diff --git a/CLAUDE.md b/CLAUDE.md
index da03542..ef17e0d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -13,7 +13,7 @@ uv sync                          # .venv with the package (editable) + dev group
 uv run pytest                    # full suite (testpaths = tests)
 uv run pytest tests/cfg/test_cfg.py::TestConfig_e2e::test_uses_spec_as_defaults   # single test
 uv run pytest -k "<keyword>"
-uv run --python 3.8 pytest       # any supported Python; uv.lock forks per version
+uv run --isolated --python 3.8 pytest   # any supported Python, without switching .venv
 
 uv run ruff check --fix          # lint + import sorting (config in pyproject.toml)
 uv run ruff format               # formatter, line length 88
diff --git a/README.md b/README.md
index 68a2d40..3d43b6a 100644
--- a/README.md
+++ b/README.md
@@ -86,7 +86,7 @@ or
 uv run pytest                                   # full suite
 uv run pytest tests/cfg/test_cfg.py::TestConfig_e2e::test_uses_spec_as_defaults
 uv run pytest -k "<keyword>"
-uv run --python 3.8 pytest                      # against another supported Python
+uv run --isolated --python 3.8 pytest           # another supported Python; --isolated keeps .venv as is
 ```
 
 # Development
diff --git a/docs/superpowers/plans/2026-10-04-modernization-hotfix-track.md b/docs/superpowers/plans/2026-10-04-modernization-hotfix-track.md
index d904814..8e4a1c5 100644
--- a/docs/superpowers/plans/2026-10-04-modernization-hotfix-track.md
+++ b/docs/superpowers/plans/2026-10-04-modernization-hotfix-track.md
@@ -305,7 +305,7 @@ Expected: `Resolved N packages`, and the sync installs into `.venv/`. The lock f
 Run:
 ```bash
 uv run pytest -q
-uv run --python 3.8 pytest -q
+uv run --isolated --python 3.8 pytest -q
 uv run ruff check && uv run ruff format --check
 git status --short
 ```
@@ -383,7 +383,7 @@ Replace everything from `# Tests` up to (not including) `# License` with:
 uv run pytest                                   # full suite
 uv run pytest tests/cfg/test_cfg.py::TestConfig_e2e::test_uses_spec_as_defaults
 uv run pytest -k "<keyword>"
-uv run --python 3.8 pytest                      # against another supported Python
+uv run --isolated --python 3.8 pytest                      # against another supported Python
 ```
 
 # Development
@@ -469,7 +469,7 @@ uv sync                          # .venv with the package (editable) + dev group
 uv run pytest                    # full suite (testpaths = tests)
 uv run pytest tests/cfg/test_cfg.py::TestConfig_e2e::test_uses_spec_as_defaults   # single test
 uv run pytest -k "<keyword>"
-uv run --python 3.8 pytest       # any supported Python; uv.lock forks per version
+uv run --isolated --python 3.8 pytest       # any supported Python; uv.lock forks per version
 
 uv run ruff check --fix          # lint + import sorting (config in pyproject.toml)
 uv run ruff format               # formatter, line length 88
@@ -886,13 +886,13 @@ Run again. Expected: all pass, 1 skipped.
 
 ```bash
 uv run ruff check && uv run ruff format --check
-uv run --python 3.8 pytest -q
-uv run --python 3.9 pytest -q
-uv run --python 3.10 pytest -q
-uv run --python 3.11 pytest -q
-uv run --python 3.12 pytest -q
-uv run --python 3.13 pytest -q
-uv run --python 3.14 pytest -q
+uv run --isolated --python 3.8 pytest -q
+uv run --isolated --python 3.9 pytest -q
+uv run --isolated --python 3.10 pytest -q
+uv run --isolated --python 3.11 pytest -q
+uv run --isolated --python 3.12 pytest -q
+uv run --isolated --python 3.13 pytest -q
+uv run --isolated --python 3.14 pytest -q
 ```
 Expected: lint clean, and every lane `N passed, 1 skipped`. Also run Task 4 step 5 with `--python 3.14`, which repeats the smoke step on the newest lane.