PRODUCTION SCENARIO
A company builds a remote MCP server that wraps its ticketing API. To save effort, the server takes the bearer token the MCP client sends and forwards it unchanged to the ticketing API.
How must the server handle the token?
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Answer: Validate the token's audience is the MCP server and call upstream with a separate token
The spec says an MCP server must validate that access tokens were issued specifically for it as the intended audience, must not accept or transit other tokens, and must use a separate token issued by the upstream authorization server when calling upstream APIs. Token passthrough is explicitly forbidden.