diff --git a/.github/workflows/core_tests.yml b/.github/workflows/core_tests.yml
index 17b3cbc2a..c0d1a808b 100644
--- a/.github/workflows/core_tests.yml
+++ b/.github/workflows/core_tests.yml
@@ -435,7 +435,7 @@ jobs:
         if: steps.cache.outputs.cache-hit != 'true'
 
       - name: Install Larch
-        run: mamba install "larch>=5.7.1"
+        run: mamba install "larch>=5.7.1,<6"
 
       - name: Install activitysim
         # installing without dependencies is faster, we trust that all needed dependencies
diff --git a/conda-environments/docbuild.yml b/conda-environments/docbuild.yml
index 275709c4e..be592acd6 100644
--- a/conda-environments/docbuild.yml
+++ b/conda-environments/docbuild.yml
@@ -23,7 +23,7 @@ dependencies:
 - git
 - jupyter-book
 - jupyterlab
-- larch >=5.5.3
+- larch >=5.5.3,<6
 - matplotlib
 - multimethod <2.0
 - myst-nb