From 59f07c3da104a8040045da7e1e74890197365a35 Mon Sep 17 00:00:00 2001
From: Aliaksandr Pinchuk <aliaksandr.pin4uk@gmail.com>
Date: Mon, 23 Feb 2026 10:42:37 +0100
Subject: [PATCH] NullPointerException when oneOf changes to anyOf in response
 schema Fixes #381

---
 .../ComposedSchemaDiffResult.java             | 20 ++++++-----
 .../openapidiff/core/OneOfDiffTest.java       |  7 ++++
 core/src/test/resources/oneOf_to_anyOf_1.yaml | 34 +++++++++++++++++++
 core/src/test/resources/oneOf_to_anyOf_2.yaml | 34 +++++++++++++++++++
 4 files changed, 86 insertions(+), 9 deletions(-)
 create mode 100644 core/src/test/resources/oneOf_to_anyOf_1.yaml
 create mode 100644 core/src/test/resources/oneOf_to_anyOf_2.yaml

diff --git a/core/src/main/java/org/openapitools/openapidiff/core/compare/schemadiffresult/ComposedSchemaDiffResult.java b/core/src/main/java/org/openapitools/openapidiff/core/compare/schemadiffresult/ComposedSchemaDiffResult.java
index 2f86318eb..336c8b290 100644
--- a/core/src/main/java/org/openapitools/openapidiff/core/compare/schemadiffresult/ComposedSchemaDiffResult.java
+++ b/core/src/main/java/org/openapitools/openapidiff/core/compare/schemadiffresult/ComposedSchemaDiffResult.java
@@ -117,16 +117,18 @@ private Map<String, Schema> getSchema(
 
   private Map<String, String> getMapping(ComposedSchema composedSchema) {
     Map<String, String> reverseMapping = new LinkedHashMap<>();
-    for (Schema<?> schema : composedSchema.getOneOf()) {
-      String ref = schema.get$ref();
-      if (ref == null) {
-        continue;
-      }
-      String schemaName = refPointer.getRefName(ref);
-      if (schemaName == null) {
-        throw new IllegalArgumentException("invalid schema: " + ref);
+    if (composedSchema.getOneOf() != null) {
+      for (Schema<?> schema : composedSchema.getOneOf()) {
+        String ref = schema.get$ref();
+        if (ref == null) {
+          continue;
+        }
+        String schemaName = refPointer.getRefName(ref);
+        if (schemaName == null) {
+          throw new IllegalArgumentException("invalid schema: " + ref);
+        }
+        reverseMapping.put(ref, schemaName);
       }
-      reverseMapping.put(ref, schemaName);
     }
 
     if (composedSchema.getDiscriminator() != null
diff --git a/core/src/test/java/org/openapitools/openapidiff/core/OneOfDiffTest.java b/core/src/test/java/org/openapitools/openapidiff/core/OneOfDiffTest.java
index 87cabc709..bc7b316ee 100644
--- a/core/src/test/java/org/openapitools/openapidiff/core/OneOfDiffTest.java
+++ b/core/src/test/java/org/openapitools/openapidiff/core/OneOfDiffTest.java
@@ -18,6 +18,8 @@ public class OneOfDiffTest {
   private final String OPENAPI_DOC8 = "oneOf_discriminator-missing_1.yaml";
   private final String OPENAPI_DOC9 = "oneOf_discriminator-missing_2.yaml";
   private final String OPENAPI_DOC10 = "unnamed_oneof_schema_1.yaml";
+  private final String OPENAPI_DOC11 = "oneOf_to_anyOf_1.yaml";
+  private final String OPENAPI_DOC12 = "oneOf_to_anyOf_2.yaml";
 
   @Test
   public void testDiffSame() {
@@ -59,4 +61,9 @@ public void testOneOfDiscrimitatorMissingSameOrder() {
   public void testOneOfDiscrimitatorMissingDifferentOrder() {
     assertOpenApiAreEquals(OPENAPI_DOC8, OPENAPI_DOC9);
   }
+
+  @Test // issue #381
+  public void testOneOfToAnyOfDoesNotThrow() {
+    assertOpenApiChangedEndpoints(OPENAPI_DOC11, OPENAPI_DOC12);
+  }
 }
diff --git a/core/src/test/resources/oneOf_to_anyOf_1.yaml b/core/src/test/resources/oneOf_to_anyOf_1.yaml
new file mode 100644
index 000000000..1db19e0d0
--- /dev/null
+++ b/core/src/test/resources/oneOf_to_anyOf_1.yaml
@@ -0,0 +1,34 @@
+openapi: 3.0.1
+info:
+  title: oneOf to anyOf repro (old)
+  version: 1.0.0
+paths:
+  /example:
+    get:
+      responses:
+        "200":
+          description: ok
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Result"
+components:
+  schemas:
+    Result:
+      oneOf:
+        - $ref: "#/components/schemas/TypeA"
+        - $ref: "#/components/schemas/TypeB"
+    TypeA:
+      type: object
+      required:
+        - a
+      properties:
+        a:
+          type: string
+    TypeB:
+      type: object
+      required:
+        - b
+      properties:
+        b:
+          type: string
diff --git a/core/src/test/resources/oneOf_to_anyOf_2.yaml b/core/src/test/resources/oneOf_to_anyOf_2.yaml
new file mode 100644
index 000000000..72ab53aa3
--- /dev/null
+++ b/core/src/test/resources/oneOf_to_anyOf_2.yaml
@@ -0,0 +1,34 @@
+openapi: 3.0.1
+info:
+  title: oneOf to anyOf repro (new)
+  version: 1.0.0
+paths:
+  /example:
+    get:
+      responses:
+        "200":
+          description: ok
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Result"
+components:
+  schemas:
+    Result:
+      anyOf:
+        - $ref: "#/components/schemas/TypeA"
+        - $ref: "#/components/schemas/TypeB"
+    TypeA:
+      type: object
+      required:
+        - a
+      properties:
+        a:
+          type: string
+    TypeB:
+      type: object
+      required:
+        - b
+      properties:
+        b:
+          type: string