to any folder, and set Antigravity → custom executable path to the extracted agy_acp_server.exe (localharness_external.exe must sit next to it). After that, resolve and checkAntigravityProvider succeed.