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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 v2.1.221

Auto mode can be told to do its work through Bash ("thrifty sonic")

Not switched on
Useful3 Signal5
Auto Mode Notable not in their notes

Auto mode can be told to use Bash for reads and edits if you set the env var.

CLAUDE_CODE_THRIFTY_SONIC makes auto mode prefer cat, grep and sed over the dedicated tools; off by default.

Feature flag
tengu_thrifty_sonic On for this account, and not off by default

The flag server returned on for the one account this site reads, and nothing in this release compiles it off by default. The compiled default is shown below, and says which it is when we cannot read one: a fifth of gates compile in a string or a number rather than on or off, and most published releases have no gate table behind them at all. No client can see what the server returns for your account.

This account: on · anonymous baseline: on · compiled default in v2.1.221: on

These values were read against a different version of Claude Code, so treat them as the nearest reading available instead of one taken on this release.

Read once, for one account on one subscription tier, against v2.1.221. It isn't a statement about your account. What a flag value here can and cannot tell you

CLAUDE_CODE_THRIFTY_SONIC
What

A new check decides whether auto mode gets a "bash first" instruction block telling the model to use Bash instead of the dedicated read and edit tools. It is off in this build unless the env var is set.

Details
  • When it fires, the auto-mode system reminder appends prose telling the model to "read files with cat, head, or sed -n, search with grep and find, and make file changes with sed, heredocs, or short scripts".
  • The auto-mode-exit reminder then says " Resume using the dedicated tools for file reads, searches, and edits."
  • The attachment carries a new bashFirst field, set only when the session has Bash plus the edit tools available and the check returns true.
  • Gate: CLAUDE_CODE_THRIFTY_SONIC wins if set, otherwise the remote gate tengu_thrifty_sonic, whose in-source fallback is false.
Evidence

tengu_thrifty_sonic

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

Related

Other releases about the same thing. Found by shared names or similar wording; neither means one caused the other.

See this entry in the whole of v2.1.221 →