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Fix HTTP fallback error handling (#1791)

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Pranav Senthilnathan 2026-09-03 16:57:31 -07:00 • committed by GitHub
commit df11d7da3b
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3 changed files with 146 additions and 24 deletions
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19
src/ModelContextProtocol.Core/Client/AutoDetectingClientSessionTransport.cs
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@ -77,16 +77,19 @@ internal sealed partial class AutoDetectingClientSessionTransport : ITransport
else if (await StreamableHttpClientSessionTransport.TryReadJsonRpcErrorAsync(response, cancellationToken).ConfigureAwait(false) is { } parsedError)
{
// A JSON-RPC error envelope in the body means the peer IS a Streamable HTTP server.
// It just rejected our specific request (e.g., -32022 UnsupportedProtocolVersion,
// -32021 MissingRequiredClientCapability, -32020 HeaderMismatch, or any other
// application-level error). Don't fall back to SSE — that would mask the real signal
// and surface a misleading "session id required" error from the SSE GET path.
// Adopt the Streamable HTTP transport and throw the structured exception so the
// connect-time fallback logic can react per spec PR #2844. Setting ActiveTransport
// first makes the catch filter below leave the now-owned transport alone.
// Adopt it before surfacing the failure so the catch filter leaves the now-owned
// transport alone, and never mask the response by attempting deprecated SSE.
LogUsingStreamableHttp(_name);
ActiveTransport = streamableHttpTransport;
throw McpSessionHandler.CreateRemoteProtocolExceptionFromError(parsedError);
if (StreamableHttpClientSessionTransport.ShouldSurfaceJsonRpcErrorAsProtocolException(response.StatusCode, parsedError))
{
throw McpSessionHandler.CreateRemoteProtocolExceptionFromError(parsedError);
}
// TryReadJsonRpcErrorAsync buffered the content, so this preserves the same response
// body and status without consuming the network stream a second time.
throw await HttpResponseMessageExtensions.CreateHttpRequestExceptionWithBodyAsync(response, cancellationToken).ConfigureAwait(false);
}
else
{

8
src/ModelContextProtocol.Core/Client/McpClientImpl.cs
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@ -367,14 +367,6 @@ internal sealed partial class McpClientImpl : McpClient
// fallback): falling back to initialize wouldn't fix a malformed envelope.
throw;
}
catch (McpProtocolException ex) when (
ex.ErrorCode == McpErrorCode.InvalidRequest &&
ex.Message.Contains(McpHttpHeaders.SessionId, StringComparison.Ordinal))
{
// Local transport validation: a 2026-07-28+ response must not carry HTTP session state.
// This is not evidence of an initialize-handshake server, so do not fall back.
throw;
}
catch (McpProtocolException)
{
// Per spec PR #2844, the fallback MUST NOT be keyed to a single error code.

12
src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs
View file

@ -80,8 +80,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
// for robustness. Servers occasionally emit them with 4xx codes other than 400.
if (!response.IsSuccessStatusCode &&
await TryReadJsonRpcErrorAsync(response, cancellationToken).ConfigureAwait(false) is { } parsedError &&
(response.StatusCode == HttpStatusCode.BadRequest ||
IsPerRequestMetadataProtocolErrorCode((McpErrorCode)parsedError.Error.Code)))
ShouldSurfaceJsonRpcErrorAsProtocolException(response.StatusCode, parsedError))
{
throw McpSessionHandler.CreateRemoteProtocolExceptionFromError(parsedError);
}
@ -89,10 +88,11 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
await response.EnsureSuccessStatusCodeWithResponseBodyAsync(cancellationToken).ConfigureAwait(false);
}
private static bool IsPerRequestMetadataProtocolErrorCode(McpErrorCode code) =>
code is McpErrorCode.UnsupportedProtocolVersion
or McpErrorCode.MissingRequiredClientCapability
or McpErrorCode.HeaderMismatch;
internal static bool ShouldSurfaceJsonRpcErrorAsProtocolException(HttpStatusCode statusCode, JsonRpcError error) =>
statusCode == HttpStatusCode.BadRequest ||
(McpErrorCode)error.Error.Code is McpErrorCode.UnsupportedProtocolVersion
or McpErrorCode.MissingRequiredClientCapability
or McpErrorCode.HeaderMismatch;
/// <summary>
/// Reads a JSON-RPC error envelope from an <c>application/json</c> response body, returning