VS Code magic variables don't work in MCP configuration

[dsent]

App Version

3.20.3

API Provider

Not Applicable / Other

Model Used

N/A

🔁 Steps to Reproduce

1. Add mcp-server-git to MCPs
2. Use ${workspaceFolder} in the list of args:
   

💥 Outcome Summary

Expected the current workspace folder to be passed as an argument to the MCP.
Got "Invalid MCP settings JSON format. Please check your settings file for syntax errors." and disappearance of the MCP from the list of installed MCPs.
If I use absolute path, everything works:

Other variables, such as ${fileWorkspaceFolder} don't work too, but the observed results vary (e.g. some of them don't make the MCP server disappear, but result in an error -32000.

Putting the setting into project or global MCP list makes no difference.
Relevant: #4017, #3970

📄 Relevant Logs or Errors (Optional)

I'm on Windows 11 24H2.

[dosubot]

Magic variable support for MCPs (like ${workspaceFolder}) was added in Roo Code 3.19.1, so it should work in 3.20.3. However, the docs don't mention any known issues or special syntax requirements for using these variables in MCP configs, and I couldn't find details about issues #4017 or #3970 in the documentation.

The "Invalid MCP settings JSON format" error usually means there's a syntax problem in your config file, not necessarily with the variable itself—sometimes unescaped characters or misplaced quotes can trigger this, especially if the variable expands to a path with backslashes on Windows. The docs also confirm that Roo Code won't modify a corrupted config file until the syntax is fixed, so double-check for any subtle JSON formatting issues, especially around how the variable is used or quoted in your args list docs.

If using an absolute path works but the variable doesn't, it's possible the variable isn't being expanded as expected, or the expansion is producing invalid JSON. As a workaround, stick with absolute paths for now, and if you want to keep experimenting, try using forward slashes in your paths or wrapping the variable in double quotes if it's not already.

If you have a minimal example of your config, I can help check for subtle syntax issues.

To reply, just mention @dosu.

---

^{How did I do? Good | Irrelevant | Incorrect | Verbose | Hallucination | Report 🐛 | Other}   

[daniel-lxs]

The issue occurs because VSCode magic variables like ${workspaceFolder} are not being properly handled when they contain Windows paths with backslashes. The variable injection process in https://github.com/RooCodeInc/Roo-Code/blob/main/src/utils/config.ts#L35-L59 converts the MCP configuration to JSON string, performs variable substitution, then parses it back. When ${workspaceFolder} is replaced with a Windows path containing unescaped backslashes (e.g., C:\Users\project), the resulting JSON becomes invalid, triggering the "Invalid MCP settings JSON format" error at https://github.com/RooCodeInc/Roo-Code/blob/main/src/services/mcp/McpHub.ts#L283-L289.

The root cause is that the injectVariables function performs direct string replacement without properly escaping special characters for JSON. When it replaces ${workspaceFolder} with a Windows path, the backslashes need to be escaped to maintain valid JSON syntax. The fix would involve escaping the replacement values before substituting them into the JSON string, particularly handling backslashes and other JSON special characters.