return0

return0 is an MCP server that connects AI development environments like Cursor to live, running Node.js and Next.js applications in production. Built for software engineers, site reliability engineers, and backend developers, it enables real-time extraction of variable states directly from deployed runtime environments. Instead of relying purely on static log traces or synthetic local reproductions, users can query production state straight from their code editor. The server reads source files locally, applies TypeScript AST analysis to ensure line numbers accurately map to the targeted code, and connects to the active runtime to retrieve live variable values, types, and timestamps. This functionality is suited for debugging serverless and edge-deployed workloads, such as Next.js applications hosted on platforms like Vercel, where attaching traditional interactive debuggers is impractical. By surfacing precise execution context during an active AI chat session, return0 allows coding agents to identify logic errors and inspect application state without interrupting live traffic.

Category: Developer Tools & Code Intelligence

Tags: debugging, ide, nextjs, nodejs, remote-debugging

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How to install and configure return0

1. Obtain an API key by signing up on the return0 platform or use demo for basic testing. 2. Open Cursor IDE and navigate to Settings (or use Ctrl+, / Cmd+,). 3. In the search bar, search for "MCP" and select MCP Servers. 4. Click Add Server or configure your mcp.json file directly. 5. Add the following JSON configuration: json { "mcpServers": { "return0": { "command": "npx", "args": ["-y", "@return-0/mcp-server"], "env": { "RETURN0_API_KEY": "your-api-key-here" } } } } 6. Save the settings. Cursor will invoke @return-0/mcp-server automatically via npx.

What you can do with return0

• Inspect live variable values: Retrieve real-time values, variable types, and evaluation timestamps for specific variables in production Node.js services without attaching a heavy debugger. - Debug serverless Next.js routes: Troubleshoot unexpected behavior in Vercel or cloud-hosted Next.js endpoints by querying live variables directly through file paths and line numbers. - Batch state extraction across files: Request runtime values across multiple source files simultaneously to observe data transformations across distinct application layers in a single query. - Diagnose production logic errors: Feed verified production runtime state into Cursor chat to assist the AI model in diagnosing and patching subtle backend edge cases.

Key facts

• https://github.com/getreturn0/mcp
• Developer Tools & Code Intelligence, Monitoring & Observability
• debugging, ide, nextjs, nodejs, remote-debugging

Part of MCP Servers

How do I install return0?

To install return0 in Cursor IDE, open Settings, locate the MCP Servers configuration, and add a new server running npx with -y and @return-0/mcp-server. You must set the RETURN0_API_KEY environment variable to your key or demo.

What can return0 do?

return0 exposes a variable_extractor tool that analyzes local TypeScript files to verify line locations and fetches real-time variable values, types, and timestamps directly from running production Node.js and Next.js applications into your AI workflow.

Which MCP clients work with return0?

The official documentation and instructions specifically cover Cursor IDE. However, any client compatible with the Model Context Protocol that supports stdio process execution and environment variable configuration can run the @return-0/mcp-server package.

How does return0 locate variables in source code?

When an extraction request is made, the return0 MCP server reads your local source files and performs TypeScript AST analysis to verify and correct line numbers before querying the production runtime for exact values.