From 80e2602b4dc93688b21bf963e4e309248a41289a Mon Sep 17 00:00:00 2001
From: Simon Podlipsky <simon@podlipsky.net>
Date: Fri, 2 Oct 2026 11:33:15 +0200
Subject: [PATCH] fix(typescript-fetch): preserve nullable primitive oneOf
 branches

---
 .../TypeScriptFetchClientCodegen.java         |  5 ++
 .../TypeScriptFetchClientCodegenTest.java     | 33 ++++++++++++
 .../oneof-nullable-primitives.yaml            | 50 +++++++++++++++++++
 3 files changed, 88 insertions(+)
 create mode 100644 modules/openapi-generator/src/test/resources/3_0/typescript-fetch/oneof-nullable-primitives.yaml

diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptFetchClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptFetchClientCodegen.java
index 411521087608..da4471838f8f 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptFetchClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptFetchClientCodegen.java
@@ -965,6 +965,11 @@ private ExtendedCodegenModel processCodeGenModel(ExtendedCodegenModel cm) {
                 .map(CodegenComposedSchemas::getOneOf)
                 .orElse(Collections.emptyList());
 
+        // Type names omit branch nullability. Null is valid only when exactly one branch accepts it.
+        if (oneOfsList.stream().filter(cp -> cp.isNullable).count() == 1) {
+            cm.oneOf.add("null");
+        }
+
         // create a set of any non-primitive, non-array types used in the oneOf schemas which will
         // need to be imported.
         cm.oneOfModels = oneOfsList.stream()
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 bd92f9d4617d..e325ecc573d8 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
@@ -17,6 +17,7 @@
 import org.openapitools.codegen.typescript.TypeScriptGroups;
 import org.openapitools.codegen.utils.ModelUtils;
 import org.testng.Assert;
+import org.testng.annotations.DataProvider;
 import org.testng.annotations.Test;
 
 import java.io.File;
@@ -752,6 +753,38 @@ public void testOneOfModelsDoNotImportPrimitiveTypes() throws IOException {
         TestUtils.assertFileContains(testDiscriminatorResponse, "export type TestDiscriminatorResponse = { discriminatorField: 'optionOne' } & OptionOne | { discriminatorField: 'optionTwo' } & OptionTwo");
     }
 
+    @DataProvider
+    public Object[][] nullableOneOfRuntimeCheckModes() {
+        return new Object[][] {{false}, {true}};
+    }
+
+    @Test(dataProvider = "nullableOneOfRuntimeCheckModes")
+    public void testNullablePrimitiveOneOfTypes(boolean withoutRuntimeChecks) throws IOException {
+        Map<String, Object> properties = new HashMap<>();
+        properties.put(TypeScriptFetchClientCodegen.WITHOUT_RUNTIME_CHECKS, withoutRuntimeChecks);
+        File output = generate(properties,
+                "src/test/resources/3_0/typescript-fetch/oneof-nullable-primitives.yaml");
+
+        for (String model : List.of("InlineNullableValue", "ReferencedNullableValue")) {
+            Path file = Paths.get(output + (withoutRuntimeChecks ? "/models/index.ts" : "/models/" + model + ".ts"));
+            TestUtils.assertFileContains(file, "export type " + model + " = null | number | string;");
+        }
+
+        // Null must match exactly one branch, rather than none or both branches.
+        for (String model : List.of("NonNullableValue", "MultipleNullableValue")) {
+            Path file = Paths.get(output + (withoutRuntimeChecks ? "/models/index.ts" : "/models/" + model + ".ts"));
+            TestUtils.assertFileContains(file, "export type " + model + " = number | string;");
+        }
+
+        Path arrayItems = Paths.get(output + (withoutRuntimeChecks
+                ? "/models/index.ts" : "/models/NullableArrayItems.ts"));
+        TestUtils.assertFileContains(arrayItems, "export type NullableArrayItems = Array<string | null> | number;");
+
+        Path values = Paths.get(output + (withoutRuntimeChecks ? "/models/index.ts" : "/models/Values.ts"));
+        TestUtils.assertFileContains(values, "array: Array<InlineNullableValue>;",
+                "map: { [key: string]: InlineNullableValue; };");
+    }
+
     /**
      * Issue #21587
      * When using oneOf, the Typescript Fetch generator should import modelled types except for
diff --git a/modules/openapi-generator/src/test/resources/3_0/typescript-fetch/oneof-nullable-primitives.yaml b/modules/openapi-generator/src/test/resources/3_0/typescript-fetch/oneof-nullable-primitives.yaml
new file mode 100644
index 000000000000..0a6e3e35253f
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/3_0/typescript-fetch/oneof-nullable-primitives.yaml
@@ -0,0 +1,50 @@
+openapi: 3.0.3
+info:
+  title: Nullable primitive oneOf values
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    NullableString:
+      type: string
+      nullable: true
+    InlineNullableValue:
+      oneOf:
+        - type: string
+          nullable: true
+        - type: number
+    ReferencedNullableValue:
+      oneOf:
+        - $ref: '#/components/schemas/NullableString'
+        - type: number
+    NonNullableValue:
+      oneOf:
+        - type: string
+        - type: number
+    MultipleNullableValue:
+      oneOf:
+        - type: string
+          nullable: true
+        - type: number
+          nullable: true
+    NullableArrayItems:
+      oneOf:
+        - type: array
+          items:
+            type: string
+            nullable: true
+        - type: number
+    Values:
+      type: object
+      required:
+        - array
+        - map
+      properties:
+        array:
+          type: array
+          items:
+            $ref: '#/components/schemas/InlineNullableValue'
+        map:
+          type: object
+          additionalProperties:
+            $ref: '#/components/schemas/InlineNullableValue'