diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4e28429..4d650ed 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,7 +10,7 @@ It is a one-line switch: `GraphQL::useExperimentalExecutor()`.<br>
 (as it may become the default one in future)
 <br>
 <br> 
-- Spec compliance: error category, debug information and extensions are displayed under `extensions` key 
+- Spec compliance: error `category` and extensions are displayed under `extensions` key when using default formatting (#389) 
 - `AbstractValidationRule` renamed to `ValidationRule` (NS `GraphQL\Validator\Rules`)
 - `AbstractQuerySecurity` renamed to `QuerySecurityRule` (NS `GraphQL\Validator\Rules`)
 - `FindBreakingChanges` renamed to `BreakingChangesFinder` (NS `GraphQL\Utils`)
diff --git a/UPGRADE.md b/UPGRADE.md
index b560ce1..cebfb03 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -1,39 +1,10 @@
 ## Upgrade v0.12.x > dev-master
 
-### Try it: Experimental Executor with improved performance
-It is disabled by default. To enable it, do the following
-```php
-<?php
-use GraphQL\Executor\Executor;
-use GraphQL\Experimental\Executor\CoroutineExecutor;
-
-Executor::setImplementationFactory([CoroutineExecutor::class, 'create']);
-```
-
-**Please post your feedback about new executor at https://github.com/webonyx/graphql-php/issues/397
-Especially if you had issues (because it may become the default in one of the next releases)**
-
-### Breaking: minimum supported version of PHP
+### Breaking (major): minimum supported version of PHP
 New minimum required version of PHP is **7.1+**
 
-### Breaking: multiple interfaces separated with & in SDL
-Before the change:
-```graphql
-type Foo implements Bar, Baz { field: Type }
-```
-
-After the change:
-```graphql
-type Foo implements Bar & Baz { field: Type }
-```
-
-To allow for an adaptive migration, use `allowLegacySDLImplementsInterfaces` option of parser:
-```php
-Parser::parse($source, [ 'allowLegacySDLImplementsInterfaces' => true])
-```
-
-### Breaking: errors formatting changed according to spec 
-Extensions assigned to errors are shown under `extensions` key
+### Breaking (major): default errors formatting changed according to spec 
+**Category** and extensions assigned to errors are shown under `extensions` key
 ```php
 $e = new Error(
     'msg',
@@ -50,6 +21,7 @@ Formatting before the change:
 'errors' => [
     [
         'message' => 'msg',
+        'category' => 'graphql',
         'foo' => 'bar'
     ]
 ]
@@ -59,11 +31,47 @@ After the change:
 'errors' => [
     [
         'message' => 'msg',
-        'extensions' => ['foo' => 'bar'],
+        'extensions' => [
+            'category' => 'graphql',
+            'foo' => 'bar',
+        ],
     ]
 ]
 ```
 
+Note: if error extensions contain `category` key - it has a priority over default category.
+
+You can always switch to [custom error formatting](https://webonyx.github.io/graphql-php/error-handling/#custom-error-handling-and-formatting) to revert to the old format.
+
+### Try it: Experimental Executor with improved performance
+It is disabled by default. To enable it, do the following
+```php
+<?php
+use GraphQL\Executor\Executor;
+use GraphQL\Experimental\Executor\CoroutineExecutor;
+
+Executor::setImplementationFactory([CoroutineExecutor::class, 'create']);
+```
+
+**Please post your feedback about new executor at https://github.com/webonyx/graphql-php/issues/397
+Especially if you had issues (because it may become the default in one of the next releases)**
+
+### Breaking: multiple interfaces separated with & in SDL
+Before the change:
+```graphql
+type Foo implements Bar, Baz { field: Type }
+```
+
+After the change:
+```graphql
+type Foo implements Bar & Baz { field: Type }
+```
+
+To allow for an adaptive migration, use `allowLegacySDLImplementsInterfaces` option of parser:
+```php
+Parser::parse($source, [ 'allowLegacySDLImplementsInterfaces' => true])
+```
+
 
 ## Upgrade v0.11.x > v0.12.x
 
diff --git a/src/Error/FormattedError.php b/src/Error/FormattedError.php
index e03b2ac..b200774 100644
--- a/src/Error/FormattedError.php
+++ b/src/Error/FormattedError.php
@@ -187,12 +187,16 @@ class FormattedError
         if ($e instanceof ClientAware) {
             $formattedError = [
                 'message'  => $e->isClientSafe() ? $e->getMessage() : $internalErrorMessage,
-                'category' => $e->getCategory(),
+                'extensions' => [
+                    'category' => $e->getCategory(),
+                ],
             ];
         } else {
             $formattedError = [
                 'message'  => $internalErrorMessage,
-                'category' => Error::CATEGORY_INTERNAL,
+                'extensions' => [
+                    'category' => Error::CATEGORY_INTERNAL,
+                ],
             ];
         }
 
@@ -210,7 +214,7 @@ class FormattedError
                 $formattedError['path'] = $e->path;
             }
             if (! empty($e->getExtensions())) {
-                $formattedError['extensions'] = $e->getExtensions();
+                $formattedError['extensions'] = $e->getExtensions() + $formattedError['extensions'];
             }
         }
 
diff --git a/tests/Executor/AbstractTest.php b/tests/Executor/AbstractTest.php
index a1423b9..eba24ec 100644
--- a/tests/Executor/AbstractTest.php
+++ b/tests/Executor/AbstractTest.php
@@ -420,7 +420,7 @@ class AbstractTest extends TestCase
                         'function or each possible type should provide an "isTypeOf" function.',
                     'locations'    => [['line' => 1, 'column' => 3]],
                     'path'         => ['foo'],
-                    'category'     => 'internal',
+                    'extensions'   => ['category' => 'internal'],
                 ],
             ],
         ];
diff --git a/tests/Executor/VariablesTest.php b/tests/Executor/VariablesTest.php
index 0751c67..c2625ae 100644
--- a/tests/Executor/VariablesTest.php
+++ b/tests/Executor/VariablesTest.php
@@ -222,7 +222,7 @@ class VariablesTest extends TestCase
                         '{"a":"foo","b":"bar","c":null}; ' .
                         'Expected non-nullable type String! not to be null at value.c.',
                     'locations' => [['line' => 2, 'column' => 21]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -239,7 +239,7 @@ class VariablesTest extends TestCase
                         'Variable "$input" got invalid value "foo bar"; ' .
                         'Expected type TestInputObject to be an object.',
                     'locations' => [['line' => 2, 'column' => 21]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -256,7 +256,7 @@ class VariablesTest extends TestCase
                         'Variable "$input" got invalid value {"a":"foo","b":"bar"}; ' .
                         'Field value.c of required type String! was not provided.',
                     'locations' => [['line' => 2, 'column' => 21]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -278,14 +278,14 @@ class VariablesTest extends TestCase
                         'Variable "$input" got invalid value {"na":{"a":"foo"}}; ' .
                         'Field value.na.c of required type String! was not provided.',
                     'locations' => [['line' => 2, 'column' => 19]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
                 [
                     'message'   =>
                         'Variable "$input" got invalid value {"na":{"a":"foo"}}; ' .
                         'Field value.nb of required type String! was not provided.',
                     'locations' => [['line' => 2, 'column' => 19]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -302,7 +302,7 @@ class VariablesTest extends TestCase
                         '{"a":"foo","b":"bar","c":"baz","extra":"dog"}; ' .
                         'Field "extra" is not defined by type TestInputObject.',
                     'locations' => [['line' => 2, 'column' => 21]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -434,7 +434,7 @@ class VariablesTest extends TestCase
                 [
                     'message'   => 'Variable "$value" of required type "String!" was not provided.',
                     'locations' => [['line' => 2, 'column' => 31]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -459,7 +459,7 @@ class VariablesTest extends TestCase
                         'Variable "$value" got invalid value null; ' .
                         'Expected non-nullable type String! not to be null.',
                     'locations' => [['line' => 2, 'column' => 31]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -511,7 +511,7 @@ class VariablesTest extends TestCase
                 'message'   => 'Argument "input" of required type "String!" was not provided.',
                 'locations' => [['line' => 3, 'column' => 9]],
                 'path'      => ['fieldWithNonNullableStringInput'],
-                'category'  => 'graphql',
+                'extensions' => ['category' => 'graphql'],
             ],
             ],
         ];
@@ -538,10 +538,10 @@ class VariablesTest extends TestCase
                 'message'   =>
                     'Variable "$value" got invalid value [1,2,3]; Expected type ' .
                     'String; String cannot represent an array value: [1,2,3]',
-                'category'  => 'graphql',
                 'locations' => [
                     ['line' => 2, 'column' => 31],
                 ],
+                'extensions' => ['category' => 'graphql'],
             ],
             ],
         ];
@@ -581,7 +581,7 @@ class VariablesTest extends TestCase
                     'variable "$foo" which was not provided a runtime value.',
                 'locations' => [['line' => 3, 'column' => 48]],
                 'path'      => ['fieldWithNonNullableStringInput'],
-                'category'  => 'graphql',
+                'extensions' => ['category' => 'graphql'],
             ],
             ],
         ];
@@ -652,7 +652,7 @@ class VariablesTest extends TestCase
                         'Variable "$input" got invalid value null; ' .
                         'Expected non-nullable type [String]! not to be null.',
                     'locations' => [['line' => 2, 'column' => 17]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -737,7 +737,7 @@ class VariablesTest extends TestCase
                         'Variable "$input" got invalid value ["A",null,"B"]; ' .
                         'Expected non-nullable type String! not to be null at value[1].',
                     'locations' => [['line' => 2, 'column' => 17]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -762,7 +762,7 @@ class VariablesTest extends TestCase
                         'Variable "$input" got invalid value null; ' .
                         'Expected non-nullable type [String!]! not to be null.',
                     'locations' => [['line' => 2, 'column' => 17]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -804,7 +804,7 @@ class VariablesTest extends TestCase
                         'Variable "$input" got invalid value ["A",null,"B"]; ' .
                         'Expected non-nullable type String! not to be null at value[1].',
                     'locations' => [['line' => 2, 'column' => 17]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -830,7 +830,7 @@ class VariablesTest extends TestCase
                         'Variable "$input" expected value of type "TestType!" which cannot ' .
                         'be used as an input type.',
                     'locations' => [['line' => 2, 'column' => 25]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -857,7 +857,7 @@ class VariablesTest extends TestCase
                         'Variable "$input" expected value of type "UnknownType!" which ' .
                         'cannot be used as an input type.',
                     'locations' => [['line' => 2, 'column' => 25]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -910,7 +910,7 @@ class VariablesTest extends TestCase
                     'Argument "input" has invalid value WRONG_TYPE.',
                 'locations' => [['line' => 2, 'column' => 50]],
                 'path'      => ['fieldWithDefaultArgumentValue'],
-                'category'  => 'graphql',
+                'extensions' => ['category' => 'graphql'],
             ],
             ],
         ];
diff --git a/tests/Server/QueryExecutionTest.php b/tests/Server/QueryExecutionTest.php
index fffa9b8..bc16ed6 100644
--- a/tests/Server/QueryExecutionTest.php
+++ b/tests/Server/QueryExecutionTest.php
@@ -260,7 +260,7 @@ class QueryExecutionTest extends ServerTestCase
             'errors' => [
                 [
                     'message'  => 'Persisted queries are not supported by this server',
-                    'category' => 'request',
+                    'extensions' => ['category' => 'request'],
                 ],
             ],
         ];
@@ -291,7 +291,7 @@ class QueryExecutionTest extends ServerTestCase
                 'errors' => [
                     [
                         'message'  => 'Batched queries are not supported by this server',
-                        'category' => 'request',
+                        'extensions' => ['category' => 'request'],
                     ],
                 ],
             ],
@@ -299,7 +299,7 @@ class QueryExecutionTest extends ServerTestCase
                 'errors' => [
                     [
                         'message'  => 'Batched queries are not supported by this server',
-                        'category' => 'request',
+                        'extensions' => ['category' => 'request'],
                     ],
                 ],
             ],
@@ -342,7 +342,7 @@ class QueryExecutionTest extends ServerTestCase
             'errors' => [
                 [
                     'message'  => 'GET supports only query operation',
-                    'category' => 'request',
+                    'extensions' => ['category' => 'request'],
                 ],
             ],
         ];
@@ -406,7 +406,7 @@ class QueryExecutionTest extends ServerTestCase
                 [
                     'message'   => 'Cannot query field "invalid" on type "Query".',
                     'locations' => [['line' => 1, 'column' => 2]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];
@@ -443,7 +443,7 @@ class QueryExecutionTest extends ServerTestCase
                 [
                     'message'   => 'Cannot query field "invalid2" on type "Query".',
                     'locations' => [['line' => 1, 'column' => 2]],
-                    'category'  => 'graphql',
+                    'extensions' => ['category' => 'graphql'],
                 ],
             ],
         ];