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Default draft protocol support: sessionless + handshake-less (SEP-2575 + SEP-2567) (#1610)

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Stephen Halter 2026-06-23 21:24:22 -07:00 • committed by GitHub
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27
src/ModelContextProtocol.Core/Client/AutoDetectingClientSessionTransport.cs
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@ -2,6 +2,7 @@ using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Abstractions;
using ModelContextProtocol.Protocol;
using System.Net;
using System.Net.Http;
using System.Threading.Channels;
namespace ModelContextProtocol.Client;
@ -73,19 +74,37 @@ internal sealed partial class AutoDetectingClientSessionTransport : ITransport
LogUsingStreamableHttp(_name);
ActiveTransport = streamableHttpTransport;
}
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., -32004 UnsupportedProtocolVersion,
// -32003 MissingRequiredClientCapability, -32001 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.
LogUsingStreamableHttp(_name);
ActiveTransport = streamableHttpTransport;
throw McpSessionHandler.CreateRemoteProtocolExceptionFromError(parsedError);
}
else
{
// If the status code is not success, fall back to SSE
// 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.
LogStreamableHttpFailed(_name, response.StatusCode);
await streamableHttpTransport.DisposeAsync().ConfigureAwait(false);
await InitializeSseTransportAsync(message, cancellationToken).ConfigureAwait(false);
}
}
catch
catch when (ActiveTransport is null)
{
// If nothing threw inside the try block, we've either set streamableHttpTransport as the
// ActiveTransport, or else we will have disposed it in the !IsSuccessStatusCode else block.
// Only dispose the Streamable HTTP transport when we didn't adopt it. If we set
// ActiveTransport above (success path OR structured-error path), the transport's
// lifetime is owned by the outer transport from this point on.
await streamableHttpTransport.DisposeAsync().ConfigureAwait(false);
throw;
}