Revision 2026-07-28 keeps some features of older revisions in a deprecated state¹. The SDK provides them behind explicit options. All options are off by default, except dynamic client registration in OAuthClient. The specification plans their removal not before 2027-07-28. A later SDK release then removes the options.

Applies to: MCP 2026-07-28, zig-sdk 0.1.0. The checks of sampling requests, the rate limit of log messages and dynamic client registration in AuthorizationServer: zig-sdk 0.2.0. RequestOptions.log_level: zig-sdk 0.4.0.

Feature	Option	Effect when on
Sampling through multi round-trip requests	Server.Options.mrtr.sampling	A handler can ask for sampling/createMessage.
Sampling with tools	Server.Options.mrtr.sampling_tools	The sampling request can carry tools.
Roots	Server.Options.mrtr.roots	A handler can ask for roots/list.
Logging	Server.Options.capabilities.logging	ctx.log sends notifications/message on a request stream when the request set a logLevel.
Dynamic client registration	AuthorizationServer.Options.dynamic_registration	The authorization server has a registration endpoint and names it in its metadata.

The rate limit limits.rate_limits.log_messages applies to the log messages. The server drops the messages of a caller over the limit and later sends one summary with the count.

The specification keeps the multi round-trip request itself: elicitation stays. The client answers the deprecated input kinds only when it declared them in Client.Options.capabilities and gave the hooks sample and list_roots.

The deprecated features also include the includeContext values thisServer and allServers of a sampling request¹. The server sends them only when the client declared the capability sampling.context. Else the request gets -32021. Omit the field or use none.

Before the server sends a sampling request, it checks the tool result rules of the messages². The client does the same check before it calls the hook sample. See Server Guide and Client Guide.

Feature	Option	Effect when on
Log level	Client.Options.log_level	Every request asks for log messages at this level and above.
Log level of one request	RequestOptions.log_level	The request asks for log messages at this level and above. It replaces Client.Options.log_level for the request.
Dynamic client registration	OAuthClient.Options.registration	The default value .dynamic registers the client at the registration endpoint of the authorization server. With .client_metadata_url, the client registers itself only when the authorization server does not support client ID metadata documents. With .pre_registered, the client never registers itself.

• The initialize handshake. A server answers it with -32601 and a message that names the supported revision.
• Sessions, the Mcp-Session-Id header and stream resumption with Last-Event-ID.
• Server-initiated requests. Everything a server needs from a client travels in an InputRequiredResult.
• The error codes -32002 and -32042.
• The HTTP+SSE transport of revision 2024-11-05¹. Use Streamable HTTP.

1. MCP-DEPRECATED-2026-07-28: The deprecated features registry of revision 2026-07-28.
2. MCP-SAMPLING-2026-07-28: Sampling, the rules for tool uses and tool results.