diff --git a/.ci/build.sh b/.ci/build.sh
index ae10cb67155..c172e513f68 100755
--- a/.ci/build.sh
+++ b/.ci/build.sh
@@ -2,6 +2,5 @@
 
 set -e
 
-python3 -m coverage erase
 make clean
 make install-coverage
diff --git a/.ci/install.sh b/.ci/install.sh
index aeb5e65145d..30590449dda 100755
--- a/.ci/install.sh
+++ b/.ci/install.sh
@@ -24,14 +24,7 @@ sudo apt-get -qq install libfreetype6-dev liblcms2-dev libtiff-dev python3-tk\
 
 python3 -m pip install --upgrade pip
 python3 -m pip install --upgrade wheel
-python3 -m pip install coverage
-python3 -m pip install defusedxml
 python3 -m pip install ipython
-python3 -m pip install olefile
-python3 -m pip install -U pytest
-python3 -m pip install -U pytest-cov
-python3 -m pip install -U pytest-timeout
-python3 -m pip install pyroma
 # optional test dependencies, only install if there's a binary package.
 python3 -m pip install --only-binary=:all: numpy || true
 python3 -m pip install --only-binary=:all: pyarrow || true
diff --git a/.ci/test.sh b/.ci/test.sh
index 87a605d84be..fb03223db6d 100755
--- a/.ci/test.sh
+++ b/.ci/test.sh
@@ -4,4 +4,5 @@ set -e
 
 python3 -c "from PIL import Image"
 
-python3 -bb -m pytest -vv -x -W always --cov PIL --cov Tests --cov-report term --cov-report xml Tests $REVERSE
+python3 -bb -m pytest -vv -x -W always Tests -m "isolated" -n0
+python3 -bb -m pytest -vv -x -W always Tests -m "not isolated" --cov PIL --cov Tests --cov-report term --cov-report xml $REVERSE
diff --git a/.github/workflows/test-mingw.yml b/.github/workflows/test-mingw.yml
index e247414c8fc..baf0d1373fb 100644
--- a/.github/workflows/test-mingw.yml
+++ b/.github/workflows/test-mingw.yml
@@ -78,7 +78,7 @@ jobs:
           pushd depends && ./install_extra_test_images.sh && popd
 
       - name: Build Pillow
-        run: CFLAGS="-coverage" python3 -m pip install .
+        run: CFLAGS="-coverage" python3 -m pip install .[tests]
 
       - name: Test Pillow
         run: |
diff --git a/.github/workflows/test-valgrind-memory.yml b/.github/workflows/test-valgrind-memory.yml
index bd244aa5a57..66ecac4bda8 100644
--- a/.github/workflows/test-valgrind-memory.yml
+++ b/.github/workflows/test-valgrind-memory.yml
@@ -56,5 +56,10 @@ jobs:
       run: |
         # The Pillow user in the docker container is UID 1001
         sudo chown -R 1001 $GITHUB_WORKSPACE
-        docker run --name pillow_container -e "PILLOW_VALGRIND_TEST=true" -v $GITHUB_WORKSPACE:/Pillow pythonpillow/${{ matrix.docker }}:${{ matrix.dockerTag }} /Pillow/depends/docker-test-valgrind-memory.sh
+        docker run --name pillow_container \
+          -e "PILLOW_VALGRIND_TEST=true" \
+          -e "PYTEST_ADDOPTS=-n0" \
+          -v $GITHUB_WORKSPACE:/Pillow \
+          pythonpillow/${{ matrix.docker }}:${{ matrix.dockerTag }} \
+          bash -c "python3 -m pip install pytest-xdist && /Pillow/depends/docker-test-valgrind-memory.sh"
         sudo chown -R runner $GITHUB_WORKSPACE
diff --git a/.github/workflows/test-valgrind.yml b/.github/workflows/test-valgrind.yml
index 81cfb84566c..6fe4935e8db 100644
--- a/.github/workflows/test-valgrind.yml
+++ b/.github/workflows/test-valgrind.yml
@@ -54,5 +54,9 @@ jobs:
       run: |
         # The Pillow user in the docker container is UID 1001
         sudo chown -R 1001 $GITHUB_WORKSPACE
-        docker run --name pillow_container -e "PILLOW_VALGRIND_TEST=true" -v $GITHUB_WORKSPACE:/Pillow pythonpillow/${{ matrix.docker }}:${{ matrix.dockerTag }}
+        docker run --name pillow_container \
+          -e "PILLOW_VALGRIND_TEST=true" \
+          -e "PYTEST_ADDOPTS=-n0" \
+          -v $GITHUB_WORKSPACE:/Pillow \
+          pythonpillow/${{ matrix.docker }}:${{ matrix.dockerTag }}
         sudo chown -R runner $GITHUB_WORKSPACE
diff --git a/Makefile b/Makefile
index 6e050c715d2..fd106110899 100644
--- a/Makefile
+++ b/Makefile
@@ -62,7 +62,7 @@ install:
 
 .PHONY: install-coverage
 install-coverage:
-	CFLAGS="-coverage -Werror=implicit-function-declaration" python3 -m pip -v install .
+	CFLAGS="-coverage -Werror=implicit-function-declaration" python3 -m pip -v install .[tests]
 	python3 selftest.py
 
 .PHONY: debug
@@ -97,12 +97,6 @@ test:
 	python3 -c "import pytest" > /dev/null 2>&1 || python3 -m pip install pytest
 	python3 -m pytest -qq
 
-.PHONY: test-p
-test-p:
-	python3 -c "import xdist" > /dev/null 2>&1 || python3 -m pip install pytest-xdist
-	python3 -m pytest -qq -n auto
-
-
 .PHONY: valgrind
 valgrind:
 	python3 -c "import pytest_valgrind" > /dev/null 2>&1 || python3 -m pip install pytest-valgrind
diff --git a/Tests/helper.py b/Tests/helper.py
index d77b4b807ec..45058a1cc26 100644
--- a/Tests/helper.py
+++ b/Tests/helper.py
@@ -216,6 +216,7 @@ def mark_if_feature_version(
     return pytest.mark.pil_noop_mark()
 
 
+@pytest.mark.isolated
 @pytest.mark.skipif(sys.platform.startswith("win32"), reason="Requires Unix or macOS")
 class PillowLeakTestCase:
     # requires unix/macOS
diff --git a/pyproject.toml b/pyproject.toml
index 91f4750e46a..93c731b7440 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -73,6 +73,7 @@ optional-dependencies.tests = [
   "pyroma>=5",
   "pytest",
   "pytest-cov",
+  "pytest-sugar",
   "pytest-timeout",
   "pytest-xdist",
   "trove-classifiers>=2024.10.12",
@@ -208,7 +209,10 @@ lint.isort.required-imports = [
 max_supported_python = "3.14"
 
 [tool.pytest.ini_options]
-addopts = "-ra --color=auto"
+addopts = "-ra --color=auto --numprocesses=auto --dist=loadscope"
+markers = [
+  "isolated: tests that are not compatible with pytest-xdist (deselect with '-m \"not isolated\"')",
+]
 testpaths = [
   "Tests",
 ]