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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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1 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
{