Follow-up to #434 (umbrella, closed) and #439 (slice 1, merged). Pacquet now skips installability-incompatible optional dependencies, but the only SupportedArchitectures value it ever sees is None — so every install behaves as if the user opted into "every variant for the host triple only". This blocks two real workflows pnpm supports:

1. Cross-architecture install — pnpm install --cpu=arm64 --os=darwin on an x64 Linux host materializes the macOS arm64 variants in node_modules. Useful for prebuilt binary packages (esbuild, swc, native add-ons) when running CI for a host arch you don't have.

2. Multi-arch dev artifacts — pnpm-workspace.yaml carries supportedArchitectures: { os: ['darwin', 'linux'], cpu: ['arm64', 'x64'] }, materializing every variant the team's devs use so a single pnpm install covers everyone.

Scope

Mirror upstream's three wiring points pinned at pnpm/pnpm@94240bc046:

• Config.supported_architectures: Option<SupportedArchitectures> deserialized from pnpm-workspace.yaml (getOptionsFromRootManifest.ts:23).
• --cpu, --libc, --os flags on install / add / update, multi-valued (types.ts:146-148). CLI values override config wholesale per axis (overrideSupportedArchitecturesWithCLI.ts).
• dedupe_current substitution and 'any' / !foo negation already land in feat(package-is-installable): platform + engine check, skip optional incompatibles (#434 slice 1) #439's check_platform; this slice only wires the input.

Real libc detection

Slice 1 stubbed host_libc() to always return "unknown" so libc checks no-op on every host (matches non-Linux behavior). This slice replaces it with a real probe — detect-libc Rust crate, or an open-coded ldd --version parse with a /lib*/libc.so* fallback — so musl / glibc constraints actually filter on Linux. "unknown" fallback remains for hosts where detection genuinely fails.

Tests to port

• optionalDependencies.ts:594 — install optional dependency for the supported architecture set by the user (nodeLinker=%s); covers config + frozen-reinstall.
• optionalDependencies.ts:618 / :633 / :648 — remove optional dependencies that are not used and variants. Depend on the current-lockfile path; partial ports as known_failures until slice 6 lands.

Out of scope

• The removeOptionalDependenciesThatAreNotUsed behavior end-to-end depends on the current lockfile (slice 6 of the umbrella); this slice only sets up the config + check input.
• .modules.yaml.skipped write/read is umbrella slice 3.
• --no-optional plumbing is umbrella slice 5.

Upstream

pnpm/pnpm@94240bc046. Pacquet slice 1 landed in #439; this is umbrella slice 2.

Written by an agent (Claude Code, claude-opus-4-7).