diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
index 31426dd375e6..4046fed3b286 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
@@ -26,7 +26,7 @@ export function instanceOf{{classname}}(value: object): value is {{classname}} {
     {{#vars}}
     {{#required}}
     {{#hasSanitizedName}}
-    if ((!('{{name}}' in value) && !('{{baseName}}' in value)) || (value['{{name}}'] === undefined && value['{{baseName}}'] === undefined)) return false;
+    if ((!('{{name}}' in (value as Record<string, any>)) && !('{{baseName}}' in (value as Record<string, any>))) || ((value as Record<string, any>)['{{name}}'] === undefined && (value as Record<string, any>)['{{baseName}}'] === undefined)) return false;
     {{/hasSanitizedName}}
     {{^hasSanitizedName}}
     if (!('{{name}}' in value) || value['{{name}}'] === undefined) return false;
@@ -37,8 +37,8 @@ export function instanceOf{{classname}}(value: object): value is {{classname}} {
     {{#-first}}
     {{#-last}}
     {{#hasSanitizedName}}
-    {{#isString}}if (value['{{name}}'] !== '{{.}}' && value['{{baseName}}'] !== '{{.}}') return false;{{/isString}}
-    {{^isString}}if (value['{{name}}'] !== {{.}} && value['{{baseName}}'] !== {{.}}) return false;{{/isString}}
+    {{#isString}}if ((value as Record<string, any>)['{{name}}'] !== '{{.}}' && (value as Record<string, any>)['{{baseName}}'] !== '{{.}}') return false;{{/isString}}
+    {{^isString}}if ((value as Record<string, any>)['{{name}}'] !== {{.}} && (value as Record<string, any>)['{{baseName}}'] !== {{.}}) return false;{{/isString}}
     {{/hasSanitizedName}}
     {{^hasSanitizedName}}
     {{#isString}}if (value['{{name}}'] !== '{{.}}') return false;{{/isString}}
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java
index 094929f2fe53..bc015a30a947 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java
@@ -466,25 +466,26 @@ public void testInstanceOfChecksDiscriminatorValue() throws IOException {
 
         // SnakeOptionOne: discriminator_field (snake_case baseName) vs discriminatorField (camelCase name)
         // instanceOf should check both casings for field presence and discriminator value
+        // (the value is widened to Record<string, any> to avoid TS2590 on wide models)
         Path snakeOptionOne = Paths.get(output + "/models/SnakeOptionOne.ts");
         TestUtils.assertFileExists(snakeOptionOne);
-        TestUtils.assertFileContains(snakeOptionOne, "'discriminatorField' in value");
-        TestUtils.assertFileContains(snakeOptionOne, "'discriminator_field' in value");
-        TestUtils.assertFileContains(snakeOptionOne, "value['discriminatorField'] !== 'snakeOptionOne'");
-        TestUtils.assertFileContains(snakeOptionOne, "value['discriminator_field'] !== 'snakeOptionOne'");
+        TestUtils.assertFileContains(snakeOptionOne, "'discriminatorField' in (value as Record<string, any>)");
+        TestUtils.assertFileContains(snakeOptionOne, "'discriminator_field' in (value as Record<string, any>)");
+        TestUtils.assertFileContains(snakeOptionOne, "(value as Record<string, any>)['discriminatorField'] !== 'snakeOptionOne'");
+        TestUtils.assertFileContains(snakeOptionOne, "(value as Record<string, any>)['discriminator_field'] !== 'snakeOptionOne'");
         // Also verify the non-enum required field checks both casings
-        TestUtils.assertFileContains(snakeOptionOne, "'someProperty' in value");
-        TestUtils.assertFileContains(snakeOptionOne, "'some_property' in value");
+        TestUtils.assertFileContains(snakeOptionOne, "'someProperty' in (value as Record<string, any>)");
+        TestUtils.assertFileContains(snakeOptionOne, "'some_property' in (value as Record<string, any>)");
 
         // DashedOptionOne: discriminator-field (dashed baseName) vs discriminatorField (camelCase name)
         Path dashedOptionOne = Paths.get(output + "/models/DashedOptionOne.ts");
         TestUtils.assertFileExists(dashedOptionOne);
-        TestUtils.assertFileContains(dashedOptionOne, "'discriminatorField' in value");
-        TestUtils.assertFileContains(dashedOptionOne, "'discriminator-field' in value");
-        TestUtils.assertFileContains(dashedOptionOne, "value['discriminatorField'] !== 'dashedOptionOne'");
-        TestUtils.assertFileContains(dashedOptionOne, "value['discriminator-field'] !== 'dashedOptionOne'");
-        TestUtils.assertFileContains(dashedOptionOne, "'someProperty' in value");
-        TestUtils.assertFileContains(dashedOptionOne, "'some-property' in value");
+        TestUtils.assertFileContains(dashedOptionOne, "'discriminatorField' in (value as Record<string, any>)");
+        TestUtils.assertFileContains(dashedOptionOne, "'discriminator-field' in (value as Record<string, any>)");
+        TestUtils.assertFileContains(dashedOptionOne, "(value as Record<string, any>)['discriminatorField'] !== 'dashedOptionOne'");
+        TestUtils.assertFileContains(dashedOptionOne, "(value as Record<string, any>)['discriminator-field'] !== 'dashedOptionOne'");
+        TestUtils.assertFileContains(dashedOptionOne, "'someProperty' in (value as Record<string, any>)");
+        TestUtils.assertFileContains(dashedOptionOne, "'some-property' in (value as Record<string, any>)");
 
         // Numeric singleton enum: value check must NOT quote the literal
         Path numericModel = Paths.get(output + "/models/NumericSingletonEnumModel.ts");
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
index 6890904e62b9..13edcbd9d92c 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
@@ -142,7 +142,7 @@ export type EnumTestEnumNumberEnum = typeof EnumTestEnumNumberEnum[keyof typeof
  * Check if a given object implements the EnumTest interface.
  */
 export function instanceOfEnumTest(value: object): value is EnumTest {
-    if ((!('enumStringRequired' in value) && !('enum_string_required' in value)) || (value['enumStringRequired'] === undefined && value['enum_string_required'] === undefined)) return false;
+    if ((!('enumStringRequired' in (value as Record<string, any>)) && !('enum_string_required' in (value as Record<string, any>))) || ((value as Record<string, any>)['enumStringRequired'] === undefined && (value as Record<string, any>)['enum_string_required'] === undefined)) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
index 822bb60636fb..c80896e71861 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
@@ -122,7 +122,7 @@ export interface FormatTest {
  */
 export function instanceOfFormatTest(value: object): value is FormatTest {
     if (!('number' in value) || value['number'] === undefined) return false;
-    if ((!('_byte' in value) && !('byte' in value)) || (value['_byte'] === undefined && value['byte'] === undefined)) return false;
+    if ((!('_byte' in (value as Record<string, any>)) && !('byte' in (value as Record<string, any>))) || ((value as Record<string, any>)['_byte'] === undefined && (value as Record<string, any>)['byte'] === undefined)) return false;
     if (!('date' in value) || value['date'] === undefined) return false;
     if (!('password' in value) || value['password'] === undefined) return false;
     return true;
diff --git a/samples/client/petstore/typescript-fetch/builds/oneOf/models/DashedOptionOne.ts b/samples/client/petstore/typescript-fetch/builds/oneOf/models/DashedOptionOne.ts
index 1a37609b9973..a6e019ea9fcd 100644
--- a/samples/client/petstore/typescript-fetch/builds/oneOf/models/DashedOptionOne.ts
+++ b/samples/client/petstore/typescript-fetch/builds/oneOf/models/DashedOptionOne.ts
@@ -47,10 +47,10 @@ export type DashedOptionOneDiscriminatorFieldEnum = typeof DashedOptionOneDiscri
  * Check if a given object implements the DashedOptionOne interface.
  */
 export function instanceOfDashedOptionOne(value: object): value is DashedOptionOne {
-    if ((!('discriminatorField' in value) && !('discriminator-field' in value)) || (value['discriminatorField'] === undefined && value['discriminator-field'] === undefined)) return false;
-    if (value['discriminatorField'] !== 'dashedOptionOne' && value['discriminator-field'] !== 'dashedOptionOne') return false;
+    if ((!('discriminatorField' in (value as Record<string, any>)) && !('discriminator-field' in (value as Record<string, any>))) || ((value as Record<string, any>)['discriminatorField'] === undefined && (value as Record<string, any>)['discriminator-field'] === undefined)) return false;
+    if ((value as Record<string, any>)['discriminatorField'] !== 'dashedOptionOne' && (value as Record<string, any>)['discriminator-field'] !== 'dashedOptionOne') return false;
     
-    if ((!('someProperty' in value) && !('some-property' in value)) || (value['someProperty'] === undefined && value['some-property'] === undefined)) return false;
+    if ((!('someProperty' in (value as Record<string, any>)) && !('some-property' in (value as Record<string, any>))) || ((value as Record<string, any>)['someProperty'] === undefined && (value as Record<string, any>)['some-property'] === undefined)) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/oneOf/models/DashedOptionTwo.ts b/samples/client/petstore/typescript-fetch/builds/oneOf/models/DashedOptionTwo.ts
index 80d2f6424fc8..ca3cd32e44dc 100644
--- a/samples/client/petstore/typescript-fetch/builds/oneOf/models/DashedOptionTwo.ts
+++ b/samples/client/petstore/typescript-fetch/builds/oneOf/models/DashedOptionTwo.ts
@@ -47,10 +47,10 @@ export type DashedOptionTwoDiscriminatorFieldEnum = typeof DashedOptionTwoDiscri
  * Check if a given object implements the DashedOptionTwo interface.
  */
 export function instanceOfDashedOptionTwo(value: object): value is DashedOptionTwo {
-    if ((!('discriminatorField' in value) && !('discriminator-field' in value)) || (value['discriminatorField'] === undefined && value['discriminator-field'] === undefined)) return false;
-    if (value['discriminatorField'] !== 'dashedOptionTwo' && value['discriminator-field'] !== 'dashedOptionTwo') return false;
+    if ((!('discriminatorField' in (value as Record<string, any>)) && !('discriminator-field' in (value as Record<string, any>))) || ((value as Record<string, any>)['discriminatorField'] === undefined && (value as Record<string, any>)['discriminator-field'] === undefined)) return false;
+    if ((value as Record<string, any>)['discriminatorField'] !== 'dashedOptionTwo' && (value as Record<string, any>)['discriminator-field'] !== 'dashedOptionTwo') return false;
     
-    if ((!('someProperty' in value) && !('some-property' in value)) || (value['someProperty'] === undefined && value['some-property'] === undefined)) return false;
+    if ((!('someProperty' in (value as Record<string, any>)) && !('some-property' in (value as Record<string, any>))) || ((value as Record<string, any>)['someProperty'] === undefined && (value as Record<string, any>)['some-property'] === undefined)) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/oneOf/models/SnakeOptionOne.ts b/samples/client/petstore/typescript-fetch/builds/oneOf/models/SnakeOptionOne.ts
index 5e2aa0dac710..fb3a19ecd15d 100644
--- a/samples/client/petstore/typescript-fetch/builds/oneOf/models/SnakeOptionOne.ts
+++ b/samples/client/petstore/typescript-fetch/builds/oneOf/models/SnakeOptionOne.ts
@@ -47,10 +47,10 @@ export type SnakeOptionOneDiscriminatorFieldEnum = typeof SnakeOptionOneDiscrimi
  * Check if a given object implements the SnakeOptionOne interface.
  */
 export function instanceOfSnakeOptionOne(value: object): value is SnakeOptionOne {
-    if ((!('discriminatorField' in value) && !('discriminator_field' in value)) || (value['discriminatorField'] === undefined && value['discriminator_field'] === undefined)) return false;
-    if (value['discriminatorField'] !== 'snakeOptionOne' && value['discriminator_field'] !== 'snakeOptionOne') return false;
+    if ((!('discriminatorField' in (value as Record<string, any>)) && !('discriminator_field' in (value as Record<string, any>))) || ((value as Record<string, any>)['discriminatorField'] === undefined && (value as Record<string, any>)['discriminator_field'] === undefined)) return false;
+    if ((value as Record<string, any>)['discriminatorField'] !== 'snakeOptionOne' && (value as Record<string, any>)['discriminator_field'] !== 'snakeOptionOne') return false;
     
-    if ((!('someProperty' in value) && !('some_property' in value)) || (value['someProperty'] === undefined && value['some_property'] === undefined)) return false;
+    if ((!('someProperty' in (value as Record<string, any>)) && !('some_property' in (value as Record<string, any>))) || ((value as Record<string, any>)['someProperty'] === undefined && (value as Record<string, any>)['some_property'] === undefined)) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/oneOf/models/SnakeOptionTwo.ts b/samples/client/petstore/typescript-fetch/builds/oneOf/models/SnakeOptionTwo.ts
index a89ac1d76292..f28bb29a556f 100644
--- a/samples/client/petstore/typescript-fetch/builds/oneOf/models/SnakeOptionTwo.ts
+++ b/samples/client/petstore/typescript-fetch/builds/oneOf/models/SnakeOptionTwo.ts
@@ -47,10 +47,10 @@ export type SnakeOptionTwoDiscriminatorFieldEnum = typeof SnakeOptionTwoDiscrimi
  * Check if a given object implements the SnakeOptionTwo interface.
  */
 export function instanceOfSnakeOptionTwo(value: object): value is SnakeOptionTwo {
-    if ((!('discriminatorField' in value) && !('discriminator_field' in value)) || (value['discriminatorField'] === undefined && value['discriminator_field'] === undefined)) return false;
-    if (value['discriminatorField'] !== 'snakeOptionTwo' && value['discriminator_field'] !== 'snakeOptionTwo') return false;
+    if ((!('discriminatorField' in (value as Record<string, any>)) && !('discriminator_field' in (value as Record<string, any>))) || ((value as Record<string, any>)['discriminatorField'] === undefined && (value as Record<string, any>)['discriminator_field'] === undefined)) return false;
+    if ((value as Record<string, any>)['discriminatorField'] !== 'snakeOptionTwo' && (value as Record<string, any>)['discriminator_field'] !== 'snakeOptionTwo') return false;
     
-    if ((!('someProperty' in value) && !('some_property' in value)) || (value['someProperty'] === undefined && value['some_property'] === undefined)) return false;
+    if ((!('someProperty' in (value as Record<string, any>)) && !('some_property' in (value as Record<string, any>))) || ((value as Record<string, any>)['someProperty'] === undefined && (value as Record<string, any>)['some_property'] === undefined)) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
index 790ff584911a..819c813af5f1 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
@@ -39,7 +39,7 @@ export interface Animal {
  * Check if a given object implements the Animal interface.
  */
 export function instanceOfAnimal(value: object): value is Animal {
-    if ((!('className' in value) && !('class_name' in value)) || (value['className'] === undefined && value['class_name'] === undefined)) return false;
+    if ((!('className' in (value as Record<string, any>)) && !('class_name' in (value as Record<string, any>))) || ((value as Record<string, any>)['className'] === undefined && (value as Record<string, any>)['class_name'] === undefined)) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
index 6890904e62b9..13edcbd9d92c 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
@@ -142,7 +142,7 @@ export type EnumTestEnumNumberEnum = typeof EnumTestEnumNumberEnum[keyof typeof
  * Check if a given object implements the EnumTest interface.
  */
 export function instanceOfEnumTest(value: object): value is EnumTest {
-    if ((!('enumStringRequired' in value) && !('enum_string_required' in value)) || (value['enumStringRequired'] === undefined && value['enum_string_required'] === undefined)) return false;
+    if ((!('enumStringRequired' in (value as Record<string, any>)) && !('enum_string_required' in (value as Record<string, any>))) || ((value as Record<string, any>)['enumStringRequired'] === undefined && (value as Record<string, any>)['enum_string_required'] === undefined)) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
index 822bb60636fb..c80896e71861 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
@@ -122,7 +122,7 @@ export interface FormatTest {
  */
 export function instanceOfFormatTest(value: object): value is FormatTest {
     if (!('number' in value) || value['number'] === undefined) return false;
-    if ((!('_byte' in value) && !('byte' in value)) || (value['_byte'] === undefined && value['byte'] === undefined)) return false;
+    if ((!('_byte' in (value as Record<string, any>)) && !('byte' in (value as Record<string, any>))) || ((value as Record<string, any>)['_byte'] === undefined && (value as Record<string, any>)['byte'] === undefined)) return false;
     if (!('date' in value) || value['date'] === undefined) return false;
     if (!('password' in value) || value['password'] === undefined) return false;
     return true;