How to stream command output from a sandbox

On Runtime, a streamed command has no output limit. A long stream resumes automatically from the correct byte. When a stream runs past the server's time limit, the SDK resumes it from a cursor. No line is lost or printed twice. The first line of a build or a test run reaches your terminal almost immediately.

Print output as it arrives

Callbacks receive each piece of stdout and stderr as text. The result still holds everything the command printed:

TypeScriptimport { Sandbox } from "withruntime";await using sbx = await Sandbox.create();const result = await sbx.exec("for i in 1 2 3; do echo line $i; sleep 1; done", {  onStdout: (text) => process.stdout.write(text),  onStderr: (text) => process.stderr.write(text),});console.log("exit", result.exitCode, result.stdoutTruncated);

Pythonimport sysfrom withruntime import Sandboxwith Sandbox.create() as sbx:    result = sbx.exec("for i in 1 2 3; do echo line $i; sleep 1; done",                      on_stdout=sys.stdout.write, on_stderr=sys.stderr.write)    print("exit", result.exit_code, result.stdout_truncated)

From a terminal, runtime sandboxes exec streams output by default. It exits with the command's own exit code, or with 124 when the command times out. Scripts that use set -e behave as expected:

Terminalid=$(runtime sandboxes create)runtime sandboxes exec "${id}" -- python3 -c 'print(sum([125, 250, 375]))'runtime sandboxes stop "${id}"

Read the events one by one

To tell the streams apart or act on the exit, iterate over the events. Each event has a type: start, stdout, stderr or exit.

TypeScriptimport { Sandbox } from "withruntime";await using sbx = await Sandbox.create();for await (const event of sbx.execStream("npm --version")) {  if (event.type === "stdout") process.stdout.write(event.data);  if (event.type === "stderr") process.stderr.write(event.data);  if (event.type === "exit") console.log("exit", event.exitCode);}

Pythonfrom withruntime import Sandboxwith Sandbox.create() as sbx:    for event in sbx.exec_stream("npm --version"):        if event["type"] == "stdout":            print(event["data"], end="")        elif event["type"] == "exit":            print("exit", event["exitCode"])

Over plain HTTP, POST /v1/sandboxes/{id}:exec with "stream": true returns NDJSON, with one event per line. A continue event carries the cursor to resume from (commands and processes).

Output limits

Output has no charge of its own. The sandbox is billed for the CPU it uses, at $0.025 per active vCPU-hour, and for its memory, at $0.0075 per GiB-hour (pricing).

Mistakes to avoid

• Reading a big log from the plain result. Past 64 KiB, the rest is dropped and stdoutTruncated is true. Check the flag, add a callback, or write the output to a file (cmd > /workspace/out.log) and read it with sbx.files.readText.
• Treating a timeout as an exception. The result returns timedOut: true (timed_out in Python) with the output so far. To throw CommandError on a non-zero exit instead, add check: true.
• A reader that stops reading. A slow reader is fine. The command waits for it. The sandbox holds the latest 1 MiB of a command's output. If a consumer is away for more than 10 seconds while more output arrives, it loses the oldest part. It then gets a truncated event, and both truncation flags are set. Keep the loop reading, or write to a file.
• Streaming a server. exec waits for the command to end. A dev server or a watcher belongs in a background process. Follow its output separately.
• A secret in the command line. Pass it in env. It is never echoed back, and journals record a hash, not the value.

Related

• Run a background process and follow its logs after you disconnect.
• Open an interactive terminal for colors and prompts.
• Run untrusted LLM code, where an agent reads output as it is produced.
• The full exec reference in JavaScript and Python.

Facts on this page were checked on 26 October 2026.