diff --git a/docs/quality.md b/docs/quality.md
index a5dc82a..a2efaee 100644
--- a/docs/quality.md
+++ b/docs/quality.md
@@ -190,6 +190,10 @@ cargo install knots --version 1.16.0 --locked --root .work/tools
 KNOTS_BIN=.work/tools/bin/knots scripts/crap-report.sh
 ```
 
+`cargo-llvm-cov` also needs the `llvm-tools-preview` toolchain component. The
+`nix develop` shell includes it. With rustup, run
+`rustup component add llvm-tools-preview` once. CI does not install it.
+
 The script builds an instrumented binary and runs the whole test suite, which
 takes several minutes, and writes `crap.md` and `crap.json` below
 `target/crap/`. CI does not run it. Add meaningful tests or simplify risky code. Never game the score
diff --git a/flake.nix b/flake.nix
index cd32a22..2320e08 100644
--- a/flake.nix
+++ b/flake.nix
@@ -19,7 +19,12 @@
             inherit system;
             overlays = [ rust-overlay.overlays.default ];
           };
-          rust = pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
+          # The pinned toolchain, plus llvm-tools for `scripts/crap-report.sh`. CI and
+          # rustup users install only the components in rust-toolchain.toml.
+          toolchain = (builtins.fromTOML (builtins.readFile ./rust-toolchain.toml)).toolchain;
+          rust = pkgs.rust-bin.fromRustupToolchain (toolchain // {
+            components = toolchain.components ++ [ "llvm-tools-preview" ];
+          });
         in {
           default = pkgs.mkShell {
             packages = [