[jeffhandley]

Release v2.4.0

MCP Skills support is introduced with experimental APIs in the new ModelContextProtocol.Extensions.Skills package. The release also strengthens OAuth handling and improves transport/session reliability/discovery. MCP Apps now match protocol property names case-sensitively.

What's Changed

• Add the experimental MCP Skills extension Add SEP-2640 Skills extension support #1856 by @girishkvs (co-authored by @PederHP)
• Align Skills with server primitives and typed request handlers Align Skills with server primitives and typed request handlers #1993 by @halter73 (co-authored by @copilot)
• Require explicit opt-in for the legacy OAuth metadata fallback Require explicit opt-in for legacy OAuth metadata fallback #1952 by @halter73 (co-authored by @copilot)
• Preserve refresh tokens when OAuth refresh responses omit replacements Preserve refresh tokens when OAuth refresh responses omit replacements #1968 by @halter73 (co-authored by @copilot)
• Preserve canonical OAuth resource identifiers during refresh fix: preserve canonical OAuth resource identifiers during refresh #1967 by @halter73 (co-authored by @copilot)
• Fix cancellation being reported as missing OAuth metadata Fix cancellation being reported as missing OAuth metadata #1891 by @Empiree
• Cancel open subscriptions/listen requests when the transport stops delivering messages Cancel open subscriptions/listen requests when the transport stops delivering messages #1933 by @pournasserian (co-authored by @copilot)
• Fix dropped notifications and cancellation when a request is canceled Fix dropped notifications/cancelled when a request is canceled #1917 by @zerotohero3
• Prevent server/discover probes from committing the negotiated protocol version fix: server/discover probe must not commit the negotiated protocol version #1956 by @PrathmeshPawar2110 (co-authored by @claude)
• Fall back to SSE when AutoDetect receives a 405 with a JSON-RPC error body fix: fall back to SSE when AutoDetect probe gets 405 with JSON-RPC error body #1849 by @ump45nose
• Fix legacy output schemas for delegating server tools Fix legacy output schemas for delegating server tools #1928 by @Oxygen56
• Match protocol property names case-sensitively Match protocol property names case-sensitively #1934 by @FenjuFu

Documentation Updates

• Clarify Skills discovery and documentation Clarify Skills discovery and documentation #2000 by @halter73 (co-authored by @copilot)
• Fix Markdown link-check warnings and update the Skills specification link Fix Markdown link-check warnings and outdated Skills specification link #1975 by @jeffhandley (co-authored by @copilot)

Test Improvements

• Prevent discovery probe timeouts in controlled test connections Prevent discovery probe timeouts in controlled test connections #1946 by @tarekgh
• Apply test discovery timeouts to Sampling_Stdio Apply the test discovery timeouts to Sampling_Stdio #1950 by @girishkvs
• Use a task that runs until cancelled in the task store cancellation test Use a task that runs until cancelled in the task store cancel test #1955 by @girishkvs
• Fix parallel integration test server startup Fix parallel integration test server startup #1982 by @jeffhandley (co-authored by @copilot)

Repository Infrastructure Updates

• Bump fast-uri from 3.1.5 to 3.1.7 Bump fast-uri from 3.1.5 to 3.1.7 in the npm_and_yarn group across 1 directory #1850
• Bump actions/deploy-pages from 5.0.0 to 5.0.1 Bump actions/deploy-pages from 5.0.0 to 5.0.1 #1858
• Support release-line docs publishing and release coordination [main] Support release-line docs publishing and release coordination #1972 by @jeffhandley (co-authored by @copilot)
• Bump docfx from 2.78.5 to 2.81.0 Bump docfx from 2.78.5 to 2.81.0 #1961
• Bump actions/setup-node from 7.0.0 to 7.1.0 Bump actions/setup-node from 7.0.0 to 7.1.0 #1927
• Bump becheran/mlc from 1.2.0 to 1.3.0 Bump becheran/mlc from 1.2.0 to 1.3.0 #1907
• Bump the other-testing group with 2 updates Bump the other-testing group with 2 updates #1960
• Bump actions/download-artifact from 8.0.1 to 8.0.2 Bump actions/download-artifact from 8.0.1 to 8.0.2 #1926
• Bump Anthropic from 12.40.0 to 13.3.0 Bump Anthropic from 12.40.0 to 13.3.0 #1962
• Bump the opentelemetry-testing group with 6 updates Bump the opentelemetry-testing group with 6 updates #1959
• Bump actions/upload-artifact from 7.0.1 to 7.0.2 Bump actions/upload-artifact from 7.0.1 to 7.0.2 #1925
• Fix CI selection of patched .NET SDKs Fix CI selection of patched .NET SDKs #1995 by @tarekgh

Acknowledgements

• @Empiree made their first contribution in Fix cancellation being reported as missing OAuth metadata #1891
• @zerotohero3 made their first contribution in Fix dropped notifications/cancelled when a request is canceled #1917
• @girishkvs made their first contribution in Add SEP-2640 Skills extension support #1856
• @FenjuFu made their first contribution in Match protocol property names case-sensitively #1934
• @PrathmeshPawar2110 made their first contribution in fix: server/discover probe must not commit the negotiated protocol version #1956
• @Oxygen56 made their first contribution in Fix legacy output schemas for delegating server tools #1928
• @pournasserian made their first contribution in Cancel open subscriptions/listen requests when the transport stops delivering messages #1933
• @nagavitalp submitted issue JSON property-name casing is inconsistent between JSON-RPC envelope and request params #1897 (resolved by Match protocol property names case-sensitively #1934)
• @oleksandrdubyna88 submitted issue Stdio server never exits after stdin EOF once a client has sent subscriptions/listen (2026-07-28) #1914 (resolved by Cancel open subscriptions/listen requests when the transport stops delivering messages #1933)
• @AdCodicem submitted issue A tool behind DelegatingMcpServerTool advertises a non-object outputSchema to pre-2026-07-28 clients (regression from 1.4.1) #1916 (resolved by Fix legacy output schemas for delegating server tools #1928)
• @gavin-wolfe submitted issue server/discover probe timeout falls back to initialize, which a slow 2.3.0 server then rejects (or silently downgrades) #1954 (resolved by fix: server/discover probe must not commit the negotiated protocol version #1956)
• @as992949791 submitted issue OAuth refresh discards cached refresh token when the response omits refresh_token #1958 (resolved by Preserve refresh tokens when OAuth refresh responses omit replacements #1968)
• @jeffhandley @tarekgh @halter73 @girishkvs @FenjuFu @PederHP @zerotohero3 @chrikrah @ump45nose @pournasserian reviewed pull requests

Full Changelog: v2.3.1...v2.4.0

Note

Drafted with GitHub Copilot assistance.

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API Compatibility Report

Package	Baseline	Generated suppressions	Tracked suppressions retained/removed	Result
ModelContextProtocol.Core	2.0.0	4	4 / 0	Pass
ModelContextProtocol	2.0.0	0	0 / 0	Pass
ModelContextProtocol.AspNetCore	2.0.0	0	0 / 0	Pass
ModelContextProtocol.Extensions.Apps	2.0.0	0	0 / 0	Pass
ModelContextProtocol.Extensions.Tasks	2.0.0	0	0 / 0	Pass
ModelContextProtocol.Extensions.Skills	None (new package)	N/A	N/A	Pack passed; ApiCompat unvalidated

Release clean and pack passed. Core's four generated suppressions match the tracked entries exactly; no suppression files were changed. Skills clears its package validation baseline and was not shipped in v2.3.1, so it has no prior-release ApiCompat baseline.

API Diff Report

Compared with v2.3.1:

• ModelContextProtocol.Core: Adds McpJsonUtilities.ProtocolOptions, CacheScopeConverter, and protected constructors for pagination base types. McpServerRequestHandler.Handler changes type; this API is experimental (MCPEXP002).
• ModelContextProtocol: No focused API surface changes.
• ModelContextProtocol.AspNetCore: No focused API surface changes.
• ModelContextProtocol.Extensions.Apps: The public GetUiCapability signature is unchanged. The diff tool renders an identical signature as removed and added; PR Match protocol property names case-sensitively #1934 changes Apps serializer behavior so protocol property names are case-sensitive.
• ModelContextProtocol.Extensions.Tasks: No focused API surface changes.
• ModelContextProtocol.Extensions.Skills: New package with an additive API surface; its APIs are experimental (MCPEXP005).

Note

This pull request description was prepared with GitHub Copilot assistance.