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Fix request-scoped draft client capabilities (#1685)

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>
This commit is contained in:
Pranav Senthilnathan 2026-07-12 13:56:10 -04:00 • committed by GitHub
commit a8d374b280
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3 changed files with 306 additions and 40 deletions
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54
src/ModelContextProtocol.Core/Server/DestinationBoundMcpServer.cs
View file

@ -5,18 +5,66 @@ using System.Text.Json.Nodes;
namespace ModelContextProtocol.Server;
#pragma warning disable MCPEXP002
internal sealed class DestinationBoundMcpServer(McpServerImpl server, ITransport? transport) : McpServer
internal sealed class DestinationBoundMcpServer(McpServerImpl server, ITransport? transport, JsonRpcMessageContext? requestContext = null) : McpServer
#pragma warning restore MCPEXP002
{
private readonly bool _isJuly2026OrLaterRequest = server.IsJuly2026OrLaterProtocolRequest(requestContext);
private readonly ClientCapabilities? _requestClientCapabilities = requestContext?.ClientCapabilities;
private readonly Implementation? _requestClientInfo = requestContext?.ClientInfo;
public override string? SessionId => transport?.SessionId ?? server.SessionId;
public override string? NegotiatedProtocolVersion => server.NegotiatedProtocolVersion;
public override ClientCapabilities? ClientCapabilities => server.ClientCapabilities;
public override Implementation? ClientInfo => server.ClientInfo;
public override McpServerOptions ServerOptions => server.ServerOptions;
public override IServiceProvider? Services => server.Services;
[Obsolete(Obsoletions.DeprecatedLogging_Message, DiagnosticId = Obsoletions.Deprecated_DiagnosticId, UrlFormat = Obsoletions.Deprecated_Url)]
public override LoggingLevel? LoggingLevel => server.LoggingLevel;
public override ClientCapabilities? ClientCapabilities
{
get
{
// In stateless transport mode, a single request does not have a persistent bidirectional channel.
// Server-to-client requests (sampling, roots, elicitation) are unsupported in this mode and the
// capability gates rely on a null ClientCapabilities value to report that unsupported-state path.
if (!server.HasStatefulTransport())
{
return null;
}
// On protocol revision 2026-07-28+, client capabilities are request-scoped (_meta on each request)
// and must not be inferred from prior requests. Missing per-request capabilities therefore means
// "no declared capabilities for this request", represented by an empty object. A fresh instance is
// returned deliberately: ClientCapabilities is a mutable DTO handed to user handlers, so a shared
// static empty instance could be mutated and leak across requests.
if (_isJuly2026OrLaterRequest)
{
return _requestClientCapabilities ?? new ClientCapabilities();
}
// Legacy protocol behavior uses session-scoped capabilities established during initialize (or
// pre-populated migration data), so ignore per-request values and return the server session state.
return server.ClientCapabilities;
}
}
public override Implementation? ClientInfo
{
get
{
// On protocol revision 2026-07-28+, client info is request-scoped (carried in each request's _meta),
// mirroring how ClientCapabilities is resolved above. Return only this request's declared value and
// do not fall back to shared session state, which under a stateful transport could belong to a
// different concurrent request.
if (_isJuly2026OrLaterRequest)
{
return _requestClientInfo;
}
// Legacy protocol behavior uses session-scoped client info established during initialize.
return server.ClientInfo;
}
}
/// <summary>
/// Gets or sets the MRTR context for the current request, if any.
/// Set by <see cref="McpServerImpl.CreateDestinationBoundServer"/> when an MRTR-aware handler invocation is in progress.

25
src/ModelContextProtocol.Core/Server/McpServer.cs
View file

@ -21,9 +21,19 @@ public abstract partial class McpServer : McpSession
/// </summary>
/// <remarks>
/// <para>
/// These capabilities are established during the initialization handshake and indicate
/// which features the client supports, such as sampling, roots, and other
/// protocol-specific functionality.
/// On protocol revisions that use the <c>initialize</c> handshake (<c>2025-11-25</c> and earlier), these
/// capabilities are established once during initialization and are session-scoped: they are available both
/// on the root <see cref="McpServer"/> and on the server exposed to request handlers.
/// </para>
/// <para>
/// On the <c>2026-07-28</c> revision and later (SEP-2575) there is no <c>initialize</c> handshake; the client
/// declares its capabilities per-request in <c>_meta</c>, and the server MUST NOT infer them from previous
/// requests. In that mode this property is only meaningful on the request-scoped server accessed via
/// the <c>Server</c> property of the <see cref="RequestContext{TParams}"/> passed to a handler; on the
/// root <see cref="McpServer"/> (for example one constructed manually over a
/// <see cref="System.IO.Stream"/>) it is <see langword="null"/>.
/// It is also <see langword="null"/> in stateless transport mode, where server-to-client requests are
/// unsupported.
/// </para>
/// <para>
/// Server implementations can check these capabilities to determine which features
@ -38,7 +48,14 @@ public abstract partial class McpServer : McpSession
/// <remarks>
/// <para>
/// This property contains identification information about the client that has connected to this server,
/// including its name and version. This information is provided by the client during initialization.
/// including its name and version.
/// </para>
/// <para>
/// On protocol revisions that use the <c>initialize</c> handshake (<c>2025-11-25</c> and earlier) this
/// information is provided once during initialization and is session-scoped. On the <c>2026-07-28</c>
/// revision and later it is carried per-request in <c>_meta</c>, so read it from the request-scoped server
/// accessed via the <c>Server</c> property of the <see cref="RequestContext{TParams}"/> passed to a handler
/// rather than from the root <see cref="McpServer"/>.
/// </para>
/// <para>
/// Server implementations can use this information for logging, tracking client versions,

43
src/ModelContextProtocol.Core/Server/McpServerImpl.cs
View file

@ -152,15 +152,17 @@ internal sealed partial class McpServerImpl : McpServer
/// <summary>
/// Wraps <paramref name="inner"/> so that, for every JSON-RPC request, a built-in filter first
/// synchronizes server-side state (<see cref="_negotiatedProtocolVersion"/>,
/// <see cref="_clientCapabilities"/>, <see cref="_clientInfo"/>) from the per-request <c>_meta</c>
/// values projected onto <see cref="JsonRpcMessageContext"/> and validates the per-request protocol
/// version, before delegating to the user-supplied incoming filters.
/// synchronizes server-side state (<see cref="_negotiatedProtocolVersion"/>, <see cref="_clientInfo"/>)
/// from the per-request <c>_meta</c> values projected onto <see cref="JsonRpcMessageContext"/> and
/// validates the per-request protocol version, before delegating to the user-supplied incoming filters.
/// </summary>
/// <remarks>
/// Under the 2026-07-28 protocol revision (SEP-2575) there is no <c>initialize</c> handshake, so these values
/// MUST be populated per-request. For legacy clients the per-request values are absent and the built-in
/// filter is a no-op (the values were captured during the initialize handler).
/// MUST be populated per-request. Per-request client capabilities and client info are consumed request-scoped
/// by <see cref="DestinationBoundMcpServer"/> and are not read from server-wide state by request handlers. The
/// shared <see cref="_clientInfo"/> write below is best-effort and used only to derive the session endpoint
/// name for logging/telemetry. For legacy clients the per-request values are absent and the built-in filter is
/// a no-op (the values were captured during the initialize handler).
/// </remarks>
private JsonRpcMessageFilter PrependMetaReadingFilter(JsonRpcMessageFilter inner)
{
@ -185,23 +187,16 @@ internal sealed partial class McpServerImpl : McpServer
SetNegotiatedProtocolVersion(protocolVersion);
}
if (context.ClientCapabilities is { } clientCapabilities && IsJuly2026OrLaterProtocol() && HasStatefulTransport())
{
// Under the 2026-07-28 revision the per-request _meta envelope carries the client's FULL
// capabilities (SEP-2575), so a plain overwrite is correct. The IsJuly2026OrLaterProtocol() gate
// makes any legacy per-request envelope a no-op (legacy capabilities stay as the
// initialize handshake established them); the HasStatefulTransport() gate keeps
// _clientCapabilities null under StreamableHttpServerTransport { Stateless = true }
// (where the same server instance handles every request, so persisting per-request
// capability state would both leak across requests and break the StatelessServerTests
// invariant that surfaces the "X is not supported in stateless mode" errors).
_clientCapabilities = clientCapabilities;
}
if (context.ClientInfo is { } clientInfo &&
(_clientInfo is null || !string.Equals(_clientInfo.Name, clientInfo.Name, StringComparison.Ordinal) ||
!string.Equals(_clientInfo.Version, clientInfo.Version, StringComparison.Ordinal)))
{
// This shared write is best-effort and used only to derive the session endpoint name for
// logging/telemetry. It is intentionally NOT read by request handlers on 2026-07-28+ sessions:
// DestinationBoundMcpServer resolves ClientInfo (and ClientCapabilities) request-scoped from
// the per-request _meta so concurrent requests never observe each other's values. Under a
// draft stateful session with differing per-request client info, the last writer wins here,
// which only affects the logged endpoint name and never the request-scoped values handlers see.
_clientInfo = clientInfo;
endpointNameNeedsRefresh = true;
}
@ -1627,7 +1622,7 @@ internal sealed partial class McpServerImpl : McpServer
private DestinationBoundMcpServer CreateDestinationBoundServer(JsonRpcRequest jsonRpcRequest)
{
var server = new DestinationBoundMcpServer(this, jsonRpcRequest.Context?.RelatedTransport);
var server = new DestinationBoundMcpServer(this, jsonRpcRequest.Context?.RelatedTransport, jsonRpcRequest.Context);
if (_mrtrContextsByRequestId.TryRemove(jsonRpcRequest.Id, out var mrtrContext))
{
@ -1740,7 +1735,7 @@ internal sealed partial class McpServerImpl : McpServer
{
// Ensure message has a Context so Items can be shared through the pipeline
message.Context ??= new();
var context = new MessageContext(new DestinationBoundMcpServer(this, message.Context.RelatedTransport), message);
var context = new MessageContext(new DestinationBoundMcpServer(this, message.Context.RelatedTransport, message.Context), message);
await current(context, cancellationToken).ConfigureAwait(false);
};
};
@ -1795,8 +1790,12 @@ internal sealed partial class McpServerImpl : McpServer
/// Used to gate the SEP-2663 Tasks extension, which only interoperates on the 2026-07-28 revision.
/// </summary>
private bool IsJuly2026OrLaterProtocolRequest(JsonRpcRequest? request) =>
IsJuly2026OrLaterProtocolRequest(request?.Context);
/// <inheritdoc cref="IsJuly2026OrLaterProtocolRequest(JsonRpcRequest?)"/>
internal bool IsJuly2026OrLaterProtocolRequest(JsonRpcMessageContext? requestContext) =>
McpHttpHeaders.IsJuly2026OrLaterProtocolVersion(
request?.Context?.ProtocolVersion ?? NegotiatedProtocolVersion);
requestContext?.ProtocolVersion ?? NegotiatedProtocolVersion);
/// <inheritdoc />
public override bool IsMrtrSupported => ClientSupportsMrtr() || HasStatefulTransport();