v2.3.0

This release improves HTTP discovery, OAuth, protocol interoperability, and shutdown reliability, preserves registration order for server primitives, and exposes a TimeProvider for deterministic client timeout tests. It also corrects optional server-identity handling; applications reading McpClient.ServerInfo should adopt nullable handling.

Breaking Changes

Refer to the C# SDK Versioning documentation for details on versioning and breaking change policies.

1. McpClient.ServerInfo is nullable — #1903
   • Its return annotation changes from Implementation to Implementation?. Existing non-nullable assignments or dereferences can produce nullable warnings and fail warnings-as-errors builds.
   • On 2026-07-28 connections, ServerInfo is null when the server omits optional identity metadata from server/discover. Legacy initialize still requires serverInfo. Reading the property before connecting still throws InvalidOperationException.
   • Migrate with a null check or a fallback such as client.ServerInfo?.Name ?? "Unknown server". Self-reported identity must not be used for security decisions.
   • This corrective source-compatibility change is intentionally included in 2.3.0.

What's Changed

• Correct protocol-version telemetry for stateless server spans #1810 by @luisangelrod
• Preserve HTTP errors and correct discovery fallback handling #1791 by @PranavSenthilnathan (co-authored by @Copilot)
• Make modern Streamable HTTP status mapping deterministic #1837 by @jstar0
  • Modern responses wait for the first protocol message before sending headers; long-running silent handlers may need progress output or an appropriate HTTP timeout. Legacy eager-header behavior is preserved.
• Accept opaque legacy metadata and correct request-scoped capabilities and discovery advertisement #1852 by @halter73 (co-authored by @Copilot)
• Avoid redundant SSE fallback after discovery is rejected with 400 or 404 #1855 by @scottt732
• Close server stdin before waiting for graceful stdio process exit #1838 by @luisangelrod
• Preserve insertion order when listing server tools, prompts, and resources #1924 by @PederHP (co-authored by Claude Opus 5.5)
• Allow missing optional server identity in discovery and make ServerInfo nullable #1903 by @Coding-Professional
• Include JSON parse details in Streamable HTTP 400 responses #1844 by @ump45nose
• Exclude OAuth from DiscoverProbeTimeout, expose McpClientOptions.TimeProvider for deterministic timeout tests, and default SSE to legacy initialization #1719 by @halter73 (co-authored by @Copilot)
  • OAuth remains subject to the overall InitializationTimeout and caller cancellation; it is excluded only from the short discovery-probe budget.
  • Modern discovery over SSE requires both explicit HttpTransportMode.Sse and ProtocolVersion = "2026-07-28"; pinning the version with AutoDetect is not equivalent.
• Preserve numeric progress tokens in RequestOptions serialization #1832 by @dfedoryshchev
• Dispose legacy SSE POST responses to release connections deterministically #1841 by @yalcinfu22 (co-authored by Claude Fable 5)

Documentation Updates

• Document standing support for new MCP specification revisions #1826 by @halter73 (co-authored by @Copilot)

Test Improvements

• Stabilize discovery-capable test connections while retaining initialization deadlines #1943 by @tarekgh

Repository Infrastructure Updates

• Add a source selector for manual documentation publishes #1828 by @jeffhandley (co-authored by @Copilot)
• Update SourceLink's private build dependency to resolve a vulnerable Git dependency #1873 by @halter73 (co-authored by @Copilot)
• Correct obsolete McpServerFactory references in Copilot instructions #1892 by @Zir0-93
• Update Microsoft.Extensions.AI and AI.Abstractions to 10.11.0 and align shared System dependencies to 10.0.13 #1940

Acknowledgements

• @scottt732 made their first contribution in #1855
• @Zir0-93 made their first contribution in #1892
• @Coding-Professional made their first contribution in #1903
• @ump45nose made their first contribution in #1844
• @dfedoryshchev made their first contribution in #1832
• @yalcinfu22 made their first contribution in #1841
• @NikiforovAll submitted #1800 (resolved by #1810)
• @bseekford-st submitted #1774 (resolved by #1852)
• @matteocontrini submitted #1783 (resolved by #1852)
• @Zir0-93 submitted #1893 (resolved by #1892)
• @FrancescoCrecchi submitted #1836 (resolved by #1838)
• @PederHP submitted #1923 (resolved by #1924)
• @anushree1808 submitted #1901 (resolved by #1903)
• @yalcinfu22 submitted #1840 (resolved by #1841)
• @halter73 @tarekgh @jeffhandley @luisangelrod @PederHP @chrikrah @PranavSenthilnathan @ump45nose reviewed pull requests

Full Changelog: v2.2.0...v2.3.0

Note

These release notes were drafted with GitHub Copilot and reviewed before publishing.