diff --git a/packages/cli-kit/src/public/node/is-global.test.ts b/packages/cli-kit/src/public/node/is-global.test.ts
index 4e2e0b91e9..53bd98352f 100644
--- a/packages/cli-kit/src/public/node/is-global.test.ts
+++ b/packages/cli-kit/src/public/node/is-global.test.ts
@@ -81,6 +81,23 @@ describe('currentProcessIsGlobal', () => {
     // Then
     expect(got).toBeFalsy()
   })
+
+  test('returns false on Windows when argv uses backslashes and projectDir uses forward slashes', () => {
+    // Given - mimic the exact Windows shape:
+    //   projectDir comes from pathe -> normalized forward slashes
+    //   argv[1] is OS-native -> backslash separated
+    const winProjectDir = 'C:/Users/me/project'
+    const winArgv1 = 'C:\\Users\\me\\project\\node_modules\\@shopify\\cli\\bin\\run.js'
+    vi.mocked(findPathUpSync).mockReturnValue(`${winProjectDir}/shopify.app.toml`)
+    const argv = ['node', winArgv1, 'shopify']
+
+    // When
+    const got = currentProcessIsGlobal(argv)
+
+    // Then - regression test for the path-separator bug that misclassified
+    // Windows local installs as global, triggering an unwanted `npm install -g`.
+    expect(got).toBeFalsy()
+  })
 })
 
 describe('inferPackageManagerForGlobalCLI', () => {
diff --git a/packages/cli-kit/src/public/node/is-global.ts b/packages/cli-kit/src/public/node/is-global.ts
index ac0a1e6895..068e61f95f 100644
--- a/packages/cli-kit/src/public/node/is-global.ts
+++ b/packages/cli-kit/src/public/node/is-global.ts
@@ -1,4 +1,4 @@
-import {cwd, dirname, joinPath, sniffForPath} from './path.js'
+import {cwd, dirname, isSubpath, joinPath, sniffForPath} from './path.js'
 import {isUnitTest} from './context/local.js'
 import {findPathUpSync, globSync} from './fs.js'
 import {realpathSync} from 'fs'
@@ -27,9 +27,17 @@ export function currentProcessIsGlobal(argv = process.argv): boolean {
 
     // From node docs: "The second element [of the array] will be the path to the JavaScript file being executed"
     const binDir = argv[1] ?? ''
+    if (!binDir) {
+      return true
+    }
 
-    // If binDir starts with projectDir, then we are running a local CLI
-    const isLocal = binDir.startsWith(projectDir.trim())
+    // If binDir lives inside projectDir, we are running a local CLI.
+    // Use isSubpath (pathe.relative under the hood) instead of a raw
+    // string startsWith: projectDir flows through normalizePath and is
+    // forward-slash on every platform, while argv[1] is OS-native, so on
+    // Windows it arrives backslash-separated and a naive startsWith would
+    // misclassify a local install as global.
+    const isLocal = isSubpath(projectDir.trim(), binDir)
 
     _isGlobal = !isLocal
     return _isGlobal