diff --git a/pyproject.toml b/pyproject.toml
index 57b28518..58fc59cc 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -51,6 +51,11 @@ vision-agents-plugins-speechify = { workspace = true }
 # vogent-turn requires numpy<2.0, so we override to use 1.26.x
 override-dependencies = ["numpy>=1.26.0,<2.0"]
 
+# curated-tokenizers 0.0.9 (kokoro -> misaki[en] -> spacy-curated-transformers)
+# ships no cp313 wheels, so it is built from source. Its build requirement is an
+# unpinned `cython>=0.25` and Cython 3.3 crashes cythonizing `_bbpe.pyx`.
+build-constraint-dependencies = ["cython<3.3"]
+
 # Store uv cache in project directory for better sandboxing
 cache-dir = ".uv-cache"
 
diff --git a/uv.lock b/uv.lock
index ef447a1e..31874bb3 100644
--- a/uv.lock
+++ b/uv.lock
@@ -53,6 +53,7 @@ members = [
     "vision-agents-plugins-xai",
 ]
 overrides = [{ name = "numpy", specifier = ">=1.26.0,<2.0" }]
+build-constraints = [{ name = "cython", specifier = "<3.3" }]
 
 [manifest.dependency-groups]
 dev = [