Service principals (Bearer dfk_...) may only reach the data plane
(threads / runs / assistants). All control-plane routes — mcp, models,
skills, channels, agents, memory, auth, management — now return 403
insufficient_scope. The check sits after the NULL/401 guard so an
invalid key on a control-plane path still returns 401, not 403. Human
cookie requests bypass the bearer branch entirely and are unaffected.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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