fix(downstream): remove CI env vars for tox instead of overriding

tox's is_ci() checks presence of CI (any value) and GITHUB_ACTIONS==true.
Setting CI=false still leaves it present, so is_ci() returns True.

Now empty-string env values mean "remove from environment" in the driver,
and tox recipe uses CI="" and GITHUB_ACTIONS="" to fully unset them.

Co-authored-by: Cursor AI <ai@cursor.sh>
Co-authored-by: Anthropic Claude Opus 4 <claude@anthropic.com>

Author: 3 people

downstream/README.md

@@ -22,8 +22,9 @@ sections plus tests:
 3. **`[[test]]`** — one or more `argv` arrays. For `uv-venv` / `stdlib-venv`, the
    driver sets **`VIRTUAL_ENV`** and prepends the venv’s `bin` (or `Scripts` on
    Windows) to **`PATH`**, so test commands can use bare names like `pytest`.
-   Optional **`env`** table sets extra environment variables for the step (e.g.
-   `env = { CI = "false" }` to suppress CI-only behaviour in a downstream suite).
+   Optional **`env`** table sets extra environment variables for the step;
+   an empty string removes the variable from the environment (e.g.
+   `env = { CI = "" }` to unset `CI` and suppress CI-only behaviour).
    Install only: `--only-install`.
 
 Run the driver (PEP 723 in `run_downstream.py`):

downstream/recipes/tox.toml

@@ -14,7 +14,8 @@ groups = ["test"]
 
 [[test]]
 argv = ["pytest"]
-# tox's is_ci() checks CI, GITHUB_ACTIONS, and many other env vars;
-# all must be unset/false so list_dependencies defaults to False and
-# test expectations (no freeze steps, spinner enabled, etc.) hold.
-env = { CI = "false", GITHUB_ACTIONS = "false" }
+# tox's is_ci() checks presence of CI, value of GITHUB_ACTIONS, and
+# other env vars.  Empty string = remove from environment, non-empty
+# overrides value.  Both must be gone for list_dependencies to default
+# to False and test expectations (no freeze steps, etc.) to hold.
+env = { CI = "", GITHUB_ACTIONS = "" }

downstream/run_downstream.py

@@ -161,6 +161,10 @@ def subprocess_env(
     venv_home: Path | None,
 ) -> dict[str, str]:
     env = {**os.environ, **dict(extra or {})}
+    # Empty-string values mean "remove from environment".
+    for key, val in list(env.items()):
+        if val == "":
+            del env[key]
     if venv_home is None:
         return env
     root = venv_home.resolve()