With Runtime in Cursor, the agent runs builds, tests and servers in a Firecracker microVM with its own kernel, not on your laptop. No API key goes into the config. The first tool call prints a link and a code. One browser approval connects it. A 2 vCPU, 4 GiB sandbox that is waiting on the model costs $0.03125 an hour. A new account gets 100 free hours of a 1 GB machine in its first 30 days, as of 25 October 2026 (pricing).
Option 1: the local bridge
Put this in ~/.cursor/mcp.json to use it in every project. To use it in one
project only, put it in .cursor/mcp.json at that project's root:
JSON{ "mcpServers": { "runtime": { "command": "npx", "args": ["-y", "withruntime", "mcp"] } }}The command is a bridge. It forwards MCP messages to Runtime's remote server over this machine's saved connection. The file holds no secret, so it is safe to commit in a project. The bridge needs Node 22.12 or later.
The first time:
- Cursor's agent sees one tool,
runtime_connect, and calls it. - It shows you a link and a code. Open the link, check that the code matches, and approve Connect agent.
- The agent calls the tool again, and every Runtime tool appears. No restart is needed.
A machine already connected with npx withruntime login skips this step.
Option 2: the remote server
Cursor also takes a URL. Runtime's endpoint speaks MCP Streamable HTTP and signs in through the browser:
JSON{ "mcpServers": { "runtime": { "url": "https://api.withruntime.com/mcp" } }}Cursor opens a Runtime page. Sign in and approve Connect for Cursor. The key it receives belongs to a new agent named after the app. The key is listed on API keys. Choose this option when you would rather not run Node locally.
Which one to choose
| You want | Use |
|---|---|
| One connection shared with the CLI and Claude Code on this machine | The bridge |
| A config file checked into a team repository | The bridge (no secret in it) |
| No local Node process | The remote server |
Ask for work
Enable the server from Customize in Cursor's sidebar. Then ask in the chat:
Start a Runtime sandbox, clone this repo into it over HTTPS, run the test suite and tell me which tests fail. Stop the sandbox afterwards.
By default, Cursor asks before it runs each MCP tool, and it shows the arguments. The tools that the agent uses most:
| Tool | Use in a coding session |
|---|---|
runtime_sandboxes_create |
A fresh machine per task; getOrCreate with a name reuses one |
runtime_sandboxes_exec |
Build and test; "background": true for a dev server |
runtime_sandboxes_process |
Follow a background server's output |
runtime_sandboxes_files_write, _read |
Move code and results in and out |
runtime_sandboxes_previews |
An HTTPS address to open the running app |
runtime_sandboxes_manage |
Pause between tasks, wake, stop |
runtime_sandboxes_fork |
Up to 100 running copies to try fixes side by side |
Mistakes and how Runtime handles them
- Putting a key in
mcp.json. A key is not needed, and a committed key leaks. The bridge uses the saved connection. For CI,RUNTIME_API_KEYfrom a secret manager overrides it. - An old Node version. The bridge needs Node 22.12 or later. Check with
node --version, or use the remote server, which needs no Node. - Approving a link you did not request. Match the code the agent showed
you. A connection can spend prepaid credit across the account until you
revoke it with
npx withruntime logoutor on the API keys page. - Unbounded agent spend. Set a daily spending limit on the connection's
row at API keys. Past the limit, requests fail with
spending_limit_reachedand charge nothing. - Tool names in rules from before 24 September 2026. Tools were renamed to
runtime_<product>_<verb>. Restart the agent so Cursor lists them again, and update any rule that names an old tool. - Sandboxes without credit and SSH clones. A sandbox without credit reaches ports 80 and 443 only. Tell the agent to clone over HTTPS.
Related
- How to add Runtime to Claude.ai
- Codex in a sandbox and Claude Code in a sandbox
- A coding agent sandbox
- Frameworks: coding agents
Sources
Checked 25 October 2026.
Facts on this page were checked on 25 October 2026.