diff --git a/src/Event/Http/HttpHandler.php b/src/Event/Http/HttpHandler.php
index ebd7092d5..6e61793c1 100644
--- a/src/Event/Http/HttpHandler.php
+++ b/src/Event/Http/HttpHandler.php
@@ -25,9 +25,9 @@ public function handle($event, Context $context): array
         $response = $this->handleRequest($httpEvent, $context);
 
         if ($httpEvent->isFormatV2()) {
-            return $response->toApiGatewayFormatV2();
+            return $response->toApiGatewayFormatV2($context->getAwsRequestId());
         }
 
-        return $response->toApiGatewayFormat($httpEvent->hasMultiHeader());
+        return $response->toApiGatewayFormat($httpEvent->hasMultiHeader(), $context->getAwsRequestId());
     }
 }
diff --git a/src/Event/Http/HttpResponse.php b/src/Event/Http/HttpResponse.php
index 80a679bc2..76c24d5d7 100644
--- a/src/Event/Http/HttpResponse.php
+++ b/src/Event/Http/HttpResponse.php
@@ -21,7 +21,7 @@ public function __construct(string $body, array $headers = [], int $statusCode =
         $this->statusCode = $statusCode;
     }
 
-    public function toApiGatewayFormat(bool $multiHeaders = false): array
+    public function toApiGatewayFormat(bool $multiHeaders = false, ?string $awsRequestId = null): array
     {
         $base64Encoding = (bool) getenv('BREF_BINARY_RESPONSES');
 
@@ -38,6 +38,8 @@ public function toApiGatewayFormat(bool $multiHeaders = false): array
             }
         }
 
+        $this->checkHeadersSize($headers, $awsRequestId);
+
         // The headers must be a JSON object. If the PHP array is empty it is
         // serialized to `[]` (we want `{}`) so we force it to an empty object.
         $headers = empty($headers) ? new \stdClass : $headers;
@@ -58,7 +60,7 @@ public function toApiGatewayFormat(bool $multiHeaders = false): array
     /**
      * See https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html#http-api-develop-integrations-lambda.response
      */
-    public function toApiGatewayFormatV2(): array
+    public function toApiGatewayFormatV2(?string $awsRequestId = null): array
     {
         $base64Encoding = (bool) getenv('BREF_BINARY_RESPONSES');
 
@@ -76,6 +78,11 @@ public function toApiGatewayFormatV2(): array
             }
         }
 
+        $this->checkHeadersSize(array_merge(
+            $headers,
+            ['Set-Cookie' => $cookies], // include cookies in the size check
+        ), $awsRequestId);
+
         // The headers must be a JSON object. If the PHP array is empty it is
         // serialized to `[]` (we want `{}`) so we force it to an empty object.
         $headers = empty($headers) ? new \stdClass : $headers;
@@ -98,4 +105,31 @@ private function capitalizeHeaderName(string $name): string
         $name = ucwords($name);
         return str_replace(' ', '-', $name);
     }
+
+    /**
+     * API Gateway v1 and v2 have a headers total max size of 10 KB.
+     * ALB has a max size of 32 KB.
+     * It's hard to calculate the exact size of headers here, so we just
+     * estimate it roughly: if above 9.5 KB we log a warning.
+     *
+     * @param array<string|string[]> $headers
+     */
+    private function checkHeadersSize(array $headers, ?string $awsRequestId): void
+    {
+        $estimatedHeadersSize = 0;
+        foreach ($headers as $name => $values) {
+            $estimatedHeadersSize += strlen($name);
+            if (is_array($values)) {
+                foreach ($values as $value) {
+                    $estimatedHeadersSize += strlen($value);
+                }
+            } else {
+                $estimatedHeadersSize += strlen($values);
+            }
+        }
+
+        if ($estimatedHeadersSize > 9_500) {
+            echo "$awsRequestId\tWARNING\tThe total size of HTTP response headers is estimated to be above 10 KB, which is the API Gateway limit. If the limit is reached, the HTTP response will be a 500 error.\n";
+        }
+    }
 }