Preserve HTTP status codes across target frameworks (#1767)

Copilot-Session: 9a49da98-c8d8-4b90-a6ed-50afe18ac981

src/Common/HttpResponseMessageExtensions.cs

@@ -69,10 +69,52 @@ internal static class HttpResponseMessageExtensions
             ? $"Response status code does not indicate success: {statusCodeInt} ({response.ReasonPhrase})."
             : $"Response status code does not indicate success: {statusCodeInt} ({response.ReasonPhrase}). Response body: {responseBody}";
 
-#if NET
-        return new HttpRequestException(message, inner: null, response.StatusCode);
-#else
-        return new HttpRequestException(message);
-#endif
+        return HttpRequestExceptionExtensions.Create(message, innerException: null, response.StatusCode);
+    }
+}
+
+/// <summary>
+/// Helpers for preserving HTTP status codes on <see cref="HttpRequestException"/> across all target frameworks.
+/// </summary>
+internal static class HttpRequestExceptionExtensions
+{
+    internal const string StatusCodeDataKey = "ModelContextProtocol.HttpStatusCode";
+
+    /// <summary>
+    /// Creates an <see cref="HttpRequestException"/> and preserves its HTTP status code.
+    /// </summary>
+    public static HttpRequestException Create(string message, Exception? innerException, HttpStatusCode? statusCode)
+    {
+#if NET
+        var exception = new HttpRequestException(message, innerException, statusCode);
+#else
+        var exception = new HttpRequestException(message, innerException);
+#endif
+
+        if (statusCode is not null)
+        {
+            exception.Data[StatusCodeDataKey] = statusCode.Value;
+        }
+
+        return exception;
+    }
+
+    /// <summary>
+    /// Gets the preserved HTTP status code from an <see cref="HttpRequestException"/>.
+    /// </summary>
+    public static HttpStatusCode? GetStatusCode(this HttpRequestException exception)
+    {
+#if NET
+        if (exception.StatusCode is { } statusCode)
+        {
+            return statusCode;
+        }
+#endif
+
+        return exception.Data[StatusCodeDataKey] switch
+        {
+            HttpStatusCode storedStatusCode => storedStatusCode,
+            _ => null,
+        };
     }
 }

src/ModelContextProtocol.Core/Client/AutoDetectingClientSessionTransport.cs

@@ -146,14 +146,10 @@ internal sealed partial class AutoDetectingClientSessionTransport : ITransport
             // keep HttpRequestException as the surfaced type so existing callers can still catch it and read StatusCode.
             await sseTransport.DisposeAsync().ConfigureAwait(false);
             LogSseFallbackFailedAfterStreamableHttp(_name, sseError);
-#if NET
-            throw new HttpRequestException(streamableHttpError.Message, sseError, streamableHttpError.StatusCode);
-#else
-            // net472 has no HttpRequestException overload that carries a status code, so this target
-            // preserves the status text in the message but not a programmatic StatusCode. Preserving the
-            // status code is intentionally net5+ only rather than an oversight.
-            throw new HttpRequestException(streamableHttpError.Message, sseError);
-#endif
+            throw HttpRequestExceptionExtensions.Create(
+                streamableHttpError.Message,
+                sseError,
+                streamableHttpError.GetStatusCode());
         }
         catch
         {