diff --git a/.github/workflows/test-snippets.yml b/.github/workflows/test-snippets.yml
index 3ae3ffb..873c456 100644
--- a/.github/workflows/test-snippets.yml
+++ b/.github/workflows/test-snippets.yml
@@ -8,6 +8,21 @@ on:
       - '*.mdx'
       - 'tests/**'
 
+# These tests create shared, named resources on one Eden AI account (custom
+# tokens, uploaded files), so two runs of the same ref must not overlap.
+# update-dates.yml pushes a dateModified bump to every .mdx PR, which starts a
+# second run seconds after the first, so serialise them per ref.
+#
+# Deliberately NOT cancel-in-progress: cleanup happens in pytest_sessionfinish,
+# so cancelling a started run orphans the tokens and uploaded file it created.
+# The next run then snapshots those as pre-existing and never deletes them,
+# leaking a little account state on every .mdx PR. Queuing costs one extra
+# minute of CI and always runs cleanup. (A still-pending run carries no
+# resources, so GitHub replacing it is harmless.)
+concurrency:
+  group: test-snippets-${{ github.ref }}
+  cancel-in-progress: false
+
 jobs:
   execute:
     name: Execution Tests
diff --git a/tests/conftest.py b/tests/conftest.py
index 6839791..6d9514f 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -79,8 +79,12 @@ def pytest_sessionstart(session: pytest.Session) -> None:
             try:
                 create_custom_token(**token_spec)
             except requests.HTTPError as exc:
-                # token may already exist
-                if not exc.response or exc.response.status_code != 400:
+                # Token may already exist — another run on the same account can
+                # create it first. Test `is None` explicitly: Response.__bool__
+                # returns self.ok, so a 400 response is falsy and `not
+                # exc.response` was short-circuiting to True, re-raising the one
+                # case this handler exists to swallow.
+                if exc.response is None or exc.response.status_code != 400:
                     raise
 
     path = _shared_state_path(session.config)