diff --git a/src/create-define-rules/create-define-rules.test.ts b/src/create-define-rules/create-define-rules.test.ts
index 0518740..aed47cf 100644
--- a/src/create-define-rules/create-define-rules.test.ts
+++ b/src/create-define-rules/create-define-rules.test.ts
@@ -32,8 +32,8 @@ describe(createDefineRules, () => {
 		expectTypeOf(rules['@morev/sass/no-unused-variables']).toEqualTypeOf<[
 			true,
 			{
-				checkRoot: true;
-				ignore: ['b'];
+				readonly checkRoot: true;
+				readonly ignore: readonly ['b'];
 			},
 		]>();
 	});
@@ -48,7 +48,7 @@ describe(createDefineRules, () => {
 		expectTypeOf<keyof typeof rules>().toEqualTypeOf<'@morev/sass/no-unused-variables'>();
 		expectTypeOf(rules['@morev/sass/no-unused-variables']).toEqualTypeOf<[
 			true,
-			{ checkRoot: true },
+			{ readonly checkRoot: true },
 		]>();
 	});
 
@@ -65,9 +65,9 @@ describe(createDefineRules, () => {
 		expectTypeOf(rules['@morev/bem/block-variable']).toEqualTypeOf<[
 			true,
 			{
-				firstChild: true;
-				severity: 'warning';
-				separators: undefined;
+				readonly firstChild: true;
+				readonly severity: 'warning';
+				readonly separators: undefined;
 			},
 		]>();
 	});
@@ -86,8 +86,8 @@ describe(createDefineRules, () => {
 		expectTypeOf(rules['@morev/bem/block-variable']).toEqualTypeOf<[
 			true,
 			{
-				firstChild: true;
-				separators: {
+				readonly firstChild: true;
+				readonly separators: {
 					readonly element: '__';
 				};
 			},
@@ -118,8 +118,8 @@ describe(createDefineRules, () => {
 		expectTypeOf(rules['@morev/bem/block-variable']).toEqualTypeOf<[
 			true,
 			{
-				firstChild: true;
-				separators: typeof separators;
+				readonly firstChild: true;
+				readonly separators: typeof separators;
 			},
 		]>();
 	});
@@ -141,8 +141,8 @@ describe(createDefineRules, () => {
 		expectTypeOf(rules['@morev/bem/block-variable']).toEqualTypeOf<[
 			true,
 			{
-				firstChild: true;
-				separators: undefined;
+				readonly firstChild: true;
+				readonly separators: undefined;
 			},
 		]>();
 	});
diff --git a/src/create-define-rules/create-define-rules.types.ts b/src/create-define-rules/create-define-rules.types.ts
index d35b5d0..ccbd3be 100644
--- a/src/create-define-rules/create-define-rules.types.ts
+++ b/src/create-define-rules/create-define-rules.types.ts
@@ -1,4 +1,4 @@
-import type { ReadonlyDeep, Simplify, WritableDeep } from 'type-fest';
+import type { ReadonlyDeep, Simplify } from 'type-fest';
 import type { PluginGlobals, RulesSchema } from '#modules/meta';
 import type { RuleWithSeparators } from './create-define-rules';
 
@@ -63,15 +63,15 @@ type GlobalSeparators<Globals extends PluginGlobals> =
  * Secondary options after runtime normalization.
  *
  * Rules that consume BEM separators receive separators from plugin globals;
- * all other rules keep their secondary options as writable output objects.
+ * all other rules keep their inferred secondary options.
  */
 type NormalizedRuleOptions<
 	RuleName,
 	Secondary,
 	Globals extends PluginGlobals,
 > = RuleName extends RuleWithSeparators
-	? Simplify<Omit<WritableDeep<Secondary>, 'separators'> & { separators: GlobalSeparators<Globals> }>
-	: WritableDeep<Secondary>;
+	? Simplify<Omit<Secondary, 'separators'> & { readonly separators: GlobalSeparators<Globals> }>
+	: Secondary;
 
 /**
  * Normalized two-item rule tuple returned by `defineRules`.