Prioritize json over cbor for stubbed client

gems/aws-sdk-core/CHANGELOG.md

@@ -1,6 +1,8 @@
 Unreleased Changes
 ------------------
 
+* Issue - Prioritizes JSON over CBOR when both are supported for stubbed clients.
+
 3.239.2 (2025-11-25)
 ------------------
 

gems/aws-sdk-core/lib/aws-sdk-core/client_stubs.rb

@@ -280,6 +280,12 @@ def data_to_http_resp(operation_name, data)
     end
 
     def protocol_helper
+      # Prioritize JSON over CBOR when CBOR is the configured protocol but both are supported. This is to match similar
+      # prioritization in service.rb code generation.
+      if @config.api.metadata['protocol'] == 'smithy-rpc-v2-cbor' && @config.api.metadata['protocols']&.include?('json')
+        return Stubbing::Protocols::Json.new
+      end
+
       case @config.api.metadata['protocol']
       when 'json'               then Stubbing::Protocols::Json
       when 'rest-json'          then Stubbing::Protocols::RestJson

gems/aws-sdk-core/spec/aws/client_spec.rb

@@ -208,6 +208,36 @@ module Aws
 
       end
 
+      describe 'protocol_helper' do
+        it 'returns configured protocol' do
+          service = ApiHelper.sample_service(metadata: { 'protocol' => 'query' })
+          client_class = ApiHelper.sample_client(service: service)
+          client = client_class.new(options)
+
+          helper = client.send(:protocol_helper)
+          expect(helper).to be_a(Aws::Stubbing::Protocols::Query)
+        end
+
+        it 'prioritizes Json over RpcV2 when both protocols are supported' do
+          service = ApiHelper.sample_service(
+            metadata: {
+              'protocol' => 'smithy-rpc-v2-cbor',
+              'protocols' => %w[smithy-rpc-v2-cbor json]
+            }
+          )
+          client_class = ApiHelper.sample_client(service: service)
+          client = client_class.new(options)
+
+          helper = client.send(:protocol_helper)
+          expect(helper).to be_a(Aws::Stubbing::Protocols::Json)
+        end
+
+        it 'raises error for unsupported protocol' do
+          expect do
+            ApiHelper.sample_service(metadata: { 'protocol' => 'unsupported' })
+          end.to raise_error(/unsupported protocol/)
+        end
+      end
     end
   end
 end