diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1447a22..7ed674e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 * Added/Changed/Deprecated/Removed/Fixed/Security: YOUR CHANGE HERE
+* Fixed: multipart adapter raises `Graphlient::Errors::FaradayServerError` on 5xx responses instead of passing HTML error pages on as the body
 * Added: automatic conversion from symbols to keywords setting
 * Added: relation.empty?, relation.blank?, relation.present? methods
 * Added: fields now can be marked as "keywords"
diff --git a/lib/active_graphql/client/adapters/graphlient_multipart_adapter.rb b/lib/active_graphql/client/adapters/graphlient_multipart_adapter.rb
index 5a3351d..ac7553e 100644
--- a/lib/active_graphql/client/adapters/graphlient_multipart_adapter.rb
+++ b/lib/active_graphql/client/adapters/graphlient_multipart_adapter.rb
@@ -12,13 +12,20 @@ module Adapters
       class GraphlientMultipartAdapter < Graphlient::Adapters::HTTP::Adapter
         def execute(document:, operation_name:, variables:, context:)
           response = execute_request(document:, operation_name:, variables:, context:)
+          raise_server_error(response) if response.status >= 500
+
           response.body
-        rescue Faraday::ClientError => e
+        rescue Faraday::ClientError, Faraday::ServerError => e
           raise Graphlient::Errors::FaradayServerError, e
         end
 
         private
 
+        # 4xx bodies may still carry GraphQL errors, so only 5xx are treated as transport failures.
+        def raise_server_error(response)
+          raise Faraday::ServerError.new("the server responded with status #{response.status}", response.to_hash)
+        end
+
         def execute_request(document:, operation_name:, variables:, context:)
           connection.post do |req|
             req.headers.merge!(context[:headers] || {})
@@ -32,7 +39,6 @@ def execute_request(document:, operation_name:, variables:, context:)
 
         def connection
           @connection ||= Faraday.new(url:, headers:) do |c|
-            c.adapter Faraday::Response::RaiseError
             c.request :multipart
             c.request :url_encoded
             c.response :json
diff --git a/spec/lib/active_graphql/client/adapters/graphlient_multipart_adapter_spec.rb b/spec/lib/active_graphql/client/adapters/graphlient_multipart_adapter_spec.rb
new file mode 100644
index 0000000..d4cc633
--- /dev/null
+++ b/spec/lib/active_graphql/client/adapters/graphlient_multipart_adapter_spec.rb
@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'active_graphql/client/adapters/graphlient_multipart_adapter'
+
+class ActiveGraphql::Client
+  RSpec.describe Adapters::GraphlientMultipartAdapter do
+    subject(:adapter) { described_class.new(url) }
+
+    let(:url) { 'http://example.com/graphql' }
+    let(:document) { GraphQL.parse('mutation { createUser { id } }') }
+
+    describe '#execute' do
+      subject(:execute) { adapter.execute(document:, operation_name: nil, variables: {}, context: {}) }
+
+      context 'when server responds with JSON' do
+        before do
+          stub_request(:post, url).to_return(
+            status: 200,
+            body: { data: { createUser: { id: 1 } } }.to_json,
+            headers: { 'Content-Type' => 'application/json' }
+          )
+        end
+
+        it 'returns parsed body' do
+          expect(execute).to eq('data' => { 'createUser' => { 'id' => 1 } })
+        end
+      end
+
+      context 'when server responds with 503 HTML page' do
+        before do
+          stub_request(:post, url).to_return(
+            status: 503,
+            body: '<html><body><h1>503 Service Unavailable</h1></body></html>',
+            headers: { 'Content-Type' => 'text/html' }
+          )
+        end
+
+        it 'raises FaradayServerError with status code' do
+          expect { execute }.to raise_error(Graphlient::Errors::FaradayServerError) do |error|
+            expect(error.status_code).to eq 503
+          end
+        end
+      end
+
+      context 'when server responds with 4xx containing GraphQL errors' do
+        before do
+          stub_request(:post, url).to_return(
+            status: 422,
+            body: { errors: [{ message: 'invalid' }] }.to_json,
+            headers: { 'Content-Type' => 'application/json' }
+          )
+        end
+
+        it 'returns body so GraphQL errors can be handled' do
+          expect(execute).to eq('errors' => [{ 'message' => 'invalid' }])
+        end
+      end
+    end
+  end
+end