diff --git a/backend/python/vllm/requirements-install.txt b/backend/python/vllm/requirements-install.txt
index 69d263f0b..df897ae3a 100644
--- a/backend/python/vllm/requirements-install.txt
+++ b/backend/python/vllm/requirements-install.txt
@@ -1,6 +1,11 @@
 # mabma does not specify it's build dependencies per PEP517, so we need to disable build isolation
 # this also means that we need to install the basic build dependencies into the venv ourselves
 # https://github.com/Dao-AILab/causal-conv1d/issues/24
+#
+# pybind11 is needed to build fastsafetensors==0.3 (transitive dep of vllm) — its
+# setup.py imports pybind11 unconditionally but doesn't declare it in
+# build-system.requires, so under --no-build-isolation we have to seed it ourselves.
 packaging
 setuptools
-wheel
\ No newline at end of file
+wheel
+pybind11
\ No newline at end of file