v0.3.0-preview.1

The v0.3.0-preview.1 release introduces spec-aligned content type renames, session ID support across transports, structured output for server tools, and improved Streamable HTTP client behavior.

Breaking Changes

Refer to the C# SDK Versioning documentation for information regarding our versioning, breaking change, and support approaches.

1. Content type renames to match spec #513
   • Content renamed to ContentBlock with subclasses TextContentBlock, ImageContentBlock, AudioContentBlock, and ResourceContentBlock; CallToolResponse renamed to CallToolResult; CancelledNotification renamed to CancelledNotificationParams; ProgressNotification renamed to ProgressNotificationParams; Reference replaced with derived types ResourceReference, ResourceTemplateReference, and PromptReference; CallToolRequestParams sealed
   • Migration: Update all type name references throughout consuming code; search-and-replace is mostly sufficient
2. Session ID support added across transports #466
   • SessionId property added to the ITransport and IMcpEndpoint interfaces, requiring all custom implementations to add this member; StreamableHttpServerTransport.InitializeRequest property replaced with OnInitRequestReceived callback
   • Migration: Add public string? SessionId => null; to custom ITransport/IMcpEndpoint implementations; replace reads of .InitializeRequest with callback via .OnInitRequestReceived

What's Changed

• Add context to complete requests #488 by @stephentoub
• Add structured output/output schema support for server-side tools #480 by @eiriktsarpalis (co-authored by @stephentoub)
• Pass session IDs to MCP endpoints #466 by @eiriktsarpalis (co-authored by @halter73)
• Make enums serialize as strings if using the reflection-based serializer #473 by @eiriktsarpalis
• Update Microsoft.Extensions.AI version and expose JSON configuration on McpServerResourceCreateOptions #511 by @sbomer (co-authored by @eiriktsarpalis)
• Update content types according to latest spec #513 by @stephentoub
• Fix visibility of polyfill #523 by @stephentoub
• Add a callback for receiving stderr output #525 by @stephentoub
• Send MCP-Protocol-Version header in Streamable HTTP client transport #500 by @halter73
• Send DELETE request when closing a Streamable HTTP session on the client #501 by @halter73
• Add 2025-06-18 as supported protocol version #529 by @stephentoub

Documentation Updates

• Remove incorrect acknowledgement from README of new Core package #482 by @halter73
• Fix QuickstartClient path #528 by @stephentoub

Repository Infrastructure Updates

• Bump version to 0.2.0-preview.4 by @halter73
• Bump version number to 0.3.0 #534 by @eiriktsarpalis

Acknowledgements

• @sbomer made their first contribution in #511
• @AArnott submitted issue #425 (resolved by #466) and issue #487 (resolved by #525)
• @rlacaci submitted issue #357 (resolved by #511)
• @ihrpr submitted issues #508, #517, and #518 (resolved by #513)
• @akeit0 submitted issue #522 (resolved by #523)
• @AArnott reviewed pull requests

Full Changelog: v0.2.0-preview.3...v0.3.0-preview.1