diff --git a/README.md b/README.md
index 1b85342..55b6e48 100644
--- a/README.md
+++ b/README.md
@@ -133,7 +133,7 @@ whose output was byte-identical to zlib-ng's at levels 1, 6 and 9 on the
 benchmark inputs and which matched its HDF5 read and write speed within 6%
 (tank, 2026-09-23, [BENCHMARKS.md](BENCHMARKS.md#deflate-backend-zlib-rs-vs-zlib-ng)). CI
 fails if a C-building crate enters their default dependency tree. C comes in
-only when you ask: `fast-deflate` (zlib-ng, needs cmake), `zstd`, `szip`,
+only when you ask: `fast-deflate` (zlib-ng, needs cmake), `zstd`, `szip` (a `clawhdf5-format` feature),
 `https` and the cloud stores (ring / aws-lc-rs), the BLAS backends,
 `clawhdf5-migrate` (bundled SQLite) and the Node.js bindings. One exception
 links rather than builds C: on macOS the default `system-zlib-decompress`