Validate MCP-Protocol-Version header in Streamable HTTP handler#1277
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stephentoub merged 10 commits intomainfrom Feb 19, 2026
Merged
Validate MCP-Protocol-Version header in Streamable HTTP handler#1277stephentoub merged 10 commits intomainfrom
stephentoub merged 10 commits intomainfrom
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If the server receives a request with an invalid or unsupported MCP-Protocol-Version header, respond with 400 Bad Request per the MCP spec. A missing header is allowed for backwards compatibility. Exposes McpSession.SupportedProtocolVersions as a public static property so the AspNetCore handler can validate against it. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
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[WIP] Add validation for MCP-Protocol-Version header
Validate MCP-Protocol-Version header in Streamable HTTP handler
Feb 15, 2026
stephentoub
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Feb 15, 2026
tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs
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…ns class, use nullable out param - Move SupportedProtocolVersions from McpSession to new ProtocolVersions static class in Protocol namespace (following RequestMethods/NotificationMethods pattern) - Revert McpSession.cs to original state - Change out string to out string? with null on success - Add ProtocolVersionsTests for the new public property Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
The SupportedProtocolVersions doesn't need to be exposed publicly. Instead, use InternalsVisibleTo to let ModelContextProtocol.AspNetCore access the internal McpSessionHandler.SupportedProtocolVersions directly. - Delete ProtocolVersions.cs public static class - Delete ProtocolVersionsTests.cs - Add InternalsVisibleTo in Core csproj for AspNetCore - Reference McpSessionHandler.SupportedProtocolVersions directly Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
…idation Define supported protocol versions as a private HashSet directly in StreamableHttpHandler instead of referencing McpSessionHandler internals or adding any cross-assembly visibility. No new public API surface needed. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
stephentoub
reviewed
Feb 15, 2026
Add abstract SupportedProtocolVersions property to McpServer, implemented by McpServerImpl (delegating to McpSessionHandler.SupportedProtocolVersions) and DestinationBoundMcpServer. Protocol version validation in StreamableHttpHandler now occurs after session resolution, querying the server instance directly. Remove private HashSet from handler. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Use TryGetValue to check version first, then TryRemove to actually delete. This prevents losing the session if validation fails. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
stephentoub
reviewed
Feb 15, 2026
…yList<string> Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
halter73
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Feb 17, 2026
…nc comments Remove the abstract SupportedProtocolVersions property from McpServer, McpServerImpl, and DestinationBoundMcpServer. Instead, keep a private HashSet in StreamableHttpHandler with the supported protocol versions, and add comments on both lists about keeping them in sync. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
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halter73
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Feb 19, 2026
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@copilot Capture brief notes on why this is a behavioral breaking change. Ideally, include a small sample client repro that would experience the change in behavior. |
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Per the 2025-06-18 MCP spec: if the server receives a request with an invalid or unsupported
MCP-Protocol-Versionheader, it MUST respond with 400 Bad Request. A missing header is not an error (backwards compatibility).Changes
StreamableHttpHandler.cs— Added protocol version validation at the top of POST, GET, and DELETE handlers. Supported versions are defined as a privateHashSet<string>directly in the handler, with a comment to keep in sync with the canonical list. Present but unsupported values → 400; absent header → allowed.McpSessionHandler.cs— Added a sync comment on the internalSupportedProtocolVersionslist referencing the copy inStreamableHttpHandler.StreamableHttpServerConformanceTests.cs— Added tests for invalid versions (400), missing header (OK), and valid header (OK) on both POST and GET.Original prompt
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