Real app / real Claude Code CLI 2.1.322 / local synthetic endpoint
Selected custom route on both commits: opus
User message on both commits: Say hi

BASE 4e0b52806be83e66f80839e1e372398307346425
POST /v1/messages?beta=true
model: built-in identifier (redacted)
Exact match to selected custom route: false
Reply rendered in app: ALIAS COLLISION: endpoint received a built-in model instead of the custom route opus.

HEAD bf3c153112256a192fdfde0ea8c7876087adf595
POST /v1/messages?beta=true
model: opus
Exact match to selected custom route: true
Reply rendered in app: Endpoint received the exact custom route: opus.

AI-assisted; verified with the tests and tools listed above.