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‎src/Microsoft.OpenApi/Models/OpenApiSchema.cs‎
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‎test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs‎
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‎test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs‎
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@@ -2,14 +2,12 @@
 
 ## [3.1.0](https://github.com/microsoft/OpenAPI.NET/compare/v3.0.3...v3.1.0) (2025-12-17)
 
-
 ### Features
 
 * Add `type: "null"` downcasting when in oneOf and anyOf for OpenAPI v3 ([782cf8d](https://github.com/microsoft/OpenAPI.NET/commit/782cf8d1ff8166e3c7be706e08dabf168b9616a4))
 
 ## [3.0.3](https://github.com/microsoft/OpenAPI.NET/compare/v3.0.2...v3.0.3) (2025-12-16)
 
-### Bug Fixes
 
 * load JSON documents that are preceded by multiple whitespace ([6461bac](https://github.com/microsoft/OpenAPI.NET/commit/6461bac01c4176424210e9ac249698f665a514a6))
 * non-seekable json streams would fail to load as a document ([2436d73](https://github.com/microsoft/OpenAPI.NET/commit/2436d7382bfbf8b9ba501d88f682e952bdf27146))
@@ -43,8 +41,20 @@
 
 * adds support for OpenAPI 3.2.0 ([765a8dd](https://github.com/microsoft/OpenAPI.NET/commit/765a8dd4d6efd1a31b6a76d282ccffa5877a845a))
 
-## [2.3.12](https://github.com/microsoft/OpenAPI.NET/compare/v2.3.11...v2.3.12) (2025-12-15)
+## [2.4.1](https://github.com/microsoft/OpenAPI.NET/compare/v2.4.0...v2.4.1) (2025-12-18)
+
+### Bug Fixes
+
+* **schema:** always serialize `additionalProperties: false` ([6651c36](https://github.com/microsoft/OpenAPI.NET/commit/6651c36ff341329c053776d65b36b1b7fa9dd3ea))
+* **schema:** always serialize `additionalProperties: false` ([e36fc95](https://github.com/microsoft/OpenAPI.NET/commit/e36fc9565bce42916eb7bf64d1f74d491dd1f407))
 
+## [2.4.0](https://github.com/microsoft/OpenAPI.NET/compare/v2.3.12...v2.4.0) (2025-12-17)
+
+### Features
+
+* Add `type: "null"` downcasting when in oneOf and anyOf for OpenAPI v3 ([782cf8d](https://github.com/microsoft/OpenAPI.NET/commit/782cf8d1ff8166e3c7be706e08dabf168b9616a4))
+
+## [2.3.12](https://github.com/microsoft/OpenAPI.NET/compare/v2.3.11...v2.3.12) (2025-12-15)
 
 ### Bug Fixes
 
 
@@ -494,9 +494,8 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
                     AdditionalProperties,
                     callback);
             }
-            // true is the default in earlier versions 3, no need to write it out
-            // boolean value is only supported for version 3 and earlier (version 2 is implemented in the other serialize method, the condition is a failsafe)
-            else if (!AdditionalPropertiesAllowed && version <= OpenApiSpecVersion.OpenApi3_0)
+            // true is the default, no need to write it out
+            else if (!AdditionalPropertiesAllowed)
             {
                 writer.WriteProperty(OpenApiConstants.AdditionalProperties, AdditionalPropertiesAllowed);
             }
 
@@ -755,8 +755,11 @@ public async Task SerializeAdditionalPropertiesAllowedAsV3PlusDefaultDoesNotEmit
             Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
         }
 
-        [Fact]
-        public async Task SerializeAdditionalPropertiesAllowedAsV3FalseEmits()
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_1)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_2)]
+        public async Task SerializeAdditionalPropertiesAllowedAsV3PlusFalseEmits(OpenApiSpecVersion version)
         {
             var expected = @"{ ""additionalProperties"": false }";
             // Given
@@ -766,7 +769,7 @@ public async Task SerializeAdditionalPropertiesAllowedAsV3FalseEmits()
             };
 
             // When
-            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
+            var actual = await schema.SerializeAsJsonAsync(version);
 
             // Then
             Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
Original file line number	Diff line number	Diff line change
`@@ -494,9 +494,8 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version`
`494`	`494`	`AdditionalProperties,`
`495`	`495`	`callback);`
`496`	`496`	`}`
`497`		`- // true is the default in earlier versions 3, no need to write it out`
`498`		`- // boolean value is only supported for version 3 and earlier (version 2 is implemented in the other serialize method, the condition is a failsafe)`
`499`		`- else if (!AdditionalPropertiesAllowed && version <= OpenApiSpecVersion.OpenApi3_0)`
	`497`	`+ // true is the default, no need to write it out`
	`498`	`+ else if (!AdditionalPropertiesAllowed)`
`500`	`499`	`{`
`501`	`500`	`writer.WriteProperty(OpenApiConstants.AdditionalProperties, AdditionalPropertiesAllowed);`
`502`	`501`	`}`