`GLIBC_x.y not found` from a Homebrew binary (Linux)

Symptom: a brew-installed binary refuses to start, blaming Homebrew’s own libc:

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.../opt/binutils/bin/as: .../opt/glibc/lib/libc.so.6: version `GLIBC_2.38' not found

Only some binaries are affected — the recently installed ones — and the error names whichever binary you happened to run, so it reads like a problem with that package. ldd disagrees and shows the system libc, because it resolves through the system loader while the binary itself runs under brew/lib/ld.so.

Root cause: the glibc keg is older than the bottles. Homebrew’s Linux bottles carry the glibc floor of the CI image that built them, and when homebrew-core moves that image it bumps the glibc formula in the same breath (Ubuntu 22.04 → 24.04, glibc 2.35 → 2.39, July 2026). Nothing upgrades an installed glibc keg on its own — it isn’t in the Brewfile — so every formula poured after the move lands with a floor the keg can’t meet. Aug 2026: seven kegs (binutils, gcc@15, texlab, tinymist, cadical, harper, juliaup) broke at once, all poured by one brew bundle run days after the builder moved.

Confirm:

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brew outdated --formula --verbose glibc      # glibc (2.35_2) < 2.39_1
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ldd --version | head -1                      # host glibc
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jq -r '.built_on.os_version' "$(brew --cellar)"/<formula>/*/INSTALL_RECEIPT.json

Fix: rerun install/linux-packages.sh. It reconciles the keg against the formula before the bundle, and checks every keg installed since the last run against what the keg provides. The broken kegs need no reinstall — they were fine all along; only the loader under them was too old.

Upgrading by hand needs one guard:

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HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK=1 brew upgrade glibc

A bare brew upgrade glibc moves the keg in ~3 minutes and then spends hours source-rebuilding every dependent whose linkage it considers stale, holding a formula lock the whole time so every other brew command fails with “has already locked”. The rebuild buys nothing: glibc is backward compatible, and the kegs built against the old one keep running.

Homebrew refuses to build a glibc newer than the host’s, so a host older than the formula can’t be fixed this way. That combination has no in-place remedy: either the host glibc moves, or the keg goes and every formula is reinstalled against the host loader. The script warns rather than pretending.

Related: brew reinstall glibc fails with Errno::ENOENT ... gcc-13. Reinstall unlinks the keg before building, which leaves brew/lib/ld.so dangling, and no brew binary — including the compiler — can start. Upgrades build the new keg first and are safe; a keg can only be rebuilt at the same version by rebuilding the prefix.