Fixing `insufficient_scope` 403s in MCP Step-Up Authorization

MCP clients can enter an authorization loop when they respond to a 403 insufficient_scope challenge by requesting only the newly challenged scope, thereby dropping scopes requested earlier; for example, alternating read and write operations can repeatedly revoke each other’s effective permissions and trigger recurring consent prompts. Under the MCP 2026-07-28 authorization specification, servers may challenge only with scopes needed for the current operation, while clients are responsible for re-authorizing with the union of all previously requested scopes and newly challenged scopes. Clients should preserve this scope set per authorization server, distinguish 403 scope failures from 401 token failures, and impose bounded retry limits keyed to the resource, operation, and challenged scopes to prevent endless retries when authorization is denied. Servers should return all scopes required for an operation in a single consistent challenge, include relevant metadata, and correctly handle scope hierarchies so broader permissions satisfy narrower requirements. When ordinary scopes cannot express fine-grained restrictions such as project-specific or time-limited access, structured authorization details or token exchange may be more appropriate, but neither replaces the client-side union requirement.

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