From fde38d8bec7af0304b0f522683ccda2d245cd860 Mon Sep 17 00:00:00 2001
From: Vincent Biret <vibiret@microsoft.com>
Date: Mon, 20 Jul 2026 08:47:45 -0400
Subject: [PATCH] feat: serialize license identifier as extension for earlier
 versions

---
 .../Models/OpenApiConstants.cs                |  5 +++
 .../Models/OpenApiLicense.cs                  |  2 ++
 src/Microsoft.OpenApi/PublicAPI.Unshipped.txt |  1 +
 .../Models/OpenApiLicenseTests.cs             | 35 +++++++++++++++++++
 4 files changed, 43 insertions(+)

diff --git a/src/Microsoft.OpenApi/Models/OpenApiConstants.cs b/src/Microsoft.OpenApi/Models/OpenApiConstants.cs
index 3a24bfb60..db8aa1d62 100644
--- a/src/Microsoft.OpenApi/Models/OpenApiConstants.cs
+++ b/src/Microsoft.OpenApi/Models/OpenApiConstants.cs
@@ -230,6 +230,11 @@ public static class OpenApiConstants
         /// </summary>
         public const string Identifier = "identifier";
 
+        /// <summary>
+        /// Field: x-oai-license-identifier
+        /// </summary>
+        public const string OaiLicenseIdentifier = "x-oai-license-identifier";
+
         /// <summary>
         /// Field: Namespace
         /// </summary>
diff --git a/src/Microsoft.OpenApi/Models/OpenApiLicense.cs b/src/Microsoft.OpenApi/Models/OpenApiLicense.cs
index 04a267bdc..6297c03a9 100644
--- a/src/Microsoft.OpenApi/Models/OpenApiLicense.cs
+++ b/src/Microsoft.OpenApi/Models/OpenApiLicense.cs
@@ -63,6 +63,7 @@ public virtual void SerializeAsV31(IOpenApiWriter writer)
         public virtual void SerializeAsV3(IOpenApiWriter writer)
         {
             WriteInternal(writer, OpenApiSpecVersion.OpenApi3_0);
+            writer.WriteProperty(OpenApiConstants.OaiLicenseIdentifier, Identifier);
             writer.WriteEndObject();
         }
 
@@ -72,6 +73,7 @@ public virtual void SerializeAsV3(IOpenApiWriter writer)
         public virtual void SerializeAsV2(IOpenApiWriter writer)
         {
             WriteInternal(writer, OpenApiSpecVersion.OpenApi2_0);
+            writer.WriteProperty(OpenApiConstants.OaiLicenseIdentifier, Identifier);
             writer.WriteEndObject();
         }
 
diff --git a/src/Microsoft.OpenApi/PublicAPI.Unshipped.txt b/src/Microsoft.OpenApi/PublicAPI.Unshipped.txt
index 7dc5c5811..e763311ca 100644
--- a/src/Microsoft.OpenApi/PublicAPI.Unshipped.txt
+++ b/src/Microsoft.OpenApi/PublicAPI.Unshipped.txt
@@ -1 +1,2 @@
 #nullable enable
+const Microsoft.OpenApi.OpenApiConstants.OaiLicenseIdentifier = "x-oai-license-identifier" -> string!
diff --git a/test/Microsoft.OpenApi.Tests/Models/OpenApiLicenseTests.cs b/test/Microsoft.OpenApi.Tests/Models/OpenApiLicenseTests.cs
index 58cdae2ce..d81dce9b4 100644
--- a/test/Microsoft.OpenApi.Tests/Models/OpenApiLicenseTests.cs
+++ b/test/Microsoft.OpenApi.Tests/Models/OpenApiLicenseTests.cs
@@ -148,6 +148,25 @@ public async Task SerializeLicenseWithIdentifierAsJsonWorks()
             Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), actual.MakeLineBreaksEnvironmentNeutral());
         }
 
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi2_0)]
+        public async Task SerializeLicenseWithIdentifierAsJsonWorksForV3AndV2(OpenApiSpecVersion version)
+        {
+            // Arrange
+            var expected =
+                @"{
+  ""name"": ""Apache 2.0"",
+  ""x-oai-license-identifier"": ""Apache-2.0""
+}";
+
+            // Act
+            var actual = await LicenseWithIdentifier.SerializeAsJsonAsync(version);
+
+            // Assert
+            Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), actual.MakeLineBreaksEnvironmentNeutral());
+        }
+
         [Fact]
         public async Task SerializeLicenseWithIdentifierAsYamlWorks()
         {
@@ -161,5 +180,21 @@ public async Task SerializeLicenseWithIdentifierAsYamlWorks()
             // Assert
             Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), actual.MakeLineBreaksEnvironmentNeutral());
         }
+
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi2_0)]
+        public async Task SerializeLicenseWithIdentifierAsYamlWorksForV3AndV2(OpenApiSpecVersion version)
+        {
+            // Arrange
+            var expected = @"name: Apache 2.0
+x-oai-license-identifier: Apache-2.0";
+
+            // Act
+            var actual = await LicenseWithIdentifier.SerializeAsYamlAsync(version);
+
+            // Assert
+            Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), actual.MakeLineBreaksEnvironmentNeutral());
+        }
     }
 }