Use parsed_body to auto parse the response as JSON.

test/controllers/direct_uploads_controller_test.rb

@@ -22,10 +22,10 @@ class ActiveStorage::S3DirectUploadsControllerTest < ActionController::TestCase
       post :create, params: { blob: {
           filename: "hello.txt", byte_size: 6, checksum: Digest::MD5.base64digest("Hello"), content_type: "text/plain" } }
 
-      details = JSON.parse(@response.body)
-
-      assert_match /#{SERVICE_CONFIGURATIONS[:s3][:bucket]}\.s3.(\S+)?amazonaws\.com/, details["upload_to_url"]
-      assert_equal "hello.txt", ActiveStorage::Blob.find_signed(details["signed_blob_id"]).filename.to_s
+      @response.parsed_body.tap do |details|
+        assert_match(/#{SERVICE_CONFIGURATIONS[:s3][:bucket]}\.s3.(\S+)?amazonaws\.com/, details["upload_to_url"])
+        assert_equal "hello.txt", ActiveStorage::Blob.find_signed(details["signed_blob_id"]).filename.to_s
+      end
     end
   end
 else
@@ -52,10 +52,10 @@ class ActiveStorage::GCSDirectUploadsControllerTest < ActionController::TestCase
       post :create, params: { blob: {
           filename: "hello.txt", byte_size: 6, checksum: Digest::MD5.base64digest("Hello"), content_type: "text/plain" } }
 
-      details = JSON.parse(@response.body)
-
-      assert_match %r{storage\.googleapis\.com/#{@config[:bucket]}}, details["upload_to_url"]
-      assert_equal "hello.txt", ActiveStorage::Blob.find_signed(details["signed_blob_id"]).filename.to_s
+      @response.parsed_body.tap do |details|
+        assert_match %r{storage\.googleapis\.com/#{@config[:bucket]}}, details["upload_to_url"]
+        assert_equal "hello.txt", ActiveStorage::Blob.find_signed(details["signed_blob_id"]).filename.to_s
+      end
     end
   end
 else