diff --git a/.github/workflows/nix.yml b/.github/workflows/nix.yml
new file mode 100644
index 000000000..d992f7433
--- /dev/null
+++ b/.github/workflows/nix.yml
@@ -0,0 +1,28 @@
+# Evaluates the Nix flake (no build, about a minute): `nix build` / `nix develop` stop at evaluation
+# when Cargo.lock gains something nix/package.nix can't vendor, and nothing else would notice.
+name: Nix
+
+on:
+  push:
+    branches: [main]
+    paths: ["flake.nix", "flake.lock", "nix/**", "Cargo.lock", ".github/workflows/nix.yml"]
+  pull_request:
+    paths: ["flake.nix", "flake.lock", "nix/**", "Cargo.lock", ".github/workflows/nix.yml"]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: nix-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  eval:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: cachix/install-nix-action@13d8dd58da0234aa297dedd986986ccb8e7f3e24 # v31.11.1
+      - name: nix flake check --no-build
+        run: nix flake check --no-build --all-systems
diff --git a/AGENTS.md b/AGENTS.md
index 8cd199ff6..297682755 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -131,8 +131,8 @@ of it. Full standard: `../craftrules/standards/never-crash.md`
 → `ui-egui`, `mcp` (L5) → apps `lightcraft` (desktop), `lightcraft-cli` (render/commands/MCP). `scenes` generates demo
 photos. `xtask` = tooling (`ci`, `layers`, `assets`, `parity`, `wasm`, `corpus`, `stats`). `flake.nix` +
 `nix/package.nix` = the Nix package (`nix build` builds both binaries with the craft-fonts input, installs the
-desktop file/icons/AppStream metadata and runs `cargo test --workspace`; `nix develop` = dev shell). Community-maintained and not
-in CI: it may lag behind the workspace; see README → Quick start.
+desktop file/icons/AppStream metadata and runs `cargo test --workspace`; `nix develop` = dev shell). Community-maintained; CI only
+evaluates it (`.github/workflows/nix.yml`), so the build may lag behind the workspace; see README → Quick start.
 
 ## Contributor credits (About window)
 
diff --git a/CLAUDE.md b/CLAUDE.md
index 4a736f12a..6099b1d2f 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -138,5 +138,5 @@ of it. Full standard: `../craftrules/standards/never-crash.md`
 → `ui-egui`, `mcp` (L5) → apps `lightcraft` (desktop), `lightcraft-cli` (render/commands/MCP). `scenes` generates demo
 photos. `xtask` = tooling (`ci`, `layers`, `assets`, `parity`, `wasm`, `corpus`, `stats`). `flake.nix` +
 `nix/package.nix` = the Nix package (`nix build` builds both binaries with the craft-fonts input, installs the
-desktop file/icons/AppStream metadata and runs `cargo test --workspace`; `nix develop` = dev shell). Community-maintained and not
-in CI: it may lag behind the workspace; see README → Quick start.
+desktop file/icons/AppStream metadata and runs `cargo test --workspace`; `nix develop` = dev shell). Community-maintained; CI only
+evaluates it (`.github/workflows/nix.yml`), so the build may lag behind the workspace; see README → Quick start.