What is MCP authorization? How OAuth works for AI agents

MCP authorization uses OAuth 2.1 to let AI agents access protected HTTP-based MCP servers on a user’s behalf with short-lived, server-specific access tokens rather than passwords or persistent API keys, while STDIO servers should instead obtain credentials from their environment. Under the current 2026-07-28 specification, the MCP server acts as an OAuth resource server that discovers and trusts a separate authorization server, validates tokens, enforces exact audience binding through the RFC 8707 resource parameter, and never forwards received tokens upstream. Clients discover Protected Resource Metadata and authorization-server metadata, use mandatory PKCE, validate callback issuers to prevent mix-up attacks, and generally register through Client ID Metadata Documents, with Dynamic Client Registration now deprecated. Servers must return 401 for missing or invalid tokens and 403 for insufficient permissions, allowing clients to perform step-up authorization by requesting the union of existing and newly required scopes. Although the specification defines token acquisition and validation requirements, it leaves application-specific decisions—such as which identities may use particular tools or resources, authorization-server operation, and unattended-agent delegation—to implementers.

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