diff --git a/.github/workflows/python-package.yml b/.github/workflows/python-package.yml
index 6d056faa..fd4cbf0b 100644
--- a/.github/workflows/python-package.yml
+++ b/.github/workflows/python-package.yml
@@ -27,9 +27,7 @@ jobs:
         enable-cache: true
         activate-environment: true
     - name: Lint with Ruff
-      run: |
-        uv tool install ruff
-        uv run ruff check --output-format=github .
+      run: uvx ruff check --output-format=github .
       continue-on-error: false
 
     - name: Install dependencies
diff --git a/pyproject.toml b/pyproject.toml
index 0e815b3d..ba5e18fe 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -87,6 +87,15 @@ target-version = "py39"
 #  "UP",
 #  "W",
 #]
+# Pin the rule set explicitly - ruff 0.16 widened its defaults from 2 to 38 rule
+# prefixes, so relying on the defaults makes lint results depend on the ruff version.
+select = [
+  "E4",
+  "E7",
+  "E9",
+  "F",
+  "I",
+]
 ignore = [
 "PLR0915",
 "PLR0912",
diff --git a/src/pyff/builtins.py b/src/pyff/builtins.py
index 9c560956..a3a69ae2 100644
--- a/src/pyff/builtins.py
+++ b/src/pyff/builtins.py
@@ -551,6 +551,11 @@ def _nop(x):
         resource_name = m.group(2)
     out = output_file
     data = req.t
+
+    # clean unused namespaces - the working document isn't always XML (eg discojson* produce JSON)
+    if isinstance(data, (etree._Element, etree._ElementTree)):
+        etree.cleanup_namespaces(data)
+
     if not req.args.get('raw'):
         data = dumptree(req.t, pretty_print=req.args.get('pretty_print'))