fix(isolation): apply the configured registry to namespaced images

internal/upstream/core/isolation.go

@@ -472,8 +472,13 @@ func (im *IsolationManager) ResolveDefaults(serverConfig *config.ServerConfig) *
 
 // buildFullImageName constructs the full image name with registry if needed
 func (im *IsolationManager) buildFullImageName(image string) string {
-	// If image already contains a registry (has a slash before the first colon), use as-is
-	if strings.Contains(image, "/") && strings.Index(image, "/") < strings.Index(image, ":") {
+	if imageHasRegistryHost(image) {
+		return image
+	}
+
+	// Without a configured registry, a namespaced Docker Hub reference
+	// (org/image) is already pullable as written.
+	if im.globalConfig.Registry == "" && strings.Contains(image, "/") {
 		return image
 	}
 
@@ -483,11 +488,6 @@ func (im *IsolationManager) buildFullImageName(image string) string {
 		registry = "docker.io"
 	}
 
-	// Don't prepend registry if image already contains one
-	if strings.Contains(image, "/") {
-		return image
-	}
-
 	// For official images (no slash), prepend library/
 	if !strings.Contains(image, "/") {
 		return fmt.Sprintf("%s/library/%s", registry, image)
@@ -496,6 +496,18 @@ func (im *IsolationManager) buildFullImageName(image string) string {
 	return fmt.Sprintf("%s/%s", registry, image)
 }
 
+// imageHasRegistryHost applies Docker's reference rule: the first path
+// component names a registry only if it contains '.' or ':' or is
+// "localhost"; otherwise it is a Docker Hub namespace (org/image).
+func imageHasRegistryHost(image string) bool {
+	i := strings.Index(image, "/")
+	if i < 0 {
+		return false
+	}
+	host := image[:i]
+	return strings.ContainsAny(host, ".:") || host == "localhost"
+}
+
 // BuildDockerArgs constructs Docker run arguments for isolation
 func (im *IsolationManager) BuildDockerArgs(serverConfig *config.ServerConfig, runtimeType string) ([]string, error) {
 	image, err := im.GetDockerImage(serverConfig, runtimeType)

internal/upstream/core/isolation_registry_namespace_test.go

@@ -0,0 +1,30 @@
+package core
+
+import "testing"
+
+func TestBuildFullImageName_ConfiguredRegistryPrefixesNamespacedImages(t *testing.T) {
+	mirror := registryOnlyManager("mirror.corp", nil)
+	cases := map[string]string{
+		"myorg/node-mcp:1":              "mirror.corp/myorg/node-mcp:1",
+		"python:3.11":                   "mirror.corp/library/python:3.11",
+		"ghcr.io/org/img:2":             "ghcr.io/org/img:2",
+		"localhost:5000/team/img":       "localhost:5000/team/img",
+		"localhost/team/img":            "localhost/team/img",
+		"registry.local/img@sha256:abc": "registry.local/img@sha256:abc",
+	}
+	for in, want := range cases {
+		if got := mirror.buildFullImageName(in); got != want {
+			t.Errorf("registry=mirror.corp: buildFullImageName(%q) = %q, want %q", in, got, want)
+		}
+	}
+
+	plain := registryOnlyManager("", nil)
+	for in, want := range map[string]string{
+		"myorg/node-mcp:1": "myorg/node-mcp:1",
+		"python:3.11":      "docker.io/library/python:3.11",
+	} {
+		if got := plain.buildFullImageName(in); got != want {
+			t.Errorf("no registry: buildFullImageName(%q) = %q, want %q", in, got, want)
+		}
+	}
+}