Vercel Sandbox commands now support stdin

Vercel Sandbox now lets you send input to running commands through standard input (stdin). Use it to exchange messages with stdio MCP servers such as agent-browser, or send data to other command-line programs while they run.

In the TypeScript SDK, set stdin: true and detached: true in runCommand(). Use writeStdin() to send input and closeStdin() when you’re done:

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import { Sandbox } from '@vercel/sandbox';
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const sandbox = await Sandbox.create();
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const cmd = await sandbox.runCommand({
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  cmd: 'cat',
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  stdin: true,
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  detached: true,
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});
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await cmd.writeStdin('hello');
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await cmd.closeStdin();
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const result = await cmd.wait();
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console.log(await result.stdout()); // "hello"

You can also set stdin to a Node.js readable stream, such as process.stdin. The SDK forwards the stream’s data and closes the command’s standard input when the stream ends.

In the Python SDK, set stdin=subprocess.PIPE and send input with server.stdin.send(). Call communicate() to close stdin, collect remaining output, and wait for the process to exit:

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import subprocess
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from vercel import sandbox
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from vercel.api import session
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async with session():
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    async with sandbox.create_sandbox() as instance:
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        server = await instance.create_process("cat", stdin=subprocess.PIPE)
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        await server.stdin.send(b"ping\n")
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        print(await server.stdout.readline())
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        print(await server.communicate())

For commands that receive all their input at once, you can pass input directly to Python’s run_process().

Read the Sandbox documentation to get started.