diff --git a/cmd/core/ensure_image_test.go b/cmd/core/ensure_image_test.go
index 5eb2414e..5dde2b4d 100644
--- a/cmd/core/ensure_image_test.go
+++ b/cmd/core/ensure_image_test.go
@@ -98,6 +98,55 @@ func TestEnsureImage_ForceWhenDigestPinned(t *testing.T) {
 	}
 }
 
+// A cloudimg ref without an http(s) scheme reaching Pull surfaces as
+// `unsupported protocol scheme` from http.Get; the shape guard short-circuits.
+func TestEnsureImage_SkipsBadShape(t *testing.T) {
+	tests := []struct {
+		name      string
+		image     string
+		imageType string
+	}{
+		{"cloudimg bare OCI ref", "simular/win10:22h2-20260510", types.ImageTypeCloudImg},
+		{"cloudimg local name", "win11", types.ImageTypeCloudImg},
+		{"cloudimg non-http scheme", "file:///foo.img", types.ImageTypeCloudImg},
+		{"oci malformed ref", "::bad::", types.ImageTypeOCI},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			f := &fakeImageBackend{typ: tt.imageType}
+			EnsureImage(t.Context(), []imagebackend.Images{f}, &types.VMConfig{
+				Config: types.Config{Image: tt.image, ImageType: tt.imageType},
+			})
+			if len(f.pullRefs) != 0 {
+				t.Errorf("Pull called %d time(s) with %v, want 0 (shape should have failed)", len(f.pullRefs), f.pullRefs)
+			}
+		})
+	}
+}
+
+// Acceptance counterpart: well-formed refs must reach Pull.
+func TestEnsureImage_AcceptsGoodShape(t *testing.T) {
+	tests := []struct {
+		name      string
+		image     string
+		imageType string
+	}{
+		{"cloudimg https url", "https://cloud-images.ubuntu.com/x.img", types.ImageTypeCloudImg},
+		{"oci tagged ref", "ghcr.io/cocoonstack/cocoon/ubuntu:24.04", types.ImageTypeOCI},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			f := &fakeImageBackend{typ: tt.imageType}
+			EnsureImage(t.Context(), []imagebackend.Images{f}, &types.VMConfig{
+				Config: types.Config{Image: tt.image, ImageType: tt.imageType},
+			})
+			if len(f.pullRefs) != 1 || f.pullRefs[0] != tt.image {
+				t.Errorf("Pull = %v, want one call for %q", f.pullRefs, tt.image)
+			}
+		})
+	}
+}
+
 func TestEnsureImage_SkipsPullWhenDigestLocal(t *testing.T) {
 	const digest = "sha256:adafd938488daa114be898848eb24b9b0afffc21ac18f8b11f3f0057644b11e1"
 	f := &fakeImageBackend{
diff --git a/cmd/core/helpers.go b/cmd/core/helpers.go
index f49f81f5..b707ad22 100644
--- a/cmd/core/helpers.go
+++ b/cmd/core/helpers.go
@@ -248,6 +248,10 @@ func EnsureImage(ctx context.Context, backends []imagebackend.Images, vmCfg *typ
 		// Pull by digest reference when available — ensures we get the exact
 		// version recorded at snapshot time, not whatever the tag points to now.
 		pullRef := digestPullRef(vmCfg.Image, vmCfg.ImageDigest, vmCfg.ImageType)
+		if shapeErr := validateRefShape(pullRef, vmCfg.ImageType); shapeErr != nil {
+			logger.Warnf(ctx, "skipping auto-pull of %s: %v — pre-pull manually if missing", pullRef, shapeErr)
+			return
+		}
 		logger.Infof(ctx, "base image not found locally, pulling %s ...", pullRef)
 		// Pinned digest with no local hit: force past cloudimg's URL-keyed cache.
 		needForce := vmCfg.ImageDigest != ""
@@ -491,6 +495,25 @@ func IsURL(ref string) bool {
 	return strings.HasPrefix(ref, "http://") || strings.HasPrefix(ref, "https://")
 }
 
+// validateRefShape catches malformed base-image refs before they hit a backend
+// that would surface a misleading downstream error. cloudimg fetches over HTTP
+// (ref must start with http:// or https://); OCI pulls via registry protocol
+// (ref must parse via name.ParseReference). A bare OCI ref leaking into the
+// cloudimg path would otherwise surface as `unsupported protocol scheme ""`.
+func validateRefShape(ref, imageType string) error {
+	switch imageType {
+	case types.ImageTypeCloudImg:
+		if !IsURL(ref) {
+			return fmt.Errorf("cloudimg ref %q is not an http(s) URL (imported or bare OCI ref?)", ref)
+		}
+	case types.ImageTypeOCI:
+		if _, err := name.ParseReference(ref); err != nil {
+			return fmt.Errorf("oci ref %q is not a valid OCI reference: %w", ref, err)
+		}
+	}
+	return nil
+}
+
 // digestPullRef pins OCI pulls by digest; returns image as-is for others.
 func digestPullRef(image, digest, imageType string) string {
 	if digest == "" || imageType != types.ImageTypeOCI {