Improve user feedback when CloudFormation reports no changes

When CloudFormation returns 'no changes' errors during apply, provide
a clearer explanation that while the template may have differences
(e.g., whitespace, comments, formatting), no actual resource changes
are needed and the stack is already in the desired state.

This addresses the confusing scenario where users see a template diff
but CloudFormation reports: 'The submitted information didn't contain
changes. Submit different information to create a change set.'

Changes:
- Add user_friendly_changeset_error method to detect common 'no changes'
  error messages and provide helpful context
- Update both create and update stack flows to use the improved message
- Add comprehensive tests covering various CloudFormation error messages

Author: orien

CHANGELOG.md

@@ -12,12 +12,14 @@ The format is based on [Keep a Changelog], and this project adheres to
 
 ### Changed
 
+- Improve message when CloudFormation claims there are no changes to apply, even when a template diff is present. ([#398])
 - Display Tags diff (stack tags) in `stack_master diff` and `stack_master apply` commands. ([#397])
 - Resolve style issues identified by RuboCop. ([#396])
 
 [Unreleased]: https://github.com/envato/stack_master/compare/v2.17.1...HEAD
 [#396]: https://github.com/envato/stack_master/pull/396
 [#397]: https://github.com/envato/stack_master/pull/397
+[#398]: https://github.com/envato/stack_master/pull/398
 
 ## [2.17.1] - 2025-12-19
 

lib/stack_master/commands/apply.rb

@@ -89,7 +89,7 @@ def create_stack_by_change_set
           @change_set = ChangeSet.create(stack_options.merge(change_set_type: 'CREATE'))
           if @change_set.failed?
             ChangeSet.delete(@change_set.id)
-            halt!(@change_set.status_reason)
+            halt!(user_friendly_changeset_error(@change_set.status_reason))
           end
 
           @change_set.display(StackMaster.stdout)
@@ -123,7 +123,7 @@ def update_stack
         @change_set = ChangeSet.create(stack_options)
         if @change_set.failed?
           ChangeSet.delete(@change_set.id)
-          halt!(@change_set.status_reason)
+          halt!(user_friendly_changeset_error(@change_set.status_reason))
         end
 
         @change_set.display(StackMaster.stdout)
@@ -230,6 +230,21 @@ def set_stack_policy
         StackMaster.stdout.puts 'done.'
       end
 
+      def user_friendly_changeset_error(status_reason)
+        # CloudFormation returns various messages when there are no changes to apply
+        if status_reason =~ /didn'?t contain changes|no changes|no updates are to be performed/i
+          <<~MESSAGE.chomp
+            #{status_reason}
+
+            While there may be differences in the template file (e.g., whitespace, comments, or
+            formatting), CloudFormation has determined that no actual resource changes are needed.
+            The stack is already in the desired state.
+          MESSAGE
+        else
+          status_reason
+        end
+      end
+
       extend Forwardable
       def_delegators :@stack_definition, :stack_name, :region
     end

spec/stack_master/commands/apply_spec.rb

@@ -127,11 +127,42 @@ def apply
       before do
         allow(StackMaster::ChangeSet).to receive(:delete)
         allow(change_set).to receive(:failed?).and_return(true)
-        allow(change_set).to receive(:status_reason).and_return('reason')
       end
 
-      it 'outputs the status reason' do
-        expect { apply }.to output(/reason/).to_stdout
+      context 'with a generic error' do
+        before do
+          allow(change_set).to receive(:status_reason).and_return('reason')
+        end
+
+        it 'outputs the status reason' do
+          expect { apply }.to output(/reason/).to_stdout
+        end
+      end
+
+      context 'with a "no changes" error from CloudFormation' do
+        before do
+          allow(change_set)
+            .to receive(:status_reason)
+            .and_return("The submitted information didn't contain changes. " \
+                        'Submit different information to create a change set.')
+        end
+
+        it 'outputs a user-friendly explanation' do
+          expect { apply }.to output(/The submitted information didn't contain changes/).to_stdout
+          expect { apply }.to output(/no actual resource changes are needed/).to_stdout
+          expect { apply }.to output(/stack is already in the desired state/).to_stdout
+        end
+      end
+
+      context 'with alternative "no changes" error message' do
+        before do
+          allow(change_set).to receive(:status_reason).and_return('No updates are to be performed.')
+        end
+
+        it 'outputs a user-friendly explanation' do
+          expect { apply }.to output(/No updates are to be performed/).to_stdout
+          expect { apply }.to output(/no actual resource changes are needed/).to_stdout
+        end
       end
     end