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Add Extensions support to ClientCapabilities and ServerCapabilities#1317

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copilot/reimplement-experimental-handling
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Add Extensions support to ClientCapabilities and ServerCapabilities#1317
Copilot wants to merge 3 commits intomainfrom
copilot/reimplement-experimental-handling

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Copilot AI commented Feb 19, 2026

  • Add Extensions_DiagnosticId, Extensions_Message, and Extensions_Url constants to Experimentals.cs (all mapping to MCPEXP001)
  • Add Extensions property to ClientCapabilities using the internal *Core property pattern ([Experimental][JsonIgnore] public + [JsonInclude][JsonPropertyName] internal)
  • Add Extensions property to ServerCapabilities using the same pattern
  • Update docs/list-of-diagnostics.md to include Extensions in the MCPEXP001 description
  • Add serialization roundtrip tests for Extensions in existing test files
  • Build and verify all tests pass (0 warnings, 0 errors)
  • Differentiate XML doc remarks: "Clients advertise..." vs "Servers advertise..."
  • Security scan (CodeQL: 0 alerts)

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…SEP-2133)

Add Extensions property to both ClientCapabilities and ServerCapabilities
using the internal *Core property pattern for experimental APIs. Extensions
use the same MCPEXP001 diagnostic ID as Tasks, with dedicated constants in
Experimentals.cs for clarity.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Copilot AI changed the title [WIP] Reimplement handling of [Experimental] with recent changes Add Extensions support to ClientCapabilities and ServerCapabilities Feb 19, 2026
Copilot AI requested a review from stephentoub February 19, 2026 22:24
/// </para>
/// <para>
/// Extensions provide a framework for extending the Model Context Protocol while maintaining
/// interoperability. Both clients and servers advertise extension support via this field during
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@copilot this comment ends the corresponding one in client capabilities should be tweaked. This one is about server, that I've is about client

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Fixed in 6ea42fc. The remarks now say "Clients advertise..." in ClientCapabilities and "Servers advertise..." in ServerCapabilities.

…ilities

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
@stephentoub stephentoub marked this pull request as ready for review February 20, 2026 00:22
@stephentoub stephentoub enabled auto-merge (squash) February 20, 2026 00:22
Copilot AI requested a review from stephentoub February 20, 2026 00:22
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