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Tighten draft protocol negotiation boundaries (#1692)

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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>
This commit is contained in:
Stephen Halter 2026-07-12 14:30:49 -07:00 • committed by GitHub
commit fd38f51e27
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1 changed files with 1755 additions and 589 deletions
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24
src/ModelContextProtocol.Core/Client/McpClientOptions.cs
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@ -52,16 +52,20 @@ public sealed class McpClientOptions
/// </summary>
/// <remarks>
/// <para>
/// Supported values are <c>2024-11-05</c>, <c>2025-03-26</c>, <c>2025-06-18</c>, <c>2025-11-25</c>,
/// and <c>2026-07-28</c>.
/// </para>
/// <para>
/// When <see langword="null"/> (the default), the client prefers the latest revision (<c>2026-07-28</c>),
/// which removed the <c>initialize</c> handshake and Streamable HTTP sessions. It probes with
/// <c>server/discover</c> and automatically falls back to the legacy <c>initialize</c> handshake,
/// downgrading to any version the server advertises, when the server does not support that revision.
/// <c>server/discover</c> and automatically falls back to the <c>initialize</c> handshake,
/// downgrading to an initialize-capable version the server advertises, when the server does not support that revision.
/// </para>
/// <para>
/// When non-<see langword="null"/>, this value is both the requested version and the minimum the client
/// will accept: the client requests exactly this version and refuses to downgrade below it, throwing an
/// <see cref="McpException"/> instead of falling back. Setting it to <c>2026-07-28</c> therefore disables
/// the automatic legacy-server fallback, and setting it to a version that still supports Streamable HTTP
/// the automatic initialize-handshake server fallback, and setting it to a version that still supports Streamable HTTP
/// sessions, such as <c>2025-11-25</c>, forces the <c>initialize</c> handshake and fails if the server
/// negotiates a different version. To try more than one version, leave this unset for automatic fallback
/// or retry the connection with a different value.
@ -90,7 +94,7 @@ public sealed class McpClientOptions
/// <summary>
/// Gets or sets the timeout applied to the <c>server/discover</c> probe that the client issues
/// before falling back to the legacy <c>initialize</c> handshake.
/// before falling back to the <c>initialize</c> handshake.
/// </summary>
/// <value>
/// The probe timeout. The default value is 5 seconds. Use
@ -102,17 +106,17 @@ public sealed class McpClientOptions
/// This timeout only has an effect when the client prefers the <c>2026-07-28</c> protocol revision, that is,
/// when <see cref="ProtocolVersion"/> is <see langword="null"/> (the default) or <c>2026-07-28</c>.
/// In that mode the client first probes the server with a
/// <c>server/discover</c> request. A legacy server that predates the <c>2026-07-28</c> revision may
/// <c>server/discover</c> request. A server that predates the <c>2026-07-28</c> revision may
/// silently drop the unknown method, so the probe is bounded by this timeout; when it elapses the
/// client concludes the server is legacy and falls back to the <c>initialize</c> handshake on the
/// same connection. When the caller pins a legacy <see cref="ProtocolVersion"/>, no probe is issued
/// client concludes the server requires <c>initialize</c> and falls back to that handshake on the
/// same connection. When the caller pins an initialize-capable <see cref="ProtocolVersion"/>, no probe is issued
/// and this value has no effect.
/// </para>
/// <para>
/// The default is intentionally short so that dual-era clients fall back quickly against legacy
/// The default is intentionally short so that dual-path clients fall back quickly against initialize-handshake
/// servers. Increase it for high-latency environments (for example, cold-start serverless peers or
/// satellite links) where a short probe could trigger the legacy fallback before a server on the new revision
/// server has had a chance to respond. The probe is always also bounded by
/// satellite links) where a short probe could trigger the initialize fallback before a server on the
/// per-request metadata revision has had a chance to respond. The probe is always also bounded by
/// <see cref="InitializationTimeout"/>, which governs the overall connect budget: if this value is
/// greater than or equal to <see cref="InitializationTimeout"/>, the probe is effectively bounded by
/// <see cref="InitializationTimeout"/> alone.