Homebrew: Cannot install under Rosetta 2 in ARM default prefix

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Which architecture is this shell, and which brew is first on PATH
arch
uname -m
which -a brew

# Run the native one explicitly to get past it now
arch -arm64 /opt/homebrew/bin/brew install jq

# Terminal or iTerm: untick "Open using Rosetta" in Get Info on the app

# Keep the two prefixes apart in your shell profile
# /opt/homebrew is arm64, /usr/local is x86_64
eval "$(/opt/homebrew/bin/brew shellenv)"

# For a formula that genuinely has no arm64 build, use the Intel prefix
arch -x86_64 /usr/local/bin/brew install some-intel-only-formula

# Confirm the environment a formula will be built in
brew config | grep -E 'Rosetta|macOS|CPU|HOMEBREW_PREFIX'

How to diagnose Apple errors

macOS developer errors concentrate around three subsystems: the command line tools (xcrun and the active developer directory), the dynamic linker (dyld, library paths, and code signing), and Gatekeeper / notarisation. Apple Silicon added a fourth axis: architecture mismatches between arm64 and x86_64 binaries, which usually surface as confusing "library not loaded" or "bad CPU type" messages rather than as an explicit architecture error.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Check the active developer directory first with xcode-select -p. After every macOS or Xcode update this can point at a path that no longer exists, breaking git, compilers and package managers all at once.
2. For dyld errors, run otool -L /path/to/binary to list what it actually wants, then verify each path exists. DYLD_PRINT_LIBRARIES=1 shows the resolution order at runtime.
3. Confirm architecture with file /path/to/binary and uname -m. On Apple Silicon, a Rosetta shell reports x86_64 and will silently install the wrong Homebrew prefix.
4. Homebrew lives at /opt/homebrew on Apple Silicon and /usr/local on Intel. Permission errors under /usr/local on an M-series Mac almost always mean a Rosetta/native mix-up.
5. For Gatekeeper and notarisation, use spctl -a -vvv to assess a bundle and xcrun notarytool log to get the actual rejection reason. The submission status alone tells you nothing useful.

Tools worth reaching for

• xcode-select -p
• otool -L
• codesign -dv --verbose=4
• spctl -a -vvv
• xcrun notarytool log

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 docs.brew.sh 📖 developer.apple.com

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• macOS: dyld: Library not loadedThe dynamic linker could not find a shared library at the path baked into the binary, often…
• macOS: errSecAuthFailed / Keychain access deniedA code-signing or credential operation could not read the Keychain, common in CI when the…
• macOS: notarization failed (notarytool)Apple's notary service rejected the app, usually for unsigned/un-hardened binaries, a missing…

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This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.