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fix(client): preserve underlying status code in AutoDetect probe (#1530)

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Mukunda Rao Katta 2026-07-22 13:31:16 -07:00 • committed by GitHub
commit ba372faece
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1 changed files with 295 additions and 23 deletions
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38
src/ModelContextProtocol.Core/Client/AutoDetectingClientSessionTransport.cs
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@ -93,11 +93,20 @@ internal sealed partial class AutoDetectingClientSessionTransport : ITransport
// Non-JSON-RPC error response: either the server doesn't speak MCP at all, or this
// is an older deployment that expects the SSE transport (which establishes its
// protocol via GET /sse rather than POST). Fall back to SSE per the original
// behavior.
// behavior. Capture the underlying error (status + body) before falling back so that,
// if SSE also fails, we can surface the real Streamable HTTP diagnostic to the caller
// instead of dropping it on the floor (see https://github.com/modelcontextprotocol/csharp-sdk/issues/1526).
LogStreamableHttpFailed(_name, response.StatusCode);
// This reads the response body a second time for the application/json case, where
// TryReadJsonRpcErrorAsync above already read it. HttpContent buffers after the first
// read, so this returns the same buffered content and is safe (not a second stream
// consumption). For the common non-JSON error responses (415, 405, plain text)
// TryReadJsonRpcErrorAsync returns early on the content type, so there is no double read.
var streamableHttpError = await HttpResponseMessageExtensions.CreateHttpRequestExceptionWithBodyAsync(response, cancellationToken).ConfigureAwait(false);
await streamableHttpTransport.DisposeAsync().ConfigureAwait(false);
await InitializeSseTransportAsync(message, cancellationToken).ConfigureAwait(false);
await InitializeSseTransportAsync(message, streamableHttpError, cancellationToken).ConfigureAwait(false);
}
}
catch when (ActiveTransport is null)
@ -110,7 +119,7 @@ internal sealed partial class AutoDetectingClientSessionTransport : ITransport
}
}
private async Task InitializeSseTransportAsync(JsonRpcMessage message, CancellationToken cancellationToken)
private async Task InitializeSseTransportAsync(JsonRpcMessage message, HttpRequestException? streamableHttpError, CancellationToken cancellationToken)
{
if (_options.KnownSessionId is not null)
{
@ -128,6 +137,24 @@ internal sealed partial class AutoDetectingClientSessionTransport : ITransport
LogUsingSSE(_name);
ActiveTransport = sseTransport;
}
catch (Exception sseError) when (streamableHttpError is not null && sseError is not OperationCanceledException)
{
// SSE fallback also failed. Surface the original Streamable HTTP error as the primary failure so the
// user sees the real server diagnostic (e.g. 415 Unsupported Media Type) instead of the unrelated
// SSE-fallback error (e.g. a 405 from a Streamable-HTTP-only server that doesn't accept GET). Preserve
// the original status code and attach the SSE failure as the inner exception so neither is lost, and
// 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
}
catch
{
await sseTransport.DisposeAsync().ConfigureAwait(false);
@ -166,4 +193,7 @@ internal sealed partial class AutoDetectingClientSessionTransport : ITransport
[LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} using SSE transport.")]
private partial void LogUsingSSE(string endpointName);
}
[LoggerMessage(Level = LogLevel.Warning, Message = "{EndpointName} SSE fallback failed after Streamable HTTP also failed; surfacing both errors.")]
private partial void LogSseFallbackFailedAfterStreamableHttp(string endpointName, Exception sseError);
}