diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index a06f2f7..65eac6d 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -163,11 +163,18 @@ without prematurely constructing application inputs or shared state.
    `--path` remains explain-only; against flags remain diff-only. Keep ordinary
    format defaults and singleton flag behavior unchanged.
 
-**Completion evidence:** the roadmap's inheritance table, including explicit
-empty and single-field overrides; default/explicit YAML and unsupported execution
-formats; unknown/all/unsupported scopes failing before factories. Verify against
-flags appear in diff help, not other operations. Successful pair preparation is
-completed in Stage 4; acquisition and delivery remain for Stage 8.
+**Completion evidence — complete:** right-side configuration and overlay fields
+inherit independently with their supplied state, while explicit empty overrides
+remain supplied. The `diff` command defaults to YAML, accepts explicit YAML, and
+rejects text and unsupported formats. Its help exposes both right-side flags;
+validate, show, sources, and explain do not, and `--path` remains explain-only.
+Comparison scope preflight defaults to `app` and rejects `all`, unknown, and
+unsupported scopes before suite factories run. Valid comparison preparation
+stores the resolved right selection without constructing application input or
+shared state. The existing show default and other operation defaults remain
+unchanged. Focused tests and the full ckconfig and dependent ckp suites pass.
+Separate typed pair preparation remains in Stage 4; acquisition and delivery
+remain in Stage 8.
 
 ## Stage 4: Prepare separate typed inputs for the selected pair
 
diff --git a/integrations/ckconfig/comparison_input_test.go b/integrations/ckconfig/comparison_input_test.go
new file mode 100644
index 0000000..b89328b
--- /dev/null
+++ b/integrations/ckconfig/comparison_input_test.go
@@ -0,0 +1,183 @@
+package ckconfig
+
+import (
+	"context"
+	"io"
+	"strings"
+	"testing"
+
+	commandkit "gitea.maximumdirect.net/eric/CommandKit"
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestComparisonSelectionInheritsFieldsIndependently(t *testing.T) {
+	leftConfig := commandkit.Supplied[string]{Value: "before.yml", Set: true}
+	leftOverlay := commandkit.Supplied[string]{Value: "base", Set: true}
+	tests := []struct {
+		name  string
+		left  Selection
+		right Selection
+		want  Selection
+	}{
+		{
+			name: "omitted fields stay omitted",
+		},
+		{
+			name: "both fields inherit supplied state",
+			left: Selection{Config: leftConfig, Overlay: leftOverlay},
+			want: Selection{Config: leftConfig, Overlay: leftOverlay},
+		},
+		{
+			name:  "explicit empty config overrides while overlay inherits",
+			left:  Selection{Config: leftConfig, Overlay: leftOverlay},
+			right: Selection{Config: commandkit.Supplied[string]{Set: true}},
+			want:  Selection{Config: commandkit.Supplied[string]{Set: true}, Overlay: leftOverlay},
+		},
+		{
+			name:  "explicit overlay overrides while config inherits",
+			left:  Selection{Config: leftConfig, Overlay: leftOverlay},
+			right: Selection{Overlay: commandkit.Supplied[string]{Value: "candidate", Set: true}},
+			want:  Selection{Config: leftConfig, Overlay: commandkit.Supplied[string]{Value: "candidate", Set: true}},
+		},
+		{
+			name:  "explicit empty overlay remains supplied",
+			left:  Selection{Config: leftConfig, Overlay: leftOverlay},
+			right: Selection{Overlay: commandkit.Supplied[string]{Set: true}},
+			want:  Selection{Config: leftConfig, Overlay: commandkit.Supplied[string]{Set: true}},
+		},
+		{
+			name:  "both explicit right fields replace left",
+			left:  Selection{Config: leftConfig, Overlay: leftOverlay},
+			right: Selection{Config: commandkit.Supplied[string]{Value: "after.yml", Set: true}, Overlay: commandkit.Supplied[string]{Value: "new", Set: true}},
+			want:  Selection{Config: commandkit.Supplied[string]{Value: "after.yml", Set: true}, Overlay: commandkit.Supplied[string]{Value: "new", Set: true}},
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := inheritComparisonSelection(test.left, test.right); got != test.want {
+				t.Fatalf("inherited selection = %#v, want %#v", got, test.want)
+			}
+		})
+	}
+}
+
+func TestComparisonHelpLimitsFlagsAndDefaultsFormatToYAML(t *testing.T) {
+	scope := Scope[struct{}, struct{}]{
+		Name: "app", Operations: []Operation{Diff, Show, Explain, Validate, Sources},
+		Acquire: func(context.Context, Request, struct{}, struct{}) (commandkit.Acquired[View], error) {
+			return commandkit.Acquired[View]{}, nil
+		},
+		Compare: func(context.Context, ComparisonRequest[struct{}], struct{}) (commandkit.Acquired[[]configkit.Difference], error) {
+			return commandkit.Acquired[[]configkit.Difference]{}, nil
+		},
+	}
+	app := newConfigTestApp(t, Command(Suite[struct{}, struct{}]{Scopes: []Scope[struct{}, struct{}]{scope}}))
+	help := func(args ...string) string {
+		t.Helper()
+		var output strings.Builder
+		app.Execute(context.Background(), args, commandkit.Streams{Out: &output, Err: io.Discard})
+		return output.String()
+	}
+
+	diffHelp := help("config", "diff", "--help")
+	if !strings.Contains(diffHelp, "--against-config") || !strings.Contains(diffHelp, "--against-overlay") {
+		t.Fatalf("diff help omits right-side selection flags:\n%s", diffHelp)
+	}
+	if !strings.Contains(diffHelp, "default: yaml") {
+		t.Fatalf("diff help does not show the YAML default:\n%s", diffHelp)
+	}
+	for _, operation := range []string{"validate", "show", "sources", "explain"} {
+		operationHelp := help("config", operation, "--help")
+		if strings.Contains(operationHelp, "--against-config") || strings.Contains(operationHelp, "--against-overlay") {
+			t.Errorf("%s exposes comparison-only flags:\n%s", operation, operationHelp)
+		}
+		if operation == "show" && !strings.Contains(operationHelp, "default: yaml") {
+			t.Errorf("show format default changed:\n%s", operationHelp)
+		}
+		if operation != "show" && !strings.Contains(operationHelp, "default: text") {
+			t.Errorf("%s format default changed:\n%s", operation, operationHelp)
+		}
+		if operation != "explain" && strings.Contains(operationHelp, "--path") {
+			t.Errorf("%s exposes the explain-only path flag:\n%s", operation, operationHelp)
+		}
+	}
+	explainHelp := help("config", "explain", "--help")
+	if !strings.Contains(explainHelp, "--path") {
+		t.Fatalf("explain flags changed:\n%s", explainHelp)
+	}
+}
+
+func TestComparisonPreparationAcceptsOnlyYAMLWithoutCreatingInputs(t *testing.T) {
+	inputCalls, stateCalls := 0, 0
+	prepared := captureSuite(Suite[struct{}, struct{}]{
+		NewInput: func() struct{} { inputCalls++; return struct{}{} },
+		NewState: func() struct{} { stateCalls++; return struct{}{} },
+		Scopes: []Scope[struct{}, struct{}]{{
+			Name: "app", Operations: []Operation{Diff},
+			Compare: func(context.Context, ComparisonRequest[struct{}], struct{}) (commandkit.Acquired[[]configkit.Difference], error) {
+				return commandkit.Acquired[[]configkit.Difference]{}, nil
+			},
+		}},
+	})
+
+	input := &configCommandInput[struct{}, struct{}]{
+		selection: Selection{Config: commandkit.Supplied[string]{Value: "before.yml", Set: true}},
+		against:   Selection{Overlay: commandkit.Supplied[string]{Value: "candidate", Set: true}},
+		format:    commandkit.Supplied[string]{Value: "yaml", Set: true},
+	}
+	if err := prepareConfigOperation(prepared, Diff, input); err != nil {
+		t.Fatalf("explicit YAML format rejected: %v", err)
+	}
+	if input.plan == nil || input.plan.format != "yaml" {
+		t.Fatalf("comparison plan = %#v", input.plan)
+	}
+	if input.plan.scopeName != "app" {
+		t.Fatalf("comparison default scope = %q, want app", input.plan.scopeName)
+	}
+	if want := (Selection{
+		Config:  commandkit.Supplied[string]{Value: "before.yml", Set: true},
+		Overlay: commandkit.Supplied[string]{Value: "candidate", Set: true},
+	}); input.against != want {
+		t.Fatalf("resolved right selection = %#v, want %#v", input.against, want)
+	}
+
+	for _, format := range []string{"text", "json"} {
+		input := &configCommandInput[struct{}, struct{}]{format: commandkit.Supplied[string]{Value: format}}
+		if err := prepareConfigOperation(prepared, Diff, input); err == nil {
+			t.Errorf("comparison format %q was accepted", format)
+		}
+	}
+	if inputCalls != 0 || stateCalls != 0 {
+		t.Fatalf("comparison preparation created input/state: %d/%d", inputCalls, stateCalls)
+	}
+}
+
+func TestComparisonAvailabilityPreflightPrecedesFactories(t *testing.T) {
+	inputCalls, stateCalls, acquireCalls, compareCalls := 0, 0, 0, 0
+	prepared := captureSuite(Suite[struct{}, struct{}]{
+		NewInput: func() struct{} { inputCalls++; return struct{}{} },
+		NewState: func() struct{} { stateCalls++; return struct{}{} },
+		Scopes: []Scope[struct{}, struct{}]{
+			{Name: "app", Operations: []Operation{Diff}, Compare: func(context.Context, ComparisonRequest[struct{}], struct{}) (commandkit.Acquired[[]configkit.Difference], error) {
+				compareCalls++
+				return commandkit.Acquired[[]configkit.Difference]{}, nil
+			}},
+			{Name: "plain", Operations: []Operation{Validate}, Acquire: func(context.Context, Request, struct{}, struct{}) (commandkit.Acquired[View], error) {
+				acquireCalls++
+				return commandkit.Acquired[View]{}, nil
+			}},
+		},
+	})
+	for _, scope := range []string{"missing", "all", "plain"} {
+		input := &configCommandInput[struct{}, struct{}]{
+			scope:  commandkit.Supplied[string]{Value: scope, Set: true},
+			format: commandkit.Supplied[string]{Value: "yaml"},
+		}
+		if err := prepareConfigOperation(prepared, Diff, input); err == nil {
+			t.Errorf("comparison scope %q was accepted", scope)
+		}
+	}
+	if inputCalls != 0 || stateCalls != 0 || acquireCalls != 0 || compareCalls != 0 {
+		t.Fatalf("failed preflight invoked suite work: input=%d state=%d acquire=%d compare=%d", inputCalls, stateCalls, acquireCalls, compareCalls)
+	}
+}
diff --git a/integrations/ckconfig/input.go b/integrations/ckconfig/input.go
index d6a63ef..fbc7bf9 100644
--- a/integrations/ckconfig/input.go
+++ b/integrations/ckconfig/input.go
@@ -10,6 +10,7 @@ import (
 
 type configCommandInput[I, S any] struct {
 	selection Selection
+	against   Selection
 	scope     commandkit.Supplied[string]
 	format    commandkit.Supplied[string]
 	path      commandkit.Supplied[string]
@@ -35,7 +36,7 @@ type configOperationPlan[I, S any] struct {
 func newConfigOperationCommand[I, S any](prepared *preparedSuite[I, S], operation Operation) commandkit.Command {
 	defaultScope := "app"
 	defaultFormat := "text"
-	if operation == Show {
+	if operation == Show || operation == Diff {
 		defaultFormat = "yaml"
 	}
 	flags := []commandkit.Flag[*configCommandInput[I, S]]{
@@ -52,6 +53,18 @@ func newConfigOperationCommand[I, S any](prepared *preparedSuite[I, S], operatio
 				(*input).selection.Overlay = value
 			}),
 	}
+	if operation == Diff {
+		flags = append(flags,
+			commandkit.ValueFlag(commandkit.FlagMeta{Name: "against-config", Usage: "Configuration source for the right side."}, (*string)(nil), parseConfigString,
+				func(input **configCommandInput[I, S], value commandkit.Supplied[string]) {
+					(*input).against.Config = value
+				}),
+			commandkit.ValueFlag(commandkit.FlagMeta{Name: "against-overlay", Usage: "Overlay source for the right side."}, (*string)(nil), parseConfigString,
+				func(input **configCommandInput[I, S], value commandkit.Supplied[string]) {
+					(*input).against.Overlay = value
+				}),
+		)
+	}
 	if operation == Explain {
 		flags = append(flags, commandkit.ValueFlag(commandkit.FlagMeta{Name: "path", Usage: "JSON Pointer path to explain.", Required: true}, (*string)(nil), parseConfigString,
 			func(input **configCommandInput[I, S], value commandkit.Supplied[string]) { (*input).path = value }))
@@ -90,6 +103,8 @@ func operationDeclaration(operation Operation) (string, string) {
 		return "sources", "List configuration sources."
 	case Explain:
 		return "explain", "Explain a configuration path."
+	case Diff:
+		return "diff", "Compare two configuration selections."
 	default:
 		return string(operation), "Inspect configuration."
 	}
@@ -103,6 +118,9 @@ func prepareConfigOperation[I, S any](prepared *preparedSuite[I, S], operation O
 	if operation == Show && format != "yaml" {
 		return fmt.Errorf("unsupported configuration show format %q", format)
 	}
+	if operation == Diff && format != "yaml" {
+		return fmt.Errorf("unsupported configuration diff format %q", format)
+	}
 
 	scopeName := "app"
 	if input.scope.Set {
@@ -151,6 +169,11 @@ func prepareConfigOperation[I, S any](prepared *preparedSuite[I, S], operation O
 		path:      path,
 		scopes:    make([]selectedScopeInput[I, S], 0, len(selected)),
 	}
+	if operation == Diff {
+		input.against = inheritComparisonSelection(input.selection, input.against)
+		input.plan = plan
+		return nil
+	}
 	for _, scope := range selected {
 		var value I
 		if prepared.newInput != nil {
@@ -172,6 +195,16 @@ func prepareConfigOperation[I, S any](prepared *preparedSuite[I, S], operation O
 	return nil
 }
 
+func inheritComparisonSelection(left, right Selection) Selection {
+	if !right.Config.Set {
+		right.Config = left.Config
+	}
+	if !right.Overlay.Set {
+		right.Overlay = left.Overlay
+	}
+	return right
+}
+
 func joinOperations(operations []Operation) string {
 	values := make([]string, len(operations))
 	for index, operation := range operations {