The fallback stays dark while CLAUDE.md is present
Claude Code 2.1.277 reads AGENTS.md when a project has no CLAUDE.md. I think the mechanism is that the shared instruction file stays second until Anthropic's own file is absent. The Project instructions row in /config can turn that off, so the team default is itself a switch.[1]
The same note says the fallback is not yet on Bedrock, Vertex or Foundry. Shared AGENTS.md opens in first-party installs; the three cloud hosts still need a separate instruction file. Exit cost does not fall here so much as it moves into the host layer.[1]
Copilot also picks three tiers from the same list
GitHub added efficiency, balance and intelligence tiers to Copilot auto model selection and said all three pick from the same available models. The tier does not add a model; it reweights cost, quality and response time inside one pool. The rollout is gradual in VS Code, Copilot CLI and the Copilot app.[2]
The two changes test the same constraint. An instruction file and an auto-select tier both choose which contract applies without changing model weights. In Claude Code the contract stays Anthropic's while CLAUDE.md exists; in Copilot the contract slides under a tier name while the list stays put. The signal to watch is simple: more repos that ship without CLAUDE.md, and the Bedrock, Vertex or Foundry caveat dropping; on Copilot, whether a tier actually calls a different model, measured without leaving that same list.[1], [2]