diff --git a/scripts/start-local.ps1 b/scripts/start-local.ps1
index 857bb23..d2fee5a 100644
--- a/scripts/start-local.ps1
+++ b/scripts/start-local.ps1
@@ -280,11 +280,36 @@ Reserved ranges: netsh interface ipv4 show excludedportrange protocol=tcp
 function Install-IfMissing {
     param([string]$Directory)
     $name = Split-Path -Leaf $Directory
-    if (Test-Path (Join-Path $Directory 'node_modules')) {
-        Write-Note "$name dependencies already installed"
-        return
+
+    # Whether node_modules matches the lockfile, not merely whether it exists.
+    #
+    # This used to ask only `Test-Path node_modules`, which meant a branch that
+    # ADDED a dependency never installed it for anyone who already had the
+    # directory — and almost everyone always does. The build then failed on
+    # "Cannot find module", naming a package that is right there in
+    # package.json, which reads as a broken checkout rather than a missing
+    # install. It cost an afternoon the first time #37 added @simplewebauthn.
+    #
+    # npm writes node_modules/.package-lock.json describing exactly what it put
+    # there, so comparing its timestamp against package-lock.json answers the
+    # real question: is what is installed what is currently asked for. A pull
+    # that changes dependencies makes the lockfile newer, and this notices.
+    $lockfile = Join-Path $Directory 'package-lock.json'
+    $installed = Join-Path $Directory 'node_modules/.package-lock.json'
+
+    if ((Test-Path $installed) -and (Test-Path $lockfile)) {
+        $lockTime = (Get-Item $lockfile).LastWriteTimeUtc
+        $installedTime = (Get-Item $installed).LastWriteTimeUtc
+        if ($installedTime -ge $lockTime) {
+            Write-Note "$name dependencies are up to date"
+            return
+        }
+        Write-Step "Installing $name dependencies (the lockfile has changed)"
     }
-    Write-Step "Installing $name dependencies"
+    else {
+        Write-Step "Installing $name dependencies"
+    }
+
     Push-Location $Directory
     try { Invoke-Checked { npm install } "$name npm install" } finally { Pop-Location }
 }