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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.213

CLAUDE_CODE_NO_MODEL_FALLBACK — Prevent Model Substitution

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.

See this entry in the whole of v2.1.213 →