diff --git a/pyproject.toml b/pyproject.toml
index 433b44fced..6193e6b264 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -125,6 +125,7 @@ override-dependencies = [
     "cryptography>=43.0.0,<47",
     "nvidia-modelopt>=0.42.0a0",
     "urllib3>=2.6.3",    # To address CVE-2026-21441
+    "onnx>=1.21.0; python_version < '3.13'",  # onnx<1.21 has missing lower bound on ml-dtypes, uses float4_e2m1fn (requires ml-dtypes>=0.5.0)
 ]
 
 [[tool.uv.index]]
diff --git a/uv.lock b/uv.lock
index dc560a59ce..4c67663691 100644
--- a/uv.lock
+++ b/uv.lock
@@ -24,6 +24,7 @@ overrides = [
     { name = "transformer-engine", git = "https://github.com/NVIDIA/TransformerEngine.git?rev=71bbefbf153418f943640df0f7373625dc93fa46" },
     { name = "triton", marker = "sys_platform == 'never'" },
     { name = "urllib3", specifier = ">=2.6.3" },
+    { name = "onnx", marker = "python_version < '3.13'", specifier = ">=1.21.0" },
 ]
 
 [[package]]