diff --git a/.github/workflows/publish-to-pypi.yml b/.github/workflows/publish-to-pypi.yml
index 6b6e9be..21ec706 100644
--- a/.github/workflows/publish-to-pypi.yml
+++ b/.github/workflows/publish-to-pypi.yml
@@ -22,10 +22,16 @@ jobs:
 
     steps:
       - uses: actions/checkout@v3
+      # Every interpreter in the tox.ini envlist must exist or tox cannot create
+      # that environment. The *last* entry becomes the default `python`, so keep
+      # 3.11 last: `uv python pin 3.11` and `make build` below are unaffected.
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: "3.11"
+          python-version: |
+            3.9
+            3.10
+            3.11
 
       - name: Install pypa/build
         run: |
diff --git a/.github/workflows/unit-test.yml b/.github/workflows/unit-test.yml
index 5486837..c508531 100644
--- a/.github/workflows/unit-test.yml
+++ b/.github/workflows/unit-test.yml
@@ -11,11 +11,16 @@ jobs:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v3
 
-      # Set up python 3.9
+      # Set up every interpreter in the tox.ini envlist. The *last* entry becomes
+      # the default `python`, so keep 3.9 last: `make test` stays on the
+      # requires-python = ">=3.9" floor, and only tox reaches for 3.10/3.11.
       - name: Prepare python env
         uses: actions/setup-python@v4
         with:
-          python-version: "3.9"
+          python-version: |
+            3.11
+            3.10
+            3.9
 
       # Install uv
       - name: Install uv
@@ -24,3 +29,11 @@ jobs:
       # Pack and publish Python SDK
       - name: Exec py client unit test
         run: make test
+
+      # Exercise tox on every PR instead of only at release time.
+      # tox >= 4.31 requires Python >= 3.10, so install it with 3.11 to resolve
+      # the same tox release the publish workflow uses.
+      - name: Install tox
+        run: python3.11 -m pip install tox
+      - name: Run tox
+        run: python3.11 -m tox
diff --git a/tox.ini b/tox.ini
index 6f73678..2568d88 100644
--- a/tox.ini
+++ b/tox.ini
@@ -13,6 +13,12 @@
 [tox]
 envlist = py{39,310,311}
 
+# Fail loudly when an interpreter in envlist is missing instead of silently
+# skipping that environment. tox <= 4.30 skips by default, tox >= 4.31 fails,
+# so pin the behaviour explicitly to keep CI results identical regardless of
+# which tox version the runner resolves.
+skip_missing_interpreters = false
+
 # Define the minimal tox version required to run;
 # if the host tox is less than this the tool with create an environment and
 # provision it with a tox that satisfies it under provision_tox_env.