You can access the original ResponseInterface object via its getter.
public function testCtorDefaults() { $response = new Response(); $response = $response->withStatus(404, 'File not found'); $e = new ResponseException($response); $this->assertEquals(404, $e->getCode()); $this->assertEquals('HTTP status code 404 (File not found)', $e->getMessage()); $this->assertSame($response, $e->getResponse()); }