diff --git a/software/pyproject.toml b/software/pyproject.toml
index ecb3d4536..4736df63b 100644
--- a/software/pyproject.toml
+++ b/software/pyproject.toml
@@ -62,7 +62,7 @@ builtin-toolchain = [
   # the lowest that has features transitively required by Glasgow.
   "yowasp-runtime>=1.42",
   # Most versions of Yosys and nextpnr work; the minimum versions listed here are known good.
-  "yowasp-yosys>=0.31.0.13",
+  "yowasp-yosys>=0.31.0.13,<0.48",
   "yowasp-nextpnr-ice40>=0.1",
 ]