fix: OpenAPI request body description uses correct parameter comment

src/OpenApi/gen/XmlCommentGenerator.Emitter.cs

@@ -386,9 +386,15 @@ public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransform
                 }
                 if (methodComment.Parameters is { Count: > 0})
                 {
+                    var requestBodyParameterName = context.Description.ParameterDescriptions
+                        .FirstOrDefault(parameterDescription =>
+                            parameterDescription.Source == Microsoft.AspNetCore.Mvc.ModelBinding.BindingSource.Body ||
+                            parameterDescription.Source == Microsoft.AspNetCore.Mvc.ModelBinding.BindingSource.FormFile ||
+                            parameterDescription.Source == Microsoft.AspNetCore.Mvc.ModelBinding.BindingSource.Form)
+                        ?.ParameterDescriptor?.Name;
+
                     foreach (var parameterComment in methodComment.Parameters)
                     {
-                        var parameterInfo = methodInfo.GetParameters().SingleOrDefault(info => info.Name == parameterComment.Name);
                         var operationParameter = operation.Parameters?.SingleOrDefault(parameter => parameter.Name == parameterComment.Name);
                         if (operationParameter is not null)
                         {
@@ -400,12 +406,12 @@ public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransform
                             }
                             targetOperationParameter.Deprecated = parameterComment.Deprecated;
                         }
-                        else
+                        else if (requestBodyParameterName is not null && string.Equals(parameterComment.Name, requestBodyParameterName, StringComparison.Ordinal))
                         {
                             var requestBody = operation.RequestBody;
                             if (requestBody is not null)
                             {
-                                requestBody.Description = parameterComment.Description;
+                                requestBody.Description ??= parameterComment.Description;
                                 if (parameterComment.Example is { } jsonString)
                                 {
                                     var content = requestBody?.Content?.Values;

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/OperationTests.MinimalApis.cs

@@ -530,4 +530,64 @@ await SnapshotTestHelper.VerifyOpenApi(compilation, document =>
             Assert.Equal("Property with only value documentation.", valueOnlyParam2.Description);
         });
     }
+
+    [Fact]
+    public async Task RequestBodyDescriptionUsesFromBodyParameterCommentNotLastParameter()
+    {
+        // Regression test for issue #65805
+        // When an endpoint has [FromBody] parameter followed by other parameters like [FromServices] or CancellationToken,
+        // the request body description should use the [FromBody] parameter's XML comment, not the last parameter's.
+        var source = """
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.HttpResults;
+
+var builder = WebApplication.CreateBuilder();
+builder.Services.AddOpenApi();
+var app = builder.Build();
+
+app.MapPost("/test", TestEndpoint.PostData);
+app.Run();
+
+public static class TestEndpoint
+{
+    /// <summary>
+    /// Process some sample input.
+    /// </summary>
+    /// <param name="data">Sample data provided by the user.</param>
+    /// <param name="logger">Logger for diagnostics and tracing.</param>
+    /// <param name="cancellation">Injected cancellation token.</param>
+    /// <returns>The number the user supplied.</returns>
+    public static async Task<Ok<int>> PostData(
+        [FromBody] SampleData data,
+        [FromServices] ILogger<SampleData> logger,
+        CancellationToken cancellation)
+    {
+        ArgumentNullException.ThrowIfNull(data);
+        logger.LogInformation("User supplied {Number} and {Text}", data.Number, data.Text);
+        await Task.Delay(1, cancellation).ConfigureAwait(false);
+        return TypedResults.Ok(data.Number);
+    }
+}
+
+public record SampleData(int Number, string Text);
+""";
+        var generator = new XmlCommentGenerator();
+        await SnapshotTestHelper.Verify(source, generator, out var compilation);
+        await SnapshotTestHelper.VerifyOpenApi(compilation, document =>
+        {
+            var postOperation = document.Paths["/test"].Operations[HttpMethod.Post];
+            var requestBody = postOperation.RequestBody;
+
+            Assert.NotNull(requestBody);
+            Assert.Equal("Sample data provided by the user.", requestBody.Description);
+            Assert.NotEqual("Injected cancellation token.", requestBody.Description);
+        });
+    }
 }