8 MCP `401 invalid_token` Errors and How to Fix Each

MCP server 401 responses usually indicate that a valid OAuth token was issued for a different audience rather than being malformed, because current MCP authorization rules require servers to validate that tokens are specifically intended for their canonical resource URI and require clients to send the RFC 8707 resource parameter during authorization and token requests. The guide outlines eight diagnostic causes: missing authorization-server discovery metadata, an incorrect or absent token audience, omitted resource parameters, subtle canonical URI differences such as trailing slashes or ports, issuer metadata drift, expiration or clock skew, reuse of client credentials with a different authorization server, and incompatibility between older and newer MCP protocol revisions. It recommends comparing protected-resource metadata, token claims, issuer values, and expiry timestamps byte-for-byte rather than visually, centralizing the canonical URI rather than deriving it from requests, and upgrading mismatched SDKs and transports. It also distinguishes legitimate authentication failures, which should return 401, from insufficient permissions, which must return 403 with insufficient_scope to avoid clients repeatedly refreshing tokens that cannot gain the needed scopes.

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