diff --git a/src/Server/Helper.php b/src/Server/Helper.php
index f4ff7a8..621a060 100644
--- a/src/Server/Helper.php
+++ b/src/Server/Helper.php
@@ -37,7 +37,7 @@ class Helper
             if (!$doc instanceof DocumentNode) {
                 $doc = Parser::parse($doc);
             }
-            if (!$op->allowsMutation() && AST::isMutation($op->operation, $doc)) {
+            if ($op->isReadOnly() && AST::isMutation($op->operation, $doc)) {
                 throw new UserError("Cannot execute mutation in read-only context");
             }
 
@@ -133,47 +133,35 @@ class Helper
         return $validationRules;
     }
 
-
     /**
      * Parses HTTP request and returns GraphQL QueryParams contained in this request.
      * For batched requests it returns an array of QueryParams.
      *
-     * @return OperationParams|OperationParams[]
-     */
-    public function parseHttpRequest()
-    {
-        list ($parsedBody, $isReadonly) = $this->parseRawBody();
-        return $this->toOperationParams($parsedBody, $isReadonly);
-    }
-
-    /**
-     * Extracts parsed body and readonly flag from HTTP request
+     * This function doesn't check validity of these params.
      *
      * If $readRawBodyFn argument is not provided - will attempt to read raw request body from php://input stream
      *
      * @param callable|null $readRawBodyFn
-     * @return array
+     * @return OperationParams|OperationParams[]
      */
-    public function parseRawBody(callable $readRawBodyFn = null)
+    public function parseHttpRequest(callable $readRawBodyFn = null)
     {
         $method = isset($_SERVER['REQUEST_METHOD']) ? $_SERVER['REQUEST_METHOD'] : null;
 
         if ($method === 'GET') {
-            $isReadonly = true;
             $request = array_change_key_case($_GET);
 
             if (isset($request['query']) || isset($request['queryid']) || isset($request['documentid'])) {
-                $body = $_GET;
+                $result = OperationParams::create($_GET, true);
             } else {
                 throw new UserError('Cannot execute GET request without "query" or "queryId" parameter');
             }
         } else if ($method === 'POST') {
-            $isReadonly = false;
             $contentType = isset($_SERVER['CONTENT_TYPE']) ? $_SERVER['CONTENT_TYPE'] : null;
 
             if (stripos($contentType, 'application/graphql') !== false) {
                 $rawBody =  $readRawBodyFn ? $readRawBodyFn() : $this->readRawBody();
-                $body = ['query' => $rawBody ?: ''];
+                $result = OperationParams::create(['query' => $rawBody ?: '']);
             } else if (stripos($contentType, 'application/json') !== false) {
                 $rawBody = $readRawBodyFn ? $readRawBodyFn() : $this->readRawBody();
                 $body = json_decode($rawBody ?: '', true);
@@ -187,8 +175,17 @@ class Helper
                         Utils::printSafeJson($body)
                     );
                 }
+                if (isset($body[0])) {
+                    $result = [];
+                    foreach ($body as $index => $entry) {
+                        $op = OperationParams::create($entry, true);
+                        $result[] = $op;
+                    }
+                } else {
+                    $result = OperationParams::create($body);
+                }
             } else if (stripos($contentType, 'application/x-www-form-urlencoded') !== false) {
-                $body = $_POST;
+                $result = OperationParams::create($_POST);
             } else if (null === $contentType) {
                 throw new UserError('Missing "Content-Type" header');
             } else {
@@ -197,41 +194,6 @@ class Helper
         } else {
             throw new UserError('HTTP Method "' . $method . '" is not supported', 405);
         }
-        return [
-            $body,
-            $isReadonly
-        ];
-    }
-
-    /**
-     * Converts parsed body to OperationParams (or list of OperationParams for batched request)
-     *
-     * @param $parsedBody
-     * @param $isReadonly
-     * @return OperationParams|OperationParams[]
-     */
-    public function toOperationParams($parsedBody, $isReadonly)
-    {
-        $assertValid = function (OperationParams $opParams, $queryNum = null) {
-            $errors = $opParams->validate();
-            if (!empty($errors[0])) {
-                $err = $queryNum ? "Error in query #$queryNum: {$errors[0]}" : $errors[0];
-                throw new UserError($err);
-            }
-        };
-
-        if (isset($parsedBody[0])) {
-            // Batched query
-            $result = [];
-            foreach ($parsedBody as $index => $entry) {
-                $op = OperationParams::create($entry, $isReadonly);
-                $assertValid($op, $index);
-                $result[] = $op;
-            }
-        } else {
-            $result = OperationParams::create($parsedBody, $isReadonly);
-            $assertValid($result);
-        }
         return $result;
     }
 
@@ -243,46 +205,81 @@ class Helper
         return file_get_contents('php://input');
     }
 
+    /**
+     * Checks validity of operation params and returns array of errors (empty array when params are valid)
+     *
+     * @param OperationParams $params
+     * @return array
+     */
+    public function validateOperationParams(OperationParams $params)
+    {
+        $errors = [];
+        if (!$params->query && !$params->queryId) {
+            $errors[] = 'GraphQL Request must include at least one of those two parameters: "query" or "queryId"';
+        }
+        if ($params->query && $params->queryId) {
+            $errors[] = 'GraphQL Request parameters "query" and "queryId" are mutually exclusive';
+        }
+
+        if ($params->query !== null && (!is_string($params->query) || empty($params->query))) {
+            $errors[] = 'GraphQL Request parameter "query" must be string, but got ' .
+                Utils::printSafeJson($params->query);
+        }
+        if ($params->queryId !== null && (!is_string($params->queryId) || empty($params->queryId))) {
+            $errors[] = 'GraphQL Request parameter "queryId" must be string, but got ' .
+                Utils::printSafeJson($params->queryId);
+        }
+
+        if ($params->operation !== null && (!is_string($params->operation) || empty($params->operation))) {
+            $errors[] = 'GraphQL Request parameter "operation" must be string, but got ' .
+                Utils::printSafeJson($params->operation);
+        }
+        if ($params->variables !== null && (!is_array($params->variables) || isset($params->variables[0]))) {
+            $errors[] = 'GraphQL Request parameter "variables" must be object, but got ' .
+                Utils::printSafeJson($params->variables);
+        }
+        return $errors;
+    }
+
     /**
      * Assertion to check that parsed body is valid instance of OperationParams (or array of instances)
      *
-     * @param $method
-     * @param $parsedBody
+     * @param OperationParams|OperationParams[] $parsedBody
+     * @throws InvariantViolation
+     * @throws UserError
      */
-    public function assertBodyIsParsedProperly($method, $parsedBody)
+    public function assertValidRequest($parsedBody)
     {
         if (is_array($parsedBody)) {
             foreach ($parsedBody as $index => $entry) {
                 if (!$entry instanceof OperationParams) {
                     throw new InvariantViolation(sprintf(
-                        '%s expects instance of %s or array of instances. Got invalid array where entry at position %d is %s',
-                        $method,
+                        'GraphQL Server: Parsed http request must be an instance of %s or array of such instances, '.
+                        'but got invalid array where entry at position %d is %s',
                         OperationParams::class,
                         $index,
                         Utils::printSafe($entry)
                     ));
                 }
-                $errors = $entry->validate();
+
+                $errors = $this->validateOperationParams($entry);
 
                 if (!empty($errors[0])) {
                     $err = $index ? "Error in query #$index: {$errors[0]}" : $errors[0];
-                    throw new InvariantViolation($err);
+                    throw new UserError($err);
                 }
             }
-        }
-
-        if ($parsedBody instanceof OperationParams) {
-            $errors = $parsedBody->validate();
+        } else if ($parsedBody instanceof OperationParams) {
+            $errors = $this->validateOperationParams($parsedBody);
             if (!empty($errors[0])) {
-                throw new InvariantViolation($errors[0]);
+                throw new UserError($errors[0]);
             }
+        } else {
+            throw new InvariantViolation(sprintf(
+                'GraphQL Server: Parsed http request must be an instance of %s or array of such instances, but got %s',
+                OperationParams::class,
+                Utils::printSafe($parsedBody)
+            ));
         }
-
-        throw new InvariantViolation(sprintf(
-            '%s expects instance of %s or array of instances, but got %s',
-            $method,
-            OperationParams::class,
-            Utils::printSafe($parsedBody)
-        ));
     }
 }
diff --git a/src/Server/OperationParams.php b/src/Server/OperationParams.php
index e81982d..d701f4f 100644
--- a/src/Server/OperationParams.php
+++ b/src/Server/OperationParams.php
@@ -39,17 +39,17 @@ class OperationParams
     /**
      * @var bool
      */
-    private $allowsMutations;
+    private $readOnly;
 
     /**
      * Creates an instance from given array
      *
      * @param array $params
-     * @param bool $allowsMutations
+     * @param bool $readonly
      *
      * @return static
      */
-    public static function create(array $params, $allowsMutations = true)
+    public static function create(array $params, $readonly = false)
     {
         $instance = new static();
         $instance->originalInput = $params;
@@ -68,7 +68,7 @@ class OperationParams
         $instance->queryId = $params['queryid'] ?: $params['documentid'];
         $instance->operation = $params['operation'];
         $instance->variables = $params['variables'];
-        $instance->allowsMutations = (bool) $allowsMutations;
+        $instance->readOnly = (bool) $readonly;
 
         return $instance;
     }
@@ -76,36 +76,6 @@ class OperationParams
     /**
      * @return array
      */
-    public function validate()
-    {
-        $errors = [];
-        if (!$this->query && !$this->queryId) {
-            $errors[] = 'GraphQL Request must include at least one of those two parameters: "query" or "queryId"';
-        }
-        if ($this->query && $this->queryId) {
-            $errors[] = 'GraphQL Request parameters "query" and "queryId" are mutually exclusive';
-        }
-
-        if ($this->query !== null && (!is_string($this->query) || empty($this->query))) {
-            $errors[] = 'GraphQL Request parameter "query" must be string, but got ' .
-                Utils::printSafeJson($this->query);
-        }
-        if ($this->queryId !== null && (!is_string($this->queryId) || empty($this->queryId))) {
-            $errors[] = 'GraphQL Request parameter "queryId" must be string, but got ' .
-                Utils::printSafeJson($this->queryId);
-        }
-
-        if ($this->operation !== null && (!is_string($this->operation) || empty($this->operation))) {
-            $errors[] = 'GraphQL Request parameter "operation" must be string, but got ' .
-                Utils::printSafeJson($this->operation);
-        }
-        if ($this->variables !== null && (!is_array($this->variables) || isset($this->variables[0]))) {
-            $errors[] = 'GraphQL Request parameter "variables" must be object, but got ' .
-                Utils::printSafeJson($this->variables);
-        }
-        return $errors;
-    }
-
     public function getOriginalInput()
     {
         return $this->originalInput;
@@ -114,8 +84,8 @@ class OperationParams
     /**
      * @return bool
      */
-    public function allowsMutation()
+    public function isReadOnly()
     {
-        return $this->allowsMutations;
+        return $this->readOnly;
     }
 }
diff --git a/src/Server/StandardServer.php b/src/Server/StandardServer.php
index 632dd0f..2cd0c1c 100644
--- a/src/Server/StandardServer.php
+++ b/src/Server/StandardServer.php
@@ -54,10 +54,9 @@ class StandardServer
     public function executeRequest($parsedBody = null)
     {
         if (null !== $parsedBody) {
-            $this->helper->assertBodyIsParsedProperly(__METHOD__, $parsedBody);
-        } else {
             $parsedBody = $this->helper->parseHttpRequest();
         }
+        $this->helper->assertValidRequest($parsedBody);
 
         $batched = is_array($parsedBody);
 
diff --git a/tests/Server/RequestParsingTest.php b/tests/Server/RequestParsingTest.php
index 62ad5a1..d47e5e9 100644
--- a/tests/Server/RequestParsingTest.php
+++ b/tests/Server/RequestParsingTest.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Tests\Server;
 
+use GraphQL\Error\InvariantViolation;
 use GraphQL\Error\UserError;
 use GraphQL\Server\Helper;
 use GraphQL\Server\OperationParams;
@@ -10,16 +11,16 @@ use GraphQL\Server\OperationParams;
  */
 class RequestParsingTest extends \PHPUnit_Framework_TestCase
 {
-    public function testParsesSimpleGraphqlRequest()
+    public function testParsesGraphqlRequest()
     {
         $query = '{my query}';
-        list ($parsedBody, $isReadonly) = $this->parseRawRequest('application/graphql', $query);
+        $parsedBody = $this->parseRawRequest('application/graphql', $query);
 
-        $this->assertSame(['query' => $query], $parsedBody);
-        $this->assertFalse($isReadonly);
+        $this->assertValidOperationParams($parsedBody, $query);
+        $this->assertFalse($parsedBody->isReadOnly());
     }
 
-    public function testParsesSimpleUrlencodedRequest()
+    public function testParsesUrlencodedRequest()
     {
         $query = '{my query}';
         $variables = ['test' => 1, 'test2' => 2];
@@ -31,12 +32,12 @@ class RequestParsingTest extends \PHPUnit_Framework_TestCase
             'operation' => $operation
         ];
 
-        list ($parsedBody, $isReadonly) = $this->parseFormUrlencodedRequest($post);
-        $this->assertSame($post, $parsedBody);
-        $this->assertFalse($isReadonly);
+        $parsedBody = $this->parseFormUrlencodedRequest($post);
+        $this->assertValidOperationParams($parsedBody, $query, null, $variables, $operation);
+        $this->assertFalse($parsedBody->isReadOnly());
     }
 
-    public function testParsesSimpleGETRequest()
+    public function testParsesGetRequest()
     {
         $query = '{my query}';
         $variables = ['test' => 1, 'test2' => 2];
@@ -48,12 +49,12 @@ class RequestParsingTest extends \PHPUnit_Framework_TestCase
             'operation' => $operation
         ];
 
-        list ($parsedBody, $isReadonly) = $this->parseGetRequest($get);
-        $this->assertSame($get, $parsedBody);
-        $this->assertTrue($isReadonly);
+        $parsedBody = $this->parseGetRequest($get);
+        $this->assertValidOperationParams($parsedBody, $query, null, $variables, $operation);
+        $this->assertTrue($parsedBody->isReadonly());
     }
 
-    public function testParsesSimpleJSONRequest()
+    public function testParsesJSONRequest()
     {
         $query = '{my query}';
         $variables = ['test' => 1, 'test2' => 2];
@@ -64,10 +65,9 @@ class RequestParsingTest extends \PHPUnit_Framework_TestCase
             'variables' => $variables,
             'operation' => $operation
         ];
-
-        list ($parsedBody, $isReadonly) = $this->parseRawRequest('application/json', json_encode($body));
-        $this->assertEquals($body, $parsedBody);
-        $this->assertFalse($isReadonly);
+        $parsedBody = $this->parseRawRequest('application/json', json_encode($body));
+        $this->assertValidOperationParams($parsedBody, $query, null, $variables, $operation);
+        $this->assertFalse($parsedBody->isReadOnly());
     }
 
     public function testParsesBatchJSONRequest()
@@ -85,9 +85,16 @@ class RequestParsingTest extends \PHPUnit_Framework_TestCase
             ],
         ];
 
-        list ($parsedBody, $isReadonly) = $this->parseRawRequest('application/json', json_encode($body));
-        $this->assertEquals($body, $parsedBody);
-        $this->assertFalse($isReadonly);
+        $parsedBody = $this->parseRawRequest('application/json', json_encode($body));
+        $this->assertInternalType('array', $parsedBody);
+        $this->assertCount(2, $parsedBody);
+
+        $this->assertValidOperationParams($parsedBody[0], $body[0]['query'], null, $body[0]['variables'], $body[0]['operation']);
+        $this->assertValidOperationParams($parsedBody[1], null, $body[1]['queryId'], $body[1]['variables'], $body[1]['operation']);
+
+        // Batched queries must be read-only (do not allow batched mutations)
+        $this->assertTrue($parsedBody[0]->isReadOnly());
+        $this->assertTrue($parsedBody[1]->isReadOnly());
     }
 
     public function testFailsParsingInvalidJsonRequest()
@@ -130,162 +137,6 @@ class RequestParsingTest extends \PHPUnit_Framework_TestCase
         $this->parseRawRequest(null, 'test', "PUT");
     }
 
-    public function testSimpleRequestShouldPass()
-    {
-        $query = '{my q}';
-        $variables = ['a' => 'b', 'c' => 'd'];
-        $operation = 'op';
-
-        $parsedBody = [
-            'query' => $query,
-            'variables' => $variables,
-            'operation' => $operation,
-        ];
-
-        $helper = new Helper();
-        $params = $helper->toOperationParams($parsedBody, false);
-        $this->assertValidOperationParams($params, $query, null, $variables, $operation);
-    }
-
-    public function testRequestWithQueryIdShouldPass()
-    {
-        $queryId = 'some-query-id';
-        $variables = ['a' => 'b', 'c' => 'd'];
-        $operation = 'op';
-
-        $parsedBody = [
-            'queryId' => $queryId,
-            'variables' => $variables,
-            'operation' => $operation,
-        ];
-
-        $helper = new Helper();
-        $params = $helper->toOperationParams($parsedBody, false);
-        $this->assertValidOperationParams($params, null, $queryId, $variables, $operation);
-    }
-
-    public function testProducesCorrectOperationParamsForBatchRequest()
-    {
-        $query = '{my q}';
-        $queryId = 'some-query-id';
-        $variables = ['a' => 'b', 'c' => 'd'];
-        $operation = 'op';
-
-        $parsedBody = [
-            [
-                'query' => $query,
-                'variables' => $variables,
-                'operation' => $operation,
-            ],
-            [
-                'queryId' => $queryId,
-                'variables' => [],
-                'operation' => null
-            ],
-        ];
-
-        $helper = new Helper();
-        $params = $helper->toOperationParams($parsedBody, false);
-
-        $this->assertTrue(is_array($params));
-        $this->assertValidOperationParams($params[0], $query, null, $variables, $operation);
-        $this->assertValidOperationParams($params[1], null, $queryId, [], null);
-    }
-
-    public function testRequiresQueryOrQueryId()
-    {
-        $parsedBody = [
-            'variables' => ['foo' => 'bar'],
-            'operation' => 'op',
-        ];
-
-        $helper = new Helper();
-
-        $this->setExpectedException(
-            UserError::class,
-            'GraphQL Request must include at least one of those two parameters: "query" or "queryId"'
-        );
-        $helper->toOperationParams($parsedBody, false);
-    }
-
-    public function testFailsWhenBothQueryAndQueryIdArePresent()
-    {
-        $parsedBody = [
-            'query' => '{my query}',
-            'queryId' => 'my-query-id',
-        ];
-
-        $helper = new Helper();
-
-        $this->setExpectedException(
-            UserError::class,
-            'GraphQL Request parameters "query" and "queryId" are mutually exclusive'
-        );
-        $helper->toOperationParams($parsedBody, false);
-    }
-
-    public function testFailsWhenQueryParameterIsNotString()
-    {
-        $parsedBody = [
-            'query' => ['t' => '{my query}']
-        ];
-
-        $helper = new Helper();
-
-        $this->setExpectedException(
-            UserError::class,
-            'GraphQL Request parameter "query" must be string, but got object with first key: "t"'
-        );
-        $helper->toOperationParams($parsedBody, false);
-    }
-
-    public function testFailsWhenQueryIdParameterIsNotString()
-    {
-        $parsedBody = [
-            'queryId' => ['t' => '{my query}']
-        ];
-
-        $helper = new Helper();
-
-        $this->setExpectedException(
-            UserError::class,
-            'GraphQL Request parameter "queryId" must be string, but got object with first key: "t"'
-        );
-        $helper->toOperationParams($parsedBody, false);
-    }
-
-    public function testFailsWhenOperationParameterIsNotString()
-    {
-        $parsedBody = [
-            'query' => '{my query}',
-            'operation' => []
-        ];
-
-        $helper = new Helper();
-
-        $this->setExpectedException(
-            UserError::class,
-            'GraphQL Request parameter "operation" must be string, but got array(0)'
-        );
-        $helper->toOperationParams($parsedBody, false);
-    }
-
-    public function testFailsWhenVariablesParameterIsNotObject()
-    {
-        $parsedBody = [
-            'query' => '{my query}',
-            'variables' => 'test'
-        ];
-
-        $helper = new Helper();
-
-        $this->setExpectedException(
-            UserError::class,
-            'GraphQL Request parameter "variables" must be object, but got "test"'
-        );
-        $helper->toOperationParams($parsedBody, false);
-    }
-
     /**
      * @param string $contentType
      * @param string $content
@@ -299,7 +150,7 @@ class RequestParsingTest extends \PHPUnit_Framework_TestCase
         $_SERVER['REQUEST_METHOD'] = $method;
 
         $helper = new Helper();
-        return $helper->parseRawBody(function() use ($content) {
+        return $helper->parseHttpRequest(function() use ($content) {
             return $content;
         });
     }
@@ -315,12 +166,14 @@ class RequestParsingTest extends \PHPUnit_Framework_TestCase
         $_POST = $postValue;
 
         $helper = new Helper();
-        return $helper->parseRawBody();
+        return $helper->parseHttpRequest(function() {
+            throw new InvariantViolation("Shouldn't read from php://input for urlencoded request");
+        });
     }
 
     /**
      * @param $getValue
-     * @return array
+     * @return OperationParams
      */
     private function parseGetRequest($getValue)
     {
@@ -328,7 +181,9 @@ class RequestParsingTest extends \PHPUnit_Framework_TestCase
         $_GET = $getValue;
 
         $helper = new Helper();
-        return $helper->parseRawBody();
+        return $helper->parseHttpRequest(function() {
+            throw new InvariantViolation("Shouldn't read from php://input for urlencoded request");
+        });
     }
 
     /**
diff --git a/tests/Server/RequestValidationTest.php b/tests/Server/RequestValidationTest.php
new file mode 100644
index 0000000..4f0c2ae
--- /dev/null
+++ b/tests/Server/RequestValidationTest.php
@@ -0,0 +1,179 @@
+<?php
+namespace GraphQL\Tests\Server;
+
+use GraphQL\Error\InvariantViolation;
+use GraphQL\Error\UserError;
+use GraphQL\Server\Helper;
+use GraphQL\Server\OperationParams;
+
+class RequestValidationTest extends \PHPUnit_Framework_TestCase
+{
+    public function testSimpleRequestShouldValidate()
+    {
+        $query = '{my q}';
+        $variables = ['a' => 'b', 'c' => 'd'];
+        $operation = 'op';
+
+        $parsedBody = OperationParams::create([
+            'query' => $query,
+            'variables' => $variables,
+            'operation' => $operation,
+        ]);
+
+        $this->assertValid($parsedBody);
+    }
+
+    public function testRequestWithQueryIdShouldValidate()
+    {
+        $queryId = 'some-query-id';
+        $variables = ['a' => 'b', 'c' => 'd'];
+        $operation = 'op';
+
+        $parsedBody = OperationParams::create([
+            'queryId' => $queryId,
+            'variables' => $variables,
+            'operation' => $operation,
+        ]);
+
+        $this->assertValid($parsedBody);
+    }
+
+    public function testBatchRequestShouldValidate()
+    {
+        $query = '{my q}';
+        $queryId = 'some-query-id';
+        $variables = ['a' => 'b', 'c' => 'd'];
+        $operation = 'op';
+
+        $parsedBody = [
+            OperationParams::create([
+                'query' => $query,
+                'variables' => $variables,
+                'operation' => $operation,
+            ]),
+            OperationParams::create([
+                'queryId' => $queryId,
+                'variables' => [],
+                'operation' => null
+            ]),
+        ];
+
+        $this->assertValid($parsedBody);
+    }
+
+    public function testThrowsOnInvalidRequest()
+    {
+        $parsedBody = 'str';
+        $this->assertInternalError(
+            $parsedBody,
+            'GraphQL Server: Parsed http request must be an instance of GraphQL\Server\OperationParams or array of such instances, but got "str"'
+        );
+
+        $parsedBody = ['str'];
+        $this->assertInternalError(
+            $parsedBody,
+            'GraphQL Server: Parsed http request must be an instance of GraphQL\Server\OperationParams or array of such instances, but got invalid array where entry at position 0 is "str"'
+        );
+    }
+
+    public function testRequiresQueryOrQueryId()
+    {
+        $parsedBody = OperationParams::create([
+            'variables' => ['foo' => 'bar'],
+            'operation' => 'op',
+        ]);
+
+        $this->assertInputError(
+            $parsedBody,
+            'GraphQL Request must include at least one of those two parameters: "query" or "queryId"'
+        );
+    }
+
+    public function testFailsWhenBothQueryAndQueryIdArePresent()
+    {
+        $parsedBody = OperationParams::create([
+            'query' => '{my query}',
+            'queryId' => 'my-query-id',
+        ]);
+
+        $this->assertInputError(
+            $parsedBody,
+            'GraphQL Request parameters "query" and "queryId" are mutually exclusive'
+        );
+    }
+
+    public function testFailsWhenQueryParameterIsNotString()
+    {
+        $parsedBody = OperationParams::create([
+            'query' => ['t' => '{my query}']
+        ]);
+
+        $this->assertInputError(
+            $parsedBody,
+            'GraphQL Request parameter "query" must be string, but got object with first key: "t"'
+        );
+    }
+
+    public function testFailsWhenQueryIdParameterIsNotString()
+    {
+        $parsedBody = OperationParams::create([
+            'queryId' => ['t' => '{my query}']
+        ]);
+
+        $this->assertInputError(
+            $parsedBody,
+            'GraphQL Request parameter "queryId" must be string, but got object with first key: "t"'
+        );
+    }
+
+    public function testFailsWhenOperationParameterIsNotString()
+    {
+        $parsedBody = OperationParams::create([
+            'query' => '{my query}',
+            'operation' => []
+        ]);
+
+        $this->assertInputError(
+            $parsedBody,
+            'GraphQL Request parameter "operation" must be string, but got array(0)'
+        );
+    }
+
+    public function testFailsWhenVariablesParameterIsNotObject()
+    {
+        $parsedBody = OperationParams::create([
+            'query' => '{my query}',
+            'variables' => 'test'
+        ]);
+
+        $this->assertInputError($parsedBody, 'GraphQL Request parameter "variables" must be object, but got "test"');
+    }
+
+    private function assertValid($parsedRequest)
+    {
+        $helper = new Helper();
+        $helper->assertValidRequest($parsedRequest);
+    }
+
+    private function assertInputError($parsedRequest, $expectedMessage)
+    {
+        try {
+            $helper = new Helper();
+            $helper->assertValidRequest($parsedRequest);
+            $this->fail('Expected exception not thrown');
+        } catch (UserError $e) {
+            $this->assertEquals($expectedMessage, $e->getMessage());
+        }
+    }
+
+    private function assertInternalError($parsedRequest, $expectedMessage)
+    {
+        try {
+            $helper = new Helper();
+            $helper->assertValidRequest($parsedRequest);
+            $this->fail('Expected exception not thrown');
+        } catch (InvariantViolation $e) {
+            $this->assertEquals($expectedMessage, $e->getMessage());
+        }
+    }
+}