diff --git a/.cargo/config.toml b/.cargo/config.toml
new file mode 100644
index 00000000..39ae6505
--- /dev/null
+++ b/.cargo/config.toml
@@ -0,0 +1,6 @@
+[env]
+# Rayon thread pool overhead exceeds benefit for small test fixtures (1x1, 2x2 grids).
+# This sets RAYON_NUM_THREADS=1 for all cargo commands in this workspace, making rayon
+# fall back to sequential execution. The compiled simulation binary (GDExtension, WASM)
+# is not affected — it reads RAYON_NUM_THREADS at runtime, not from cargo config.
+RAYON_NUM_THREADS = "1"