diff --git a/src/relay/fs-handler.ts b/src/relay/fs-handler.ts
index c9f358fd325..bb2ca22974c 100644
--- a/src/relay/fs-handler.ts
+++ b/src/relay/fs-handler.ts
@@ -8,10 +8,10 @@ import {
   rename,
   cp,
   rm,
-  realpath,
-  access
+  realpath
 } from 'fs/promises'
-import { extname, join } from 'path'
+import { extname } from 'path'
+import { execFile } from 'child_process'
 import type { RelayDispatcher } from './dispatcher'
 import type { RelayContext } from './context'
 import { expandTilde } from './context'
@@ -233,13 +233,16 @@ export class FsHandler {
     if (rgAvailable) {
       return listFilesWithRg(rootPath)
     }
-    // Why: git ls-files only works inside git repos. Check for .git first so
-    // we don't misclassify a git repo with only ignored files as a non-git
-    // folder — that would surface .gitignore'd files via the readdir fallback.
-    const isGitRepo = await access(join(rootPath, '.git')).then(
-      () => true,
-      () => false
-    )
+    // Why: git ls-files only works inside git repos. Use rev-parse to detect
+    // git ancestry — unlike checking for a local .git entry, this works from
+    // subdirectories of a checkout (e.g. /repo/packages/app added as a folder).
+    // Without this, a git subdirectory would fall through to readdir and
+    // surface .gitignore'd build artifacts.
+    const isGitRepo = await new Promise<boolean>((resolve) => {
+      execFile('git', ['rev-parse', '--is-inside-work-tree'], { cwd: rootPath }, (err) =>
+        resolve(!err)
+      )
+    })
     if (isGitRepo) {
       return listFilesWithGit(rootPath)
     }