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Follow-ups to #1610: de-draft 2026-07-28 terminology, renumber error codes, ProtocolVersion floor (#1671)

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Stephen Halter 2026-06-25 14:07:52 -07:00 • committed by GitHub
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src/ModelContextProtocol.Core/Client/McpClientOptions.cs
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@ -52,52 +52,23 @@ public sealed class McpClientOptions
/// </summary>
/// <remarks>
/// <para>
/// When non-<see langword="null"/>, this version is requested from the server. Setting it to a
/// legacy (non-draft) version such as <c>2025-11-25</c> opts out of the draft revision and forces
/// the <c>initialize</c> handshake; the handshake then fails if the server's negotiated version
/// does not match.
/// 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.
/// </para>
/// <para>
/// When <see langword="null"/> (the default), the client prefers the draft revision
/// (<see cref="McpSession.DraftProtocolVersion"/>): it probes with <c>server/discover</c> and
/// automatically falls back to a legacy <c>initialize</c> handshake, downgrading to any version
/// the server advertises, when the server does not support the draft revision.
/// 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
/// 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.
/// </para>
/// </remarks>
public string? ProtocolVersion { get; set; }
/// <summary>
/// Gets or sets the minimum protocol version the client will accept during version negotiation.
/// </summary>
/// <remarks>
/// <para>
/// When negotiating with a server that advertises multiple supported versions, or when falling back
/// to a legacy server, the client will refuse any version older than this minimum and surface an
/// <see cref="McpException"/> instead.
/// </para>
/// <para>
/// This is useful when the client requires features (such as the draft revision's removal of the
/// <c>initialize</c> handshake or <c>Mcp-Session-Id</c>) that are not available in older protocol
/// revisions. Because the client already prefers the draft revision by default, setting this to
/// <see cref="McpSession.DraftProtocolVersion"/> disables the automatic legacy-server fallback
/// that otherwise switches to the <c>initialize</c> handshake.
/// </para>
/// <para>
/// If <see langword="null"/> (the default), the client falls back to any version the server
/// advertises, including legacy versions such as 2025-11-25.
/// </para>
/// <example>
/// <code>
/// // The draft revision is already the default; pin the minimum to refuse the legacy fallback.
/// var clientOptions = new McpClientOptions
/// {
/// MinProtocolVersion = McpSession.DraftProtocolVersion,
/// };
/// </code>
/// </example>
/// </remarks>
public string? MinProtocolVersion { get; set; }
/// <summary>
/// Gets or sets a timeout for the client-server initialization handshake sequence.
/// </summary>
@ -128,10 +99,10 @@ public sealed class McpClientOptions
/// </value>
/// <remarks>
/// <para>
/// This timeout only has an effect when the client prefers the draft protocol revision &#8212; that is,
/// when <see cref="ProtocolVersion"/> is <see langword="null"/> (the default) or
/// <see cref="McpSession.DraftProtocolVersion"/>. In that mode the client first probes the server
/// with a <c>server/discover</c> request. A legacy server that predates the draft revision may
/// 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
/// 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
@ -140,7 +111,7 @@ public sealed class McpClientOptions
/// <para>
/// The default is intentionally short so that dual-era clients fall back quickly against legacy
/// 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 draft-capable
/// 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
/// <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