fix: propagate auth errors (401/403) immediately instead of falling b…#1444
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xue-cai wants to merge 1 commit intomodelcontextprotocol:mainfrom
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fix: propagate auth errors (401/403) immediately instead of falling b…#1444xue-cai wants to merge 1 commit intomodelcontextprotocol:mainfrom
xue-cai wants to merge 1 commit intomodelcontextprotocol:mainfrom
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…ack to SSE When AutoDetectingClientSessionTransport receives a 401 or 403 from the initial Streamable HTTP POST, it should NOT fall back to SSE transport. Auth errors are not transport-related — the server understood the request but rejected the credentials. Falling back to SSE would: 1. Fail with the same credentials (or a different transport error) 2. Mask the real authentication error from the caller This was discovered investigating Azure AI Search MCP endpoints returning 403 on Streamable HTTP POST, which caused the SDK to fall back to SSE GET, resulting in a confusing 405 error that hid the real auth failure. The fix adds an explicit check for 401/403 before the SSE fallback path, and throws HttpRequestException with the auth status code immediately. Code reference: AutoDetectingClientSessionTransport.InitializeAsync https://github.com/modelcontextprotocol/csharp-sdk/blob/v0.9.0-preview.2/src/ModelContextProtocol.Core/Client/AutoDetectingClientSessionTransport.cs#L61 Testing: # Build (two warnings suppressed are pre-existing on main, not related to this change): dotnet build '/p:NoWarn=NU1903%3BMCPEXP001' # Run all transport tests on all 3 target frameworks (73 tests × 3 = 219 total, 0 failures): dotnet test tests/ModelContextProtocol.Tests/ '/p:NoWarn=NU1903%3BMCPEXP001' --no-build --filter "FullyQualifiedName~Transport" --framework net10.0 dotnet test tests/ModelContextProtocol.Tests/ '/p:NoWarn=NU1903%3BMCPEXP001' --no-build --filter "FullyQualifiedName~Transport" --framework net9.0 dotnet test tests/ModelContextProtocol.Tests/ '/p:NoWarn=NU1903%3BMCPEXP001' --no-build --filter "FullyQualifiedName~Transport" --framework net8.0 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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…ack to SSE
When AutoDetectingClientSessionTransport receives a 401 or 403 from the initial Streamable HTTP POST, it should NOT fall back to SSE transport. Auth errors are not transport-related — the server understood the request but rejected the credentials. Falling back to SSE would:
This was discovered investigating Azure AI Search MCP endpoints returning 403 on Streamable HTTP POST, which caused the SDK to fall back to SSE GET, resulting in a confusing 405 error that hid the real auth failure.
The fix adds an explicit check for 401/403 before the SSE fallback path, and throws HttpRequestException with the auth status code immediately.
Code reference:
AutoDetectingClientSessionTransport.InitializeAsync
https://github.com/modelcontextprotocol/csharp-sdk/blob/v0.9.0-preview.2/src/ModelContextProtocol.Core/Client/AutoDetectingClientSessionTransport.cs#L61
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