diff --git a/devenv.nix b/devenv.nix
index 3afdbc5..bc2e2d5 100644
--- a/devenv.nix
+++ b/devenv.nix
@@ -15,10 +15,13 @@ let
     pkg-config
     protobuf
     cargo-cross
-    # opusic-sys (via symphonia-adapter-libopus) compiles libopus from
-    # source with CMake; ninja is picked up automatically if present.
+    # opusic-sys (via symphonia-adapter-libopus) compiles libopus from source
+    # with CMake, using the gnumake generator from stdenv. Do NOT add ninja:
+    # its mere presence flips cmake's generator (Make -> Ninja), which then
+    # conflicts with any build dir already cached under the other generator
+    # ("Does not match the generator used previously"). Keeping only cmake
+    # makes the generator deterministic.
     cmake
-    ninja
     # Stream-URL sidecar for the ytdy provider: YouTube caps tokenless
     # stream URLs at ~1 MiB and yt-dlp is the only maintained cipher
     # solver (architecture/youtube-rustypipe.md, D2-revised).