Add 2025-03-26 OAuth backward compatibility for client conformance#1374
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jeffhandley wants to merge 4 commits intomodelcontextprotocol:mainfrom
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Add 2025-03-26 OAuth backward compatibility for client conformance#1374jeffhandley wants to merge 4 commits intomodelcontextprotocol:mainfrom
jeffhandley wants to merge 4 commits intomodelcontextprotocol:mainfrom
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Implement legacy OAuth fallback so the client can authenticate against MCP servers that predate Protected Resource Metadata (RFC 9728): - When PRM discovery fails, synthesize minimal metadata using the MCP server's origin as the authorization server - When auth server metadata discovery also fails, fall back to the default endpoint paths (/authorize, /token, /register) specified by the MCP 2025-03-26 spec - Conditionally omit the 'resource' parameter from authorization and token requests when operating in legacy mode - Skip resource-match verification only for synthesized (not fetched) PRM Enable the two previously-commented-out client conformance test scenarios: auth/2025-03-26-oauth-metadata-backcompat auth/2025-03-26-oauth-endpoint-fallback Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
halter73
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Feb 24, 2026
src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs
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halter73
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Feb 24, 2026
Refactor GetAuthServerMetadataAsync to accept an allowDefaultFallback parameter and return BuildDefaultAuthServerMetadata directly instead of throwing and catching McpException at the call site. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Add two tests to AuthTests.cs covering legacy server scenarios: - CanAuthenticate_WithLegacyServerWithoutProtectedResourceMetadata: Server lacks RFC 9728 PRM but serves auth server metadata at well-known URLs on the MCP server origin. - CanAuthenticate_WithLegacyServerUsingDefaultEndpointFallback: Server lacks both PRM and auth server metadata, forcing fallback to default /authorize, /token, /register endpoint paths. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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halter73
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| "issuer": "{{OAuthServerUrl}}", | ||
| "authorization_endpoint": "{{OAuthServerUrl}}/authorize", | ||
| "token_endpoint": "{{OAuthServerUrl}}/token", | ||
| "registration_endpoint": "{{OAuthServerUrl}}/register", |
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I think we should be using McpServerUrl rather than OAuthServerUrl for the above to really replicate the legacy scenario. It looks like we're proxying these endpoints using CanAuthenticate_WithLegacyServerUsingDefaultEndpointFallback. I think we should do the same here, and that should be possible.
In reality, this would fail due to a resource mismatch if the OAuth server validated it (we don't because TestOAuthServer.RequireResource = false is set in this test). I think it's better to test the fallback scenario where we skip using the resource. Maybe we should only update the RequireResource` logic in test server to only pass if the resource is missing rather than invalid.
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Implement legacy OAuth fallback so the client can authenticate against MCP servers that predate Protected Resource Metadata (RFC 9728):
Enable the two previously-commented-out client conformance test scenarios:
auth/2025-03-26-oauth-metadata-backcompat
auth/2025-03-26-oauth-endpoint-fallback