What
A new environment variable that locks Claude Code to exactly the configured model and disallows any automatic fallback to a different model — including during compaction.
Usage
export CLAUDE_CODE_NO_MODEL_FALLBACK=true
claude --model claude-opus-4-5 ...Details
- When set, the availability fallback chain collapses to
[primary]only — no alternate models are tried - Compaction is disabled with the message "Compaction unavailable: CLAUDE_CODE_NO_MODEL_FALLBACK is set and model substitution is disabled · unset it to allow the swap"
- If code attempts a model-fallback pivot while the flag is active, a tripwire throws an error identifying the call site (this is a developer safety net, not a normal user-visible error)
- The flag is intended for environments where model consistency is a hard requirement (compliance, billing, reproducibility)
Evidence
Tripwire implementation (search for "CLAUDE_CODE_NO_MODEL_FALLBACK tripwire: a model-fallback pivot was attempted", "CLAUDE_CODE_NO_MODEL_FALLBACK")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.