v1.2.0

This release completes the SEP-2575 stateless lifecycle of the 2026-07-28 specification, together with the SEP-2322, SEP-2549, and SEP-2243 features that revision builds on. Several entries under "Changed" are incompatible with 1.1.0 and ship in a minor release under the spec-conformance and security exceptions described in VERSIONING.md.

Added

• Handle the SEP-2575 modern request envelope in the server core (#475)
• Serve both lifecycle eras over stdio with an era lock per SEP-2575 (#478)
• Serve the sessionless modern path over Streamable HTTP per SEP-2575 (#479)
• Finalize server/discover and add client modern lifecycle support per SEP-2575 (#480)
• Let handlers return multi round-trip input_required results per SEP-2322 (#481)
• Add MCP::Elicitation::EnumSchema builders per SEP-1330 (#482)
• Stamp the required resultType on modern results per SEP-2322 (#487)
• Enforce the modern lifecycle admission rules per SEP-2575 (#489)
• Stream modern request notifications and honor the envelope logLevel per SEP-2575 (#490)
• Expose the user-defined server_context in instrumentation data (#493)
• Serve the subscriptions/listen notification stream per SEP-2575 (#495)
• Add opt-in requestState sealing via MCP::Server::RequestStateSecurity (#496)
• Mirror x-mcp-header tool parameters into Mcp-Param-* headers per SEP-2243 (#498)
• Stamp the required cache hints on modern cacheable results per SEP-2549 (#499)
• Drive multi round-trip input_required results on the client per SEP-2322 (#500)
• Fulfill input_required results on the legacy wire per SEP-2322 (#501)

Changed

• Align modern envelope validation with the finalized specification (#491)
• Require the Mcp-Method header on the modern path (#492)
• Bound server-to-client requests with a timeout (#502)
• Refuse server-to-client requests in the modern lifecycle per SEP-2575 (#503)
• Bound the total wait across SSE reconnection attempts (#504)
• Bound automatic pagination in the MCP client (#505)
• Reject modern-removed methods before the connection era locks (#511)
• Stop negotiating modern protocol versions through the initialize handshake (#516)

Deprecated

• Warn on modern client connects that declare the Roots or Sampling capabilities deprecated per SEP-2577 (#406, #516)

Fixed

• Stop leaking exception messages to clients via JSON-RPC error data (#486)
• Return Invalid Params for unknown prompts and missing prompt arguments (#517)
• Restrict OAuth discovery to same-origin metadata URLs and refuse private-network destinations

v1.1.0

Added

• Support 2026-07-28 as the Latest Protocol Version (#476)
• Expose server tool annotations on MCP::Client::Tool (#445)

Fixed

• Preserve explicit tool response content (#469)

v1.0.0

First stable release. The public API is now stable: breaking changes ship only in major releases,
apart from the narrow exceptions documented in VERSIONING.md.

Added

• Add ROADMAP.md outlining the path toward SEP-1730 (#465)
• Add VERSIONING.md documenting the versioning and breaking-change policy (#466)

v0.25.0

Added

• Support SEP-990 Cross-App Access via ID-JAG and the jwt-bearer grant (#454)
• Support client-side sampling via MCP::Client#on_sampling (#458)

Fixed

• Reject notifications carrying an unknown or expired session (#455)
• Handle an initialize request sent without an ID (#456)
• Bound client-side message buffering in the HTTP transport (#459)
• Lowercase response header names in StreamableHTTPTransport (#460)

v0.24.0

Added

• Support SSE reconnection per SEP-1699 in the HTTP client transport (#426)
• Validate the RFC 9207 iss authorization response parameter per SEP-2468 (#431)
• Support private_key_jwt client authentication for the client_credentials grant (#432)
• Recognize multi round-trip input_required results per SEP-2322 (#433)
• Add opt-in ttlMs / cacheScope cache hints to List and Read results per SEP-2549 (#436)
• Add server/discover and stateless lifecycle error codes per SEP-2575 (#438)
• Associate server-to-client requests with the originating client request per SEP-2260 (#440)
• Add server-side MCP Apps helpers per SEP-1865 (#441)
• Support client-side elicitation with SEP-1034 schema defaults (#443)
• Support class-based Resource and ResourceTemplate definitions (#447)

Changed

• Bind stored client credentials to the authorization server issuer per SEP-2352 (#439)

Fixed

• Handle non-object JSON-RPC messages without raising (#448)
• Perform SSE stream writes outside the session mutex (#449)
• Validate required params for initialize and resources/subscribe (#451)

v0.23.0

Added

• Add a session-ownership hook to StreamableHTTPTransport

Changed

• Require calling MCP::Client#connect before sending requests on stdio transport (#427)

Deprecated

• Annotate Roots, Sampling, and Logging APIs as deprecated per SEP-2577 (#429)

Fixed

• Fix an incorrect result: null response to an id-bearing notification message (#435)
• Validate Host and Origin headers to prevent DNS rebinding per MCP 2025-11-25
• Bound stateful session retention to prevent an initialize-flood DoS
• Bound stdio frame reads with max_line_bytes
• Bound request body and frame reads to prevent memory-exhaustion DoS

v0.22.0

Added

• Add audience role validation for MCP::Annotations per MCP specification (#422)
• Send SEP-2243 Mcp-Method and Mcp-Name headers per MCP specification (#423)
• Support client-side notifications/cancelled per MCP specification (#425)

Changed

• Conform Tool Schemas to JSON Schema 2020-12 per SEP-2106 (#417)

Fixed

• Fix a SyntaxError on Ruby 2.7.0 caused by arguments forwarding syntax (#419)

v0.21.0

Added

• Support capability extensions per SEP-2133 (#405)
• Add range validation for MCP::Annotations#priority per MCP specification (#410)
• Isolate stateless requests in ephemeral sessions per SEP-2567 (#415)

Changed

• Set OIDC application_type on Dynamic Client Registration per SEP-837 (#408)
• Fall back to legacy 2025-03-26 OAuth discovery for servers without PRM (#414)

v0.20.0

Added

• Support W3C Trace Context Propagation via _meta per SEP-414 (#397)
• Support OAuth client_credentials grant in OAuth client (#399)
• Add annotations field to MCP::Resource and MCP::ResourceTemplate per MCP specification (#403)

Changed

• Re-run OAuth flow on 403 insufficient_scope (step-up) (#368)
• Speed up Tool::Schema validation by 5x to 100x (#369)
• Use JSON-RPC error envelope for StreamableHTTPTransport errors (#371)
• Pin RFC 8414 default well-known suffix per SEP-2351 (#395)
• Default missing MCP-Protocol-Version to 2025-03-26 in StreamableHTTPTransport (#392)

Fixed

• Preserve the request ID in invalid request error responses (#400)
• Standardize Resource Not Found errors on -32602 with URI data per SEP-2164 (#402)

v0.19.0

Added

• Support Client ID Metadata Documents in OAuth client (#361)
• Request offline_access scope when supported (#365)
• Add size field to MCP::Resource per MCP specification (#393)