Inspect string input's format if not explicitly provided

Author: MaggieKimani1

src/Microsoft.OpenApi/Reader/OpenApiModelFactory.cs

@@ -169,7 +169,7 @@ public static ReadResult Parse(string input,
                                        string format = null,
                                        OpenApiReaderSettings settings = null)
         {
-            format ??= OpenApiConstants.Json;
+            format ??= InspectInputFormat(input);
             settings ??= new OpenApiReaderSettings();
 
             // Copy string into MemoryStream
@@ -193,15 +193,14 @@ public static T Parse<T>(string input,
                                  string format = null,
                                  OpenApiReaderSettings settings = null) where T : IOpenApiElement
         {
-            format ??= OpenApiConstants.Json;
+            format ??= InspectInputFormat(input);
             settings ??= new OpenApiReaderSettings();
             var stream = new MemoryStream(Encoding.UTF8.GetBytes(input));
             return Load<T>(stream, version, format, out diagnostic, settings);
         }
 
         private static async Task<ReadResult> InternalLoadAsync(Stream input, string format, OpenApiReaderSettings settings, CancellationToken cancellationToken = default)
         {
-            Utils.CheckArgumentNull(format, nameof(format));
             var reader = OpenApiReaderRegistry.GetReader(format);
             var readResult = await reader.ReadAsync(input, settings, cancellationToken);
 
@@ -289,5 +288,10 @@ SecurityException or
             }
             return (null, null);
         }
+
+        private static string InspectInputFormat(string input)
+        {
+            return input.StartsWith("{", StringComparison.OrdinalIgnoreCase) || input.StartsWith("[", StringComparison.OrdinalIgnoreCase) ? OpenApiConstants.Json : OpenApiConstants.Yaml;
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiDocumentTests.cs

@@ -106,6 +106,21 @@ public void ParseDocumentFromInlineStringShouldSucceed()
                 });
         }
 
+        [Fact]
+        public void ParseInlineStringWithoutProvidingFormatSucceeds()
+        {
+            var stringOpenApiDoc = """
+openapi: 3.1.0
+info:
+  title: Sample API
+  version: 1.0.0
+paths: {}
+""";
+
+            var readResult = OpenApiDocument.Parse(stringOpenApiDoc);
+            readResult.OpenApiDocument.Info.Title.Should().Be("Sample API");
+        }
+
         [Fact]
         public async Task ParseBasicDocumentWithMultipleServersShouldSucceed()
         {