variable_extractorLABSCO SUMMARY
Aimed squarely at the bug that only happens in production.
The pitch is narrow and honest: one tool, one job — pull the real value of a variable out of a deployed app, which is exactly the thing traditional debugging cannot reach on a serverless deployment. Because it resolves line numbers through AST analysis rather than trusting the number you typed, a stale line reference degrades into the right variable rather than the wrong one.
What it is
The MCP server for return0, a service for debugging production Node.js applications in real time. It connects the editor to the running application, so a question about what a variable actually holds in production is answered from the live process rather than from logs or a local reproduction.
What you get
- The runtime value, type and timestamp of named variables pulled from a running production application, requested as a list of files each carrying the variable names and their line numbers —
variable_extractor - Line numbers corrected before the lookup: the server reads the source file and uses TypeScript AST analysis to locate the variable accurately even when the line has moved
Requirements
A return0 API key in RETURN0_API_KEY; the documentation at getreturn0.com/docs covers how to obtain one, and demo works for testing. Node.js, since the server is launched with npx -y @return-0/mcp-server. The package is @return-0/mcp-server (1.0.2) and installs on first launch. It reads your source files from the local filesystem, so it expects to run on the machine holding the code.
Setup effort
One command plus a key — npx -y @return-0/mcp-server, then supply credentials
USE CASES
01
Explaining a bug that only reproduces in production
Name the file and the line, and read back what the variable actually contains on the deployed instance instead of inferring it from log output.
02
Checking what a request handler really received
Pull the parsed payload variable from a live API route to see the shape production is sending, not the shape the types claim.
03
Watching several variables across one code path at once
Request multiple files and variables in a single call and compare their values and timestamps side by side.
1 tool
Listed in project docs. Your client may show a different list.
BEFORE YOU INSTALL
What to know first
Tools you get
1 tool named in the project's documentation.
Requires
- npx on PATH
- RETURN0_API_KEY in your environment
Where it runs
The documented client runs the server on your machine.
INSTALLATION
Connect your client
Server setup
TERMINAL
npx -y @return-0/mcp-serverMCP Clients
Cursor
CURSOR · JSON
{
"mcpServers": {
"return0": {
"command": "npx",
"args": [
"-y",
"@return-0/mcp-server"
],
"env": {
"RETURN0_API_KEY": "<YOUR_RETURN0_API_KEY>"
}
}
}
}
