diff --git a/pyproject.toml b/pyproject.toml
index 2d857dc6d..4d01a6fd6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -160,6 +160,7 @@ serialize = ["{major}.{minor}"]
 # sysmon, default on 3.14, is too slow: https://github.com/coveragepy/coveragepy/issues/2172
 core = "ctrace"
 branch = true
+parallel = true
 include = ["scrapy/*"]
 omit = ["tests/*"]
 disable_warnings = ["include-ignored"]
diff --git a/tests/sitecustomize.py b/tests/sitecustomize.py
new file mode 100644
index 000000000..25532bafc
--- /dev/null
+++ b/tests/sitecustomize.py
@@ -0,0 +1,2 @@
+import coverage
+coverage.process_startup()
diff --git a/tox.ini b/tox.ini
index c2d49fab8..fddb0d1cb 100644
--- a/tox.ini
+++ b/tox.ini
@@ -53,6 +53,8 @@ deps =
     {[test-requirements]deps}
     pytest >= 8.4.1  # https://github.com/pytest-dev/pytest/pull/13502
     deltatest-cli
+setenv =
+    COVERAGE_PROCESS_STARTUP = {toxinidir}/pyproject.toml
 passenv =
     PYTEST_ADDOPTS
     S3_TEST_FILE_URI
@@ -63,6 +65,7 @@ passenv =
     GCS_PROJECT_ID
     DELTA_*
     GITHUB_*
+    COVERAGE_PROCESS_STARTUP
 #allow tox virtualenv to upgrade pip/wheel/setuptools
 download = true
 commands =