Behat bug?

behay.yml

default:
    suites:
      default:
        contexts:
          - FeatureContext
          - behatch:rest
          - behatch:browser
    extensions:
        Behat\Symfony2Extension:
          kernel:
            env: test
            debug: true
        Behat\MinkExtension:
          base_url:   ~
          default_session: my_session
          sessions:
            my_session:
              symfony2: ~
        VIPSoft\DoctrineDataFixturesExtension\Extension:
          lifetime:    scenario
          autoload:    true
          directories: ~
          fixtures:    ~
        Sanpi\Behatch\Extension: ~

composer.json

        "behat/behat": "*",
        "sensio/generator-bundle": "~2.3",
        "phpspec/phpspec": "~2.0@dev",
        "behat/symfony2-extension": "dev-master",
        "behatch/contexts": "dev-master",
        "behat/mink": "1.*@stable",
        "behat/mink-extension": "*",
        "behat/mink-browserkit-driver":  "*",
        "behat/mink-goutte-driver": "*",
        "behat/mink-sahi-driver": "*",

config_test.yml

imports:
    - { resource: config_dev.yml }

framework:
    test: ~
    session:
        storage_id: session.storage.native
        handler_id: session.handler.native_file
    profiler:
        collect: false

web_profiler:
    toolbar: false
    intercept_redirects: false

swiftmailer:
    disable_delivery: true


doctrine:
    dbal:
        driver:   "pdo_sqlite"
        path:     "%kernel.cache_dir%/test.db"
        charset:  UTF8

liip_functional_test:
    cache_sqlite_db: true

Error

Cannot change the ID of an active session (500 Internal Server Error)

feature

  Scenario: Puedo borrar un objeto Tag por su ID
    Given I count the "Tag" objects
    And I select one "Tag" ID
    When I POST to "/admin/tag/delete" with:
      |name         | type                | value           |
      |tag_id       | string              | {Tag-selected}  |
      |_token       | csrf                |                 |

    Then print last response
    Then the response status code should be 200
    And I have 1 "less" "Tag" objects

FeatureContext.php

class FeatureContext extends MinkContext implements Context, SnippetAcceptingContext, KernelAwareContext {
    
.....

     /**
     * @When I POST to :url a form :form_name with:
     */
    public function iPostToAFormWith($url, $form_name, TableNode $parametersTable)
    {
        $driver = $this->getSession()->getDriver();
        if (!$driver instanceof BrowserKitDriver) {
            throw new \Exception('This step is only supported by the BrowserKitDriver');
        }
        $params = array();
        $files = array();
        $client = $driver->getClient();
        foreach ($parametersTable as $row) {
            switch ($row['type']) {
                case "string":
                    $text = $this->replaceIdsInString($row['value']);
                    $params[$form_name][$row['name']] = $text;
                    break;
                case "boolean":
                    $params[$form_name][$row['name']] = $row['value'];
                    break;
                case "file":
                    $fileUpload = new UploadedFile($row['value'], 'the_file_name');
                    $files[$form_name][$row['name']] = $fileUpload;
                    break;
                case "csrf":
                    $token = $client->getContainer()->get('form.csrf_provider')->generateCsrfToken($form_name);
                    $params[$form_name][$row['name']] = $token;
                    break;
            }
        }
        $client->request('POST', $url, $params, $files);
    }
    
....

If csrf token is set then error is raised

This comes from Symfony, not behat: https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/HttpFoundation/Session/Storage/Proxy/AbstractProxy.php#L123

Do you have any custom listeners that require session?

see symfony/symfony#13450

Background:
Given I am authenticated as "admin"

/**
* @given I am authenticated as :username
*/
public function iAmAuthenticatedAs($username)
{
$driver = $this->getSession()->getDriver();
if (!$driver instanceof BrowserKitDriver) {
throw new \Exception('This step is only supported by the BrowserKitDriver');
}

    $client = $driver->getClient();
    $client->getCookieJar()->set(new Cookie(session_name(), true));

    $session = $this->client->getContainer()->get('session');

    $user = $this->kernel->getContainer()->get('fos_user.user_manager')->findUserByUsername($username);
    $providerKey = $this->kernel->getContainer()->getParameter('fos_user.firewall_name');

    $token = new UsernamePasswordToken($user, null, $providerKey, $user->getRoles());
    $session->set('_security_'.$providerKey, serialize($token));
    $session->save();

    $cookie = new Cookie($session->getName(), $session->getId());
    $this->client->getCookieJar()->set($cookie);
}