diff --git a/packages/quicktype-core/src/rewrites/FlattenUnions.ts b/packages/quicktype-core/src/rewrites/FlattenUnions.ts
index 4357e7e7d2..aab4d85119 100644
--- a/packages/quicktype-core/src/rewrites/FlattenUnions.ts
+++ b/packages/quicktype-core/src/rewrites/FlattenUnions.ts
@@ -59,6 +59,16 @@ export function flattenUnions(
             .join(",");
     }
 
+    function containsIntersection(t: Type, seen: Set<number>): boolean {
+        if (seen.has(t.index)) return false;
+        seen.add(t.index);
+
+        if (t instanceof IntersectionType) return true;
+        if (!(t instanceof UnionType)) return false;
+
+        return iterableSome(t.members, (m) => containsIntersection(m, seen));
+    }
+
     function replace(
         types: ReadonlySet<Type>,
         builder: GraphRewriteBuilder<Type>,
@@ -86,7 +96,9 @@ export function flattenUnions(
                 trefs.map((tref) => derefTypeRef(tref, graph)),
             );
             if (
-                iterableSome(typesToUnify, (t) => t instanceof IntersectionType)
+                iterableSome(typesToUnify, (t) =>
+                    containsIntersection(t, new Set()),
+                )
             ) {
                 trefs = trefs.map((tref) =>
                     builder.reconstituteType(derefTypeRef(tref, graph)),
diff --git a/test/inputs/schema/nested-intersection-union.1.fail.union.json b/test/inputs/schema/nested-intersection-union.1.fail.union.json
new file mode 100644
index 0000000000..cedeb3f00b
--- /dev/null
+++ b/test/inputs/schema/nested-intersection-union.1.fail.union.json
@@ -0,0 +1,8 @@
+{
+    "input": [
+        {
+            "content": "valid item"
+        },
+        ["this array is not a valid item"]
+    ]
+}
diff --git a/test/inputs/schema/nested-intersection-union.1.json b/test/inputs/schema/nested-intersection-union.1.json
new file mode 100644
index 0000000000..3018c7a100
--- /dev/null
+++ b/test/inputs/schema/nested-intersection-union.1.json
@@ -0,0 +1,20 @@
+{
+    "input": [
+        {
+            "content": "hello",
+            "id": "item-1",
+            "output": "42",
+            "summary": "all fields present"
+        },
+        {
+            "content": "just a message"
+        },
+        {
+            "id": "item-2",
+            "output": "function result"
+        },
+        {
+            "summary": "just reasoning"
+        }
+    ]
+}
diff --git a/test/inputs/schema/nested-intersection-union.schema b/test/inputs/schema/nested-intersection-union.schema
new file mode 100644
index 0000000000..0453188e30
--- /dev/null
+++ b/test/inputs/schema/nested-intersection-union.schema
@@ -0,0 +1,58 @@
+{
+    "$schema": "http://json-schema.org/draft-07/schema#",
+    "type": "object",
+    "additionalProperties": false,
+    "properties": {
+        "input": {
+            "type": "array",
+            "items": {
+                "oneOf": [
+                    { "$ref": "#/definitions/Message" },
+                    { "$ref": "#/definitions/Item" }
+                ]
+            }
+        }
+    },
+    "required": ["input"],
+    "definitions": {
+        "Message": {
+            "type": "object",
+            "properties": {
+                "content": { "type": "string" }
+            },
+            "required": ["content"]
+        },
+        "Item": {
+            "oneOf": [
+                { "$ref": "#/definitions/FunctionOutput" },
+                { "$ref": "#/definitions/ReasoningItem" }
+            ]
+        },
+        "FunctionOutput": {
+            "allOf": [
+                { "$ref": "#/definitions/BaseItem" },
+                {
+                    "type": "object",
+                    "properties": {
+                        "output": { "type": "string" }
+                    },
+                    "required": ["output"]
+                }
+            ]
+        },
+        "BaseItem": {
+            "type": "object",
+            "properties": {
+                "id": { "type": "string" }
+            },
+            "required": ["id"]
+        },
+        "ReasoningItem": {
+            "type": "object",
+            "properties": {
+                "summary": { "type": "string" }
+            },
+            "required": ["summary"]
+        }
+    }
+}
diff --git a/test/languages.ts b/test/languages.ts
index 5d971b30ca..503ea71c6d 100644
--- a/test/languages.ts
+++ b/test/languages.ts
@@ -545,6 +545,7 @@ export const CJSONLanguage: Language = {
         "class-with-additional.schema",
         "go-schema-pattern-properties.schema",
         "multi-type-enum.schema",
+        "nested-intersection-union.schema",
         /* Constraints (min/max and regex) are not supported (for the current implementation, can be added later, should abord parsing and return NULL) */
         "minmaxlength.schema",
         "optional-const-ref.schema",
@@ -615,6 +616,8 @@ export const CPlusPlusLanguage: Language = {
     skipSchema: [
         // uses too much memory
         "keyword-unions.schema",
+        // The generated deserializer accepts non-object values when all class properties are optional.
+        "nested-intersection-union.schema",
         // Recursive top-level unions produce aliases that can refer to later aliases.
         "recursive-union-flattening.schema",
     ],
@@ -1016,6 +1019,9 @@ I havea no idea how to encode these tests correctly.
         "integer-string.schema",
         "intersection.schema",
         ...skipsUntypedUnions,
+        // The test driver prints the circe DecodingFailure and exits 0, so
+        // expected-failure samples cannot be detected.
+        "nested-intersection-union.schema",
         "date-time-or-string.schema",
         "implicit-one-of.schema",
         "go-schema-pattern-properties.schema",
@@ -1155,6 +1161,9 @@ export const KotlinLanguage: Language = {
         // which is not represented in the types (implicit-class-array-union);
         // class-map-union: KlaxonException: Couldn't find a suitable constructor for class UnionValue to initialize with {}
         ...skipsUntypedUnions,
+        // Deserializes an array where a union of two classes is expected
+        // instead of rejecting it.
+        "nested-intersection-union.schema",
         "class-with-additional.schema",
         "go-schema-pattern-properties.schema",
         // IllegalArgumentException
@@ -1240,6 +1249,9 @@ export const KotlinJacksonLanguage: Language = {
         // which is not represented in the types (implicit-class-array-union);
         // class-map-union: KlaxonException: Couldn't find a suitable constructor for class UnionValue to initialize with {}
         ...skipsUntypedUnions,
+        // Deserializes an array where a union of two classes is expected
+        // instead of rejecting it.
+        "nested-intersection-union.schema",
         "class-with-additional.schema",
         "go-schema-pattern-properties.schema",
         // IllegalArgumentException
@@ -1451,6 +1463,9 @@ export const HaskellLanguage: Language = {
     skipSchema: [
         "any.schema",
         ...skipsUntypedUnions,
+        // The test driver encodes the Maybe result, so a failed decode prints
+        // "null" and exits 0 — expected-failure samples cannot be detected.
+        "nested-intersection-union.schema",
         "direct-union.schema",
         ...skipsEnumValueValidation,
         "go-schema-pattern-properties.schema",