TOX_OVERRIDE for pass_env shows in config but doesn't actually pass env vars

[gaborbernat]

Setting TOX_OVERRIDE="testenv.pass_env+=NIX_LD,NIX_LD_LIBRARY_PATH" makes tox c show the variables in pass_env, but they are not actually passed through to the tox environment at runtime.

Setting it directly in tox.ini works fine. The override mechanism appears to update the config view but not the actual behavior.

Ref: #3225

[gaborbernat]

I investigated this issue and can confirm that TOX_OVERRIDE works correctly in tox 4.50.0:

Test with INI config (tox.ini):

[testenv]
package = skip
commands = python -c "import os; print('CUSTOM_VAR:', os.environ.get('CUSTOM_VAR', 'NOT_SET'))"

CUSTOM_VAR="test_value" TOX_OVERRIDE="testenv.pass_env+=CUSTOM_VAR" tox r

Result: ✅ Works correctly

• tox c -k pass_env shows CUSTOM_VAR in the list
• Runtime output shows CUSTOM_VAR: test_value

Additional notes:

1. NIX_LD variables are now in defaults: As of tox 4.24.0 (November 2024), NIX_LD and NIX_LD_LIBRARY_PATH are included in the default pass_env list, so they pass through automatically without needing any override.

2. Key aliases work: The fix in 🐛 fix(config): resolve overrides with alias config keys #3736 (February 2026) ensures pass_env and passenv are treated as aliases when resolving overrides.

3. Config format matters: If you're using TOML config (tox.toml/pyproject.toml), you need TOML-style section syntax like tool.tox.env_run_base.pass_env+=VAR, not testenv.pass_env+=VAR. This is expected behavior.

Could you verify which tox version and config format you're using? If you're still experiencing this with tox 4.50.0, please provide a minimal reproducible example.