diff --git a/setup.py b/setup.py
index 63b38ba985..028983d0c1 100644
--- a/setup.py
+++ b/setup.py
@@ -35,7 +35,7 @@ def main():
                       'graph': ['pygraphviz'],
                       'plot': ['matplotlib'],
                       'isi': ['nltk<3.6', 'unidecode'],
-                      'api': ['flask>=3.0,4.0', 'flask_restx<0.4', 'flask_cors',
+                      'api': ['flask>=3.0,<4.0', 'flask_restx<0.4', 'flask_cors',
                               'docstring-parser', 'gunicorn'],
                       # scikit-learn 1.5.0 breaks DisambManager.run_adeft_disambiguation
                       # see: https://github.com/gyorilab/adeft/issues/80