diff --git a/pyproject.toml b/pyproject.toml
index 1655b565e..489876f78 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -23,7 +23,7 @@ dependencies = [
   # core
   "cffi",
   "scons",
-  "pycapnp",
+  "pycapnp==2.1.0",
   "Cython",
   "setuptools",
   "numpy >=2.0",