Include response body in HttpRequestException for transport client errors (#1193)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

src/Common/HttpResponseMessageExtensions.cs

@@ -0,0 +1,66 @@
+using System.Net;
+using System.Net.Http;
+
+namespace ModelContextProtocol;
+
+/// <summary>
+/// Extension methods for <see cref="HttpResponseMessage"/>.
+/// </summary>
+internal static class HttpResponseMessageExtensions
+{
+    private const int MaxResponseBodyLength = 1024;
+
+    /// <summary>
+    /// Throws an <see cref="HttpRequestException"/> if the <see cref="HttpResponseMessage.IsSuccessStatusCode"/> property is <see langword="false"/>.
+    /// Unlike <see cref="HttpResponseMessage.EnsureSuccessStatusCode"/>, this method includes the response body in the exception message
+    /// to help diagnose issues when the server returns error details in the response body.
+    /// </summary>
+    /// <param name="response">The HTTP response message to check.</param>
+    /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    /// <exception cref="HttpRequestException">The response status code does not indicate success.</exception>
+    public static async Task EnsureSuccessStatusCodeWithResponseBodyAsync(this HttpResponseMessage response, CancellationToken cancellationToken = default)
+    {
+        if (!response.IsSuccessStatusCode)
+        {
+            string? responseBody = null;
+            try
+            {
+                using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+                cts.CancelAfter(TimeSpan.FromSeconds(5));
+                responseBody = await response.Content.ReadAsStringAsync(cts.Token).ConfigureAwait(false);
+
+                if (responseBody.Length > MaxResponseBodyLength)
+                {
+                    responseBody = responseBody.Substring(0, MaxResponseBodyLength) + "...";
+                }
+            }
+            catch
+            {
+                // Ignore all errors reading the response body (e.g., stream closed, timeout, cancellation) - we'll throw without it.
+            }
+
+            throw CreateHttpRequestException(response, responseBody);
+        }
+    }
+
+    /// <summary>
+    /// Creates an <see cref="HttpRequestException"/> for a non-success response, including the response body in the message.
+    /// </summary>
+    /// <param name="response">The HTTP response message.</param>
+    /// <param name="responseBody">The response body content, if available.</param>
+    /// <returns>An <see cref="HttpRequestException"/> with the response details.</returns>
+    public static HttpRequestException CreateHttpRequestException(HttpResponseMessage response, string? responseBody)
+    {
+        int statusCodeInt = (int)response.StatusCode;
+        string message = string.IsNullOrEmpty(responseBody)
+            ? $"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
+    }
+}