I was curious if there was a way to configure Github copilot to NOT have access to certain files in a project and found that - it’s not necessarily straightforward. It turns out that there are no dotfiles you can set up or a setting to explicitly tell copilot not to have access to certain files in your project, which seems problematic from a security perspective. However, there is a way to “disable” Github copilot by setting the language mode to “plaintext” in VSCode.

Configuring Language Mode

There are three ways to configure the language mode.

File in focus (shortcuts)

With the env var file open and in focus, do the following

Open the Command Palette by pressing Cmd+Shift+P.
Type Change Language Mode and select it.
In the language mode dropdown, type plaintext and select it.

Open the Command Pallete

Change Language Model

Verify Plaintext is selected

File in focus (gui)

With the env var file open and in focus, do the following

At the bottom right of the status bar, click on the language mode selector.
Select Configure File Association for ‘.env’
Type plaintext and select it.

Click on Language Mode Selector at the bottom right

Select Configure File Association for '.env'

Type plaintext and select it

User Settings (JSON)

Open the Command Palette by pressing Cmd+Shift+P.
Type open user settings and select it.
With the settings.json file open, add the following:

"files.associations": {
    ".env": "plaintext"
}

Open Command Palette and type "open user settings"

User Settings (UI)

Open the Command Palette by pressing Cmd+Shift+,.
Search associations
Add .env as an item and set it to plaintext

Adding .env to settings in UI

Bonus: Wildcard env var files

Setting .env as the file association to your settings will explicitely tell Github Copilot to disable itself for the .env file, however it will NOT disable itself for other file like .env.local or .env.development. So it’s a best practice to add a wildcard AFTER the v like so:

"files.associations": {
    ".env*": "plaintext"
}

Conclusion

There is no way to explicitly tell Github Copilot to ignore certain files. That is, unless, you have a Github Copilot Business account - which then affords you this ability. With the above technique, there are some considerations per Github’s documentation:

In Copilot Chat in Visual Studio Code and Visual Studio, content exclusions are not applied when you use the @github chat participant in your question.

It's possible that Copilot may use semantic information from an excluded file if the information is provided by the IDE in a non-excluded file. Examples of such content include type information and hover-over definitions for symbols used in code.

Excluding content from Github Copilot - Limitations of content exclusions