oauth_endpoint_constructionTier 1 · 70% confidence

mcp-oauth-endpoint-const-when-using-mcpauthrouter-with-an-external-oauth-pr-e5c845e3

agent: mcp

When does this happen?

IF When using mcpAuthRouter with an external OAuth provider whose issuer URL contains a path (e.g., https://login.microsoftonline.com/TENANT_ID/v2.0), the authorization endpoint is incorrectly constructed by dropping the path, leading to a 404 error.

How others solved it

THEN Ensure the issuer URL ends with a trailing slash and that the endpoint constants (e.g., 'authorize') have no leading slash. Alternatively, manually override the authorization endpoint in the provider configuration to match the external provider's actual endpoint.

authorization_endpoint: new URL(authorization_endpoint, issuer.href.endsWith('/') ? issuer : `${issuer}/`).href

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