dawn wants the usage query from a macOS app, and JavaScript will not run there. This is the query alone — the rules that turn a provider body into limits — as a C ABI library in Zig, with the TypeScript implementation left in place as the reference. The core is pure on purpose: it never opens a socket, reads a clock, or touches the filesystem. - `usage_parse(provider, body, now_ms)`, `usage_build_request(provider, token, account_id)`, `usage_credentials_from_access_token(jwt)`, `usage_supported_providers()`, `usage_abi_version()`, `usage_free()`. JSON in, JSON out, so the shapes are the contract already documented in packages/ai/docs/usage-query.md. Failures come back as `{"error":"..."}` and NULL only ever means allocation failed; a host with its own token needs no auth store. - Host owns the socket. A usage query is one GET, and doing it here would mean shipping TLS with its own trust store and proxy behaviour; a host with NSURLSession already has that, so it asks for the request, sends it, and hands back the body. - `native/usage/tests/c_smoke.c` is compiled by the toolchain and linked against the installed static library, so `zig build test` exercises the ABI the way a native caller does — including a NULL argument and a provider without a feed. - `packages/ai/test/usage-zig-parity.test.ts` computes both implementations from the same recorded bodies at test time (one captured live, redacted of user_id/account_id/email), so neither can drift from the other quietly. - Zig is pinned to 0.16.0 with a verified checksum: 0.16 moved args, stdio, fs, and clock APIs, and the pure core compiled unchanged through that while only the debug CLI needed edits, which is the boundary doing its job. `npm run check` now includes `check:zig`, and CI installs the pinned toolchain. Verified: zig unit tests plus the C ABI smoke test, byte-identical output against the TypeScript reference on both fixtures, `npm run check` clean.