MCP-0.15

Changes for version 0.15 - 2026-07-30

Updated to MCP protocol revision 2026-07-28, which makes the protocol stateless. This is a breaking change for every user of this module.
Removed the initialize handshake, protocol sessions, and the Mcp-Session-Id header. Every request now declares the protocol version and client capabilities it was made with in _meta.
Removed MCP::Server::Session, and the sessions and session_timeout attributes from MCP::Server::Transport::HTTP. GET and DELETE requests are now answered with status 405.
Removed initialize_session, delete_session, ping, and session_id from MCP::Client.
Removed the ping method and the RESOURCE_NOT_FOUND constant. Prompts, resources, and tools that were not found now use INVALID_PARAMS, and unknown methods are answered with status 404.
Added MCP::Server::Legacy, a temporary fallback that answers the initialize handshake of protocol revisions older than 2026-07-28, so clients that have not been updated yet can still list and call tools, prompts, and resources. It also adds a legacy attribute to MCP::Server::Context, and will be removed again in a future release.
Added the server/discover method, which replaces initialize, and an instructions attribute to MCP::Server.
Added a discover method to MCP::Client.
Added subscriptions/listen, which replaces the GET stream and resources/subscribe, to both transports.
Added MCP::Server::Subscription class, and a subscriptions attribute to both transports.
Added a listen method and a notification event to MCP::Client.
Added support for multi-round tool requests, with an input_required method on MCP::Primitive that seals its request state with HMAC-SHA256, and matching input_responses and request_state options for call_tool, get_prompt, and read_resource in MCP::Client.
Added state_secret and state_timeout attributes to MCP::Server.
Added input_responses, principal, raw_request_state, state_binding, state_secret, and state_timeout attributes, and request_state and seal_state methods, to MCP::Server::Context.
Added routing header validation to MCP::Server::Transport::HTTP, covering MCP-Protocol-Version, Mcp-Method, Mcp-Name, and the Mcp-Param-* headers declared with x-mcp-header in a tool input schema.
Added an origins attribute to MCP::Server::Transport::HTTP, to protect against DNS rebinding attacks.
Added cache hints to discovery, list, and read results, with cache_scope and cache_ttl attributes on MCP::Server and MCP::Primitive.
Added a resultType field and server info to every result.
Added a log attribute to MCP::Server, and the INTERNAL_ERROR constant.
Fixed a bug where an exception thrown by a prompt, resource, or tool, or a rejected promise returned by one, was not turned into a JSON-RPC error. Over stdio it terminated the server, and over HTTP it produced a response without a body and leaked the notification stream. The exception is now logged and answered with INTERNAL_ERROR.
Added JSON Schema 2020-12 support to MCP::Tool, which is now the default dialect, together with a max_schema_depth attribute and validation of structuredContent against output_schema. This requires JSON::Schema::Tiny 0.034.
Added client_capabilities, client_info, log_level, and protocol_version attributes, and a notify_log method, to MCP::Server::Context.
Added a cancelled event, and cancel and is_cancelled methods, to MCP::Server::Context, so long running tools can stop as soon as a client goes away. MCP::Server::Context is now a Mojo::EventEmitter.
Added HEADER_MISMATCH, META_CLIENT_CAPABILITIES, META_CLIENT_INFO, META_LOG_LEVEL, META_PROTOCOL_VERSION, META_SERVER_INFO, META_SUBSCRIPTION_ID, MISSING_CLIENT_CAPABILITY, SUPPORTED_VERSIONS, and UNSUPPORTED_PROTOCOL_VERSION constants to MCP::Constants.
Added capabilities and protocol_version attributes to MCP::Client, which renegotiates once when a server rejects its protocol version.
Improved progress and log notifications to be delivered on the response stream of the request they belong to, so they no longer require streaming to be enabled and work with pre-forking web servers.
Added a tutorial to the MCP documentation, introducing tools, prompts, resources, and everything around them.
Added examples/perldoc_stdio.pl, the server built in the tutorial.