v0.4.1-preview.1

Breaking Changes

The v0.4.1-preview.1 release includes breaking changes included in #892 and #969.

1. #892 audits and standardizes MCP protocol types for consistency, including API changes for:
2. Property mutability
3. Required properties
4. Collection types used for properties
5. #969 implements SEP-1330: Elicitation Enum Schema Improvements and Standards Compliance, which involved a breaking change to the specification:
6. EnumSchema is marked [Obsolete] using the default CS0618 diagnostic, and it becomes an alias for LegacyTitledEnumSchema, which is also marked as [Obsolete] using the default CS0618 diagnostic

Path to 1.0.0

We are working toward the first stable version of the C# SDK as version 1.0.0, which will have compliance with Version 2025-11-25 of the MCP specification. We will publish at least one more pre-release version of the SDK before declaring the stable version, and breaking changes to Remove obsolete interfaces and extension methods (#774) are expected as part of stablizing the API.

Refer to the new C# SDK Versioning documentation for information regarding our versioning, breaking change, and support approaches once we publish the 1.0.0 release.

What's Changed

• Add default value support for all primitive types in elicitation schemas (SEP-1034) by @Copilot in #924
• Audit protocol types by @MackinnonBuck in #892
• Implement SEP-986: Specify Format for Tool Names by @stephentoub in #945
• Add public constructors to McpClient types for reusing cached definitions by @Copilot in #938
• Tweak how McpClientTool.InvokeAsync exposes some content by @stephentoub in #941
• Ignore unexpected properties when deserializing a ContentBlock by @jeffhandley in #956
• Remove prerelease requirement from global.json by @eiriktsarpalis in #961
• Add Description property to Implementation type per MCP spec by @Copilot in #966
• Remove TODOs related to AllowOutOfOrderMetadataProperty workarounds. by @eiriktsarpalis in #977
• Make JsonRpcMessageWithId.RequestId non-required by @MackinnonBuck in #984
• Implement SEP-1330: Standards-compliant enum schemas for elicitation by @Copilot in #969
• Add production-ready incremental source generator for automatic Description attributes from XML comments by @Copilot in #899
• Add tools support to sampling by @stephentoub in #976
• Use JsonElement.Parse for string-to-JsonElement deserialization by @Copilot in #1002
• Fix race condition in StdioClientTransport on .NET Framework by @Copilot in #996
• Tokens can be cached beyond the lifetime of the (http) transport. by @halllo in #834
• Add ability for client to resume session by @halter73 in #1029

Documentation Updates

• Update ModelId in QuickStartClient to non-deprecated version by @PederHP in #926
• Add file-based MCP server sample by @PederHP in #931
• Align error handling documentation with SEP-1303 by @Copilot in #967
• Fix a couple issues in the client sample readme by @ericstj in #994
• Fix documentation typos and incorrect type references by @Copilot in #993
• Introduce versioning, breaking changes, and support document by @jeffhandley in #986
• Address additional feedback on Versioning doc by @jeffhandley in #999
• Tweak the file-based MCP server sample by @eiriktsarpalis in #1019
• Writing style edits by @gewarren in #1024

Repository Infrastructure Updates

• Update Microsoft.Extensions.AI packages to version 9.10.1 by @Copilot in #916
• Update to .NET SDK to 10.rc2 by @eiriktsarpalis in #883
• Bump danielpalme/ReportGenerator-GitHub-Action from 5.4.17 to 5.4.18 by @dependabot[bot] in #920
• Bump actions/upload-artifact from 4.6.2 to 5.0.0 by @dependabot[bot] in #918
• Bump actions/download-artifact from 5.0.0 to 6.0.0 by @dependabot[bot] in #919
• fixed donet version from 10.0.100-rc.1.25451.107 to 10.0.100-rc.2 by @eduardomb-aw in #927
• Update to .NET 10.x packages by @stephentoub in #957
• Fix tests in Dev Container by @halter73 in #965
• Update to MTP by @Youssef1313 in #958
• Bump actions/checkout from 5.0.0 to 5.0.1 by @dependabot[bot] in #973
• Bump danielpalme/ReportGenerator-GitHub-Action from 5.4.18 to 5.5.0 by @dependabot[bot] in #972
• Bump the opentelemetry-testing group with 6 updates by @dependabot[bot] in #974
• Revert "Update to MTP" by @eiriktsarpalis in #989
• Bump the serilog-testing group with 2 updates by @dependabot[bot] in #992
• Bump the testing-frameworks group with 3 updates by @dependabot[bot] in #991
• Skip flaky test on Mono where stderr callbacks are unsupported by @Copilot in #980
• Revert "Revert "Update to MTP"" by @Youssef1313 in #990
• Add Microsoft.CodeAnalysis.* to dependabot ignore list by @Copilot in #1009
• Revert "Update to MTP (#958)" by @halter73 in #1011
• Bump the testing-frameworks group with 3 updates by @dependabot[bot] in #1013
• Bump the serilog-testing group with 1 update by @dependabot[bot] in #1014
• Bump version to 0.4.1-preview.1 by @jeffhandley in #1031

New Contributors

• @eduardomb-aw made their first contribution in #927
• @Youssef1313 made their first contribution in #958
• @ericstj made their first contribution in #994
• @halllo made their first contribution in #834
• @gewarren made their first contribution in #1024

Full Changelog: v0.4.0-preview.3...v0.4.1-preview.1