Sample of how I mocked EightPoints guzzle Clients

file1.json

[
    {
        "method": "GET",
        "uri": "/api/v1/customers/e9f09b2b-2a71-4f62-9d1d-05d98f933016",
        "status": 200,
        "data": {
            "id": "e9f09b2b-2a71-4f62-9d1d-05d98f933016"
        }
    }
]

GuzzleClientAware.php

<?php

use GuzzleHttp\HandlerStack;
use GuzzleHttp\Promise\FulfilledPromise;
use GuzzleHttp\Psr7\Response;
use InvalidArgumentException;
use Psr\Container\ContainerInterface;
use Psr\Http\Message\RequestInterface;
use Symfony\Component\Finder\Finder;
use Symfony\Component\Finder\SplFileInfo;

trait GuzzleClientAware
{
    /**
     * @param string|array $path
     *
     * @return array
     *
     * @throws InvalidArgumentException
     */
    public function parseJsonFromPath($path): array
    {
        if (is_array($path)) {
            $decodedJsons = array_map([$this, 'parseJsonFromPath'], $path);
            $json         = call_user_func_array('array_merge', $decodedJsons);
        } else {
            if (is_dir($path)) {
                $finder = new Finder();
                $finder
                    ->in($path)
                    ->files()
                    ->name('*.json')
                    ->sortByName()
                ;

                $files = $finder->getIterator();

                $decodedJsons = array_map(function (SplFileInfo $file) {
                    return json_decode($file->getContents(), true);
                }, iterator_to_array($files));

                $json = call_user_func_array('array_merge', $decodedJsons);
            } elseif (is_file($path)) {
                $json = json_decode(file_get_contents($path), true);
            } else {
                throw new InvalidArgumentException("Path `${path}` does not seems to be valid.");
            }
        }

        return $json;
    }

    /**
     * @param array $parsedJson
     *
     * @return array
     */
    public function parseData(array $parsedJson): array
    {
        $result = [];

        foreach ($parsedJson as $row) {
            $key = $row['method'] . '_' . $row['uri'];

            if (!array_key_exists($key, $result)) {
                $result[$key] = [];
            }

            $result[$key][] = $row;
        }

        return $result;
    }

    /**
     * @param ContainerInterface $container
     * @param string             $clientName
     * @param array              $data
     */
    public function updateGuzzleClient(ContainerInterface $container, string $clientName, array $data): void
    {
        $stackCallback = function (RequestInterface $request, array $options) use ($data) {
            $matches = null;
            preg_match('#(?:.*\.php)?(?P<relative_path>\/.*)#', $request->getUri()->getPath(), $matches);
            $relativePath = $matches['relative_path'];

            $key = $request->getMethod() . '_' . $relativePath;

            if (!array_key_exists($key, $data)) {
                throw new InvalidArgumentException("`$key` not found.");
            }

            if (1 === count($data[$key])) {
                $response = reset($data[$key]);
            } else {
                $response = array_shift($data[$key]);
            }

            return new FulfilledPromise(
                new Response($response['status'], [], json_encode($response['data']))
            );
        };

        $currentClient = $container->get($clientName);

        $currentConfig = $currentClient->getConfig();
        /** @var HandlerStack $stack */
        $stack = $currentConfig['handler'];
        $stack->setHandler($stackCallback);
    }
}

MyControllerTest.php

<?php

use Symfony\Bundle\FrameworkBundle\Client;
use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;

class MyControllerTest extends WebTestCase
{
    use GuzzleClientAware;
    
    /**
     * @var Client
     */
    private $client;  
  
    public function setUp()
    {
        $this->client = static::createClient();
      
        $this->client->disableReboot();
        $this->client->enableProfiler();

        $container = $this->client->getContainer();

        $this->updateGuzzleClient($container, 'CLIENT_NAME', $this->parseData($this->parseJsonFromPath([
            __DIR__ . '/../Resources/data/CLIENT_NAME/file1.json',
            __DIR__ . '/../Resources/data/CLIENT_NAME/dir1'
        ])));
    }
}