github modelcontextprotocol/go-sdk v1.4.0
on GitHub

6 hours ago

This release marks the completion of the full 2025-11-25 specification implementation, by introducing the support for Sampling with Tools and experimental client-side OAuth support. It also contains multiple bug fixes and improvements. Thanks to all contributors!

Client-side OAuth support

This release introduces experimental support for OAuth on the client side of the SDK. It aims to support the full scope of the current MCP specification for authorization. To use it, you need to compile the SDK with the -tags mcp_go_client_oauth flag. Some changes may still be applied to this new API, based on developer feedback. The functionality is planned to become stable in v1.5.0 release, expected by the end of March 2026. More details can be found at https://github.com/modelcontextprotocol/go-sdk/blob/main/docs/protocol.md#client.

Sampling with Tools

Starting from this release, the server use the new CreateMessageWithTools method to create a sampling request to the client that contains tools that can be used by the client. On the client side, CreateMessageWithToolsHandler may be used to handle such requests and issue ToolUse responses to the server.

Behavior changes

We have two important behavior changes that were introduced to fix a bug or improve security posture. They can be temporarily turned off by specifying a special MCPGODEBUG environment variable when running the SDK. Different options can be added together, separated by a comma.

Introduced DNS rebinding protection (MCPGODEBUG=disablelocalhostprotection=1)

The requests arriving via a localhost address (127.0.0.1, [::1]) that have a non-localhost Host header will be rejected to protect against DNS rebinding attacks. The option to remove this protection will be removed in v1.6.0.

  • feat: add automatic DNS rebinding protection for localhost servers by @pcarleton in #760

Removed JSON content escaping when marshaling (MCPGODEBUG=jsonescaping=1):

By default encoding/json escapes the contents of the objects, which causes some servers to fail. We switched to no escaping by default. The option to bring back the escaping will be removed in v1.6.0.

Bug fixes

Security vulnerability caused by the case insensitive parsing behavior of encoding/json has been submitted (also release as a cherry pick in v1.3.1). Security advisory has been posted.

Other fixes:

Enhancements

Notably, the SDK now supports the extensions field in client and server capabilities, which should enable creation of MCP Apps.

  • mcp: add Extensions field to capabilities per SEP-2133 by @ymmt2005 in #794

Other enhancements:

  • mcp: enforce retry limit when SSE stream makes no progress by @majiayu000 in #742
  • mcp: export session missing error by @CocaineCong in #771
  • fix: add JSON tags to ElicitationCapabilities fields by @awschmeder in #774
  • mcp: improve http transports error handling and make transport work with any size message by @alexbumbacea in #734
  • examples: bind auth-middleware server to localhost by default by @TheodorNEngoy in #784

Repository organization

Some effort was put into better organization of the repository, as well as making sure it's up to date and secure. As a highlight, the repository is not integrated with OSSF Scorecard with a positive score of 8.7. Additionally, the full conformance test suite is now run on every PR and push to main.

New Contributors

Full Changelog: v1.3.0...v1.4.0

Check out latest releases or
releases around modelcontextprotocol/go-sdk v1.4.0

Don't miss a new go-sdk release

NewReleases is sending notifications on new releases.

Get notifications