diff --git a/mcp/client_command_test.go b/mcp/client_command_test.go
index 0d244c50..1ce5baea 100644
--- a/mcp/client_command_test.go
+++ b/mcp/client_command_test.go
@@ -28,7 +28,7 @@ func TestRenderClientListEscapesMarkdownCells(t *testing.T) {
 }
 
 func TestAddOAuthNoVerifyPersistsConfiguration(t *testing.T) {
-	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	isolateConfigHome(t)
 	output, err := executeMCPCommand(
 		"add", "private", "https://example.com/mcp",
 		"--oauth-client-id", "client-1",
@@ -53,7 +53,7 @@ func TestAddOAuthNoVerifyPersistsConfiguration(t *testing.T) {
 }
 
 func TestAddNoBrowserRequiresOAuth(t *testing.T) {
-	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	isolateConfigHome(t)
 	_, err := executeMCPCommand("add", "public", "https://example.com/mcp", "--no-browser", "--no-verify")
 	if err == nil || !strings.Contains(err.Error(), "requires --oauth") {
 		t.Fatalf("error = %v", err)
@@ -61,8 +61,7 @@ func TestAddNoBrowserRequiresOAuth(t *testing.T) {
 }
 
 func TestAddRejectsInvalidNameBeforeOAuthStorageBinding(t *testing.T) {
-	configHome := t.TempDir()
-	t.Setenv("XDG_CONFIG_HOME", configHome)
+	isolateConfigHome(t)
 	_, err := executeMCPCommand("add", "../config", "https://example.com/mcp", "--oauth", "--no-verify")
 	if err == nil || !strings.Contains(err.Error(), "invalid server name") {
 		t.Fatalf("error = %v", err)
@@ -74,7 +73,7 @@ func TestAddRejectsInvalidNameBeforeOAuthStorageBinding(t *testing.T) {
 }
 
 func TestAddRejectsLiteralOAuthSecret(t *testing.T) {
-	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	isolateConfigHome(t)
 	_, err := executeMCPCommand(
 		"add", "private", "https://example.com/mcp", "--oauth-client-id", "client", "--oauth-client-secret", "literal", "--no-verify",
 	)
diff --git a/mcp/client_e2e_test.go b/mcp/client_e2e_test.go
index b5ec1341..8a70c050 100644
--- a/mcp/client_e2e_test.go
+++ b/mcp/client_e2e_test.go
@@ -13,7 +13,7 @@ import (
 )
 
 func TestClientCommandsStdioAndOfflineHelp(t *testing.T) {
-	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	isolateConfigHome(t)
 	workingDir, err := os.Getwd()
 	if err != nil {
 		t.Fatal(err)
diff --git a/mcp/client_oauth_test.go b/mcp/client_oauth_test.go
index 5f9f5fb8..cdca552f 100644
--- a/mcp/client_oauth_test.go
+++ b/mcp/client_oauth_test.go
@@ -138,7 +138,7 @@ func testOAuthLoginUsesOIDCDiscoveryAndRefreshes(t *testing.T, metadataOverride
 	defer httpServer.Close()
 	baseURL = httpServer.URL
 
-	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	isolateConfigHome(t)
 	registry := NewServerRegistry("testapp")
 	oauthConfig := &OAuthClientConfig{Scopes: append([]string(nil), scopes...)}
 	if metadataOverride {
diff --git a/mcp/client_registry_test.go b/mcp/client_registry_test.go
index 2b0158a4..66526954 100644
--- a/mcp/client_registry_test.go
+++ b/mcp/client_registry_test.go
@@ -10,6 +10,18 @@ import (
 	"time"
 )
 
+// isolateConfigHome points NewServerRegistry at a per-test directory on every
+// platform. os.UserConfigDir only reads XDG_CONFIG_HOME on unix-like systems;
+// on darwin it derives ~/Library/Application Support from HOME, so setting only
+// XDG_CONFIG_HOME leaks registry, cache, and OAuth state into the real user
+// config directory and makes later runs fail on stale state.
+func isolateConfigHome(t *testing.T) {
+	t.Helper()
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+	t.Setenv("XDG_CONFIG_HOME", filepath.Join(home, ".config"))
+}
+
 func TestServerConfigValidate(t *testing.T) {
 	tests := []struct {
 		name string
diff --git a/mcp/client_run_test.go b/mcp/client_run_test.go
index 350733dc..0fb5dad7 100644
--- a/mcp/client_run_test.go
+++ b/mcp/client_run_test.go
@@ -10,7 +10,7 @@ import (
 )
 
 func TestRunShortHelpListsServers(t *testing.T) {
-	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	isolateConfigHome(t)
 	registry := NewServerRegistry("testapp")
 	if err := registry.Add("demo", ServerConfig{Type: "stdio", Command: "server"}); err != nil {
 		t.Fatal(err)