Fix: enum with nullable property to automatically include null value in validation

helpers/schema_compiler.go

@@ -207,6 +207,26 @@ func transformNullableSchema(schema map[string]interface{}) map[string]interface
 		schema["oneOf"] = oneOfSlice
 	}
 
+	// Handle enum values - add null if nullable but not already in enum
+	enum, hasEnum := schema["enum"]
+	if hasEnum {
+		if enumSlice, ok := enum.([]interface{}); ok {
+			// Check if null is already in enum
+			hasNull := false
+			for _, v := range enumSlice {
+				if v == nil {
+					hasNull = true
+					break
+				}
+			}
+			// Add null if not present
+			if !hasNull {
+				enumSlice = append(enumSlice, nil)
+				schema["enum"] = enumSlice
+			}
+		}
+	}
+
 	return schema
 }
 

helpers/schema_compiler_test.go

@@ -704,3 +704,98 @@ func TestTransformTypeForCoercion_EdgeCases(t *testing.T) {
 	result = transformTypeForCoercion([]interface{}{"string"})
 	assert.Equal(t, []interface{}{"string"}, result)
 }
+
+func TestTransformNullableSchema_EnumWithoutNull(t *testing.T) {
+	// Test case: nullable: true with enum that doesn't contain null
+	// Expected: null should be automatically added to the enum
+	schema := map[string]interface{}{
+		"type": "string",
+		"enum": []interface{}{
+			"active",
+			"inactive",
+			"pending",
+			"archived",
+		},
+		"nullable": true,
+	}
+
+	result := transformNullableSchema(schema)
+
+	// nullable keyword should be removed
+	_, hasNullable := result["nullable"]
+	assert.False(t, hasNullable)
+
+	// type should be converted to array including null
+	schemaType, ok := result["type"]
+	require.True(t, ok)
+
+	typeArray, ok := schemaType.([]interface{})
+	require.True(t, ok)
+	assert.Contains(t, typeArray, "string")
+	assert.Contains(t, typeArray, "null")
+
+	// enum should contain null
+	enum, ok := result["enum"]
+	require.True(t, ok)
+
+	enumSlice, ok := enum.([]interface{})
+	require.True(t, ok)
+	assert.Len(t, enumSlice, 5) // original 4 values + null
+	assert.Contains(t, enumSlice, "active")
+	assert.Contains(t, enumSlice, "inactive")
+	assert.Contains(t, enumSlice, "pending")
+	assert.Contains(t, enumSlice, "archived")
+	assert.Contains(t, enumSlice, nil)
+}
+
+func TestTransformNullableSchema_EnumWithNull(t *testing.T) {
+	// Test case: nullable: true with enum that already contains null
+	// Expected: null should NOT be added twice
+	schema := map[string]interface{}{
+		"type": "string",
+		"enum": []interface{}{
+			"active",
+			"inactive",
+			"pending",
+			"archived",
+			nil,
+		},
+		"nullable": true,
+	}
+
+	result := transformNullableSchema(schema)
+
+	// nullable keyword should be removed
+	_, hasNullable := result["nullable"]
+	assert.False(t, hasNullable)
+
+	// type should be converted to array including null
+	schemaType, ok := result["type"]
+	require.True(t, ok)
+
+	typeArray, ok := schemaType.([]interface{})
+	require.True(t, ok)
+	assert.Contains(t, typeArray, "string")
+	assert.Contains(t, typeArray, "null")
+
+	// enum should still contain only one null (not duplicated)
+	enum, ok := result["enum"]
+	require.True(t, ok)
+
+	enumSlice, ok := enum.([]interface{})
+	require.True(t, ok)
+	assert.Len(t, enumSlice, 5) // original 5 values (no duplication)
+	assert.Contains(t, enumSlice, "active")
+	assert.Contains(t, enumSlice, "inactive")
+	assert.Contains(t, enumSlice, "pending")
+	assert.Contains(t, enumSlice, "archived")
+
+	// Count how many nulls are in the enum
+	nullCount := 0
+	for _, v := range enumSlice {
+		if v == nil {
+			nullCount++
+		}
+	}
+	assert.Equal(t, 1, nullCount, "enum should contain exactly one null value")
+}

test_specs/nullable_enum.yaml

@@ -0,0 +1,131 @@
+openapi: 3.0.2
+info:
+  title: Nullable Enum Test API
+  version: 1.0.0
+  description: Test specification for nullable enum validation
+paths:
+  /status:
+    get:
+      summary: Get status with nullable enum
+      operationId: getStatus
+      responses:
+        '200':
+          description: Successful response
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/StatusResponse'
+    post:
+      summary: Create status
+      operationId: createStatus
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/StatusRequest'
+      responses:
+        '201':
+          description: Created
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/StatusResponse'
+  /items:
+    get:
+      summary: Get items with nullable enum in array
+      operationId: getItems
+      responses:
+        '200':
+          description: Successful response
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Item'
+components:
+  schemas:
+    StatusResponse:
+      type: object
+      required:
+        - id
+      properties:
+        id:
+          type: integer
+          format: int64
+        status:
+          type: string
+          description: Status field with nullable enum (no null in enum)
+          enum:
+            - active
+            - inactive
+            - pending
+            - archived
+          nullable: true
+        priority:
+          type: string
+          description: Priority field with nullable enum (null already in enum)
+          enum:
+            - high
+            - medium
+            - low
+            - null
+          nullable: true
+        category:
+          type: string
+          description: Non-nullable enum
+          enum:
+            - public
+            - private
+            - internal
+    StatusRequest:
+      type: object
+      required:
+        - status
+      properties:
+        status:
+          type: string
+          enum:
+            - active
+            - inactive
+            - pending
+            - archived
+          nullable: true
+        priority:
+          type: string
+          enum:
+            - high
+            - medium
+            - low
+            - null
+          nullable: true
+    Item:
+      type: object
+      required:
+        - id
+        - name
+      properties:
+        id:
+          type: integer
+          format: int64
+        name:
+          type: string
+        status:
+          type: string
+          description: Nested nullable enum
+          enum:
+            - available
+            - sold
+            - reserved
+          nullable: true
+        metadata:
+          type: object
+          properties:
+            visibility:
+              type: string
+              description: Deeply nested nullable enum
+              enum:
+                - visible
+                - hidden
+              nullable: true