Stream output

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OverviewWhen to use streamingStream command executionStream to clientStream process logsHandle errorsBest practicesRelated resources

This guide shows you how to handle real-time output from commands, processes, and code execution.

Coming soon: Sandbox SDK 1.0

This page documents streaming helpers on today's stable @cloudflare/sandbox package.

In the 1.0 preview (@next), stream with process handle methods such as logs() after exec(argv). Refer to Process execution or the Processes API.

When to use streaming

Use streaming when you need:

Real-time feedback - Show progress as it happens
Long-running operations - Builds, tests, installations that take time
Interactive applications - Chat bots, code execution, live demos
Large output - Process output incrementally instead of all at once
User experience - Prevent users from waiting with no feedback

Use non-streaming (exec()) for:

Quick operations - Commands that complete in seconds
Small output - When output fits easily in memory
Post-processing - When you need complete output before processing

Stream command execution

Use execStream() to get real-time output:

import { getSandbox, parseSSEStream } from "@cloudflare/sandbox";

const sandbox = getSandbox(env.Sandbox, "my-sandbox");

const stream = await sandbox.execStream("npm run build");

for await (const event of parseSSEStream(stream)) {
	switch (event.type) {
		case "stdout":
			console.log(event.data);
			break;

		case "stderr":
			console.error(event.data);
			break;

		case "complete":
			console.log("Exit code:", event.exitCode);
			break;

		case "error":
			console.error("Failed:", event.error);
			break;
	}
}

import { getSandbox, parseSSEStream, type ExecEvent } from '@cloudflare/sandbox';

const sandbox = getSandbox(env.Sandbox, 'my-sandbox');

const stream = await sandbox.execStream('npm run build');

for await (const event of parseSSEStream<ExecEvent>(stream)) {
  switch (event.type) {
    case 'stdout':
      console.log(event.data);
      break;

    case 'stderr':
      console.error(event.data);
      break;

    case 'complete':
      console.log('Exit code:', event.exitCode);
      break;

    case 'error':
      console.error('Failed:', event.error);
      break;
  }
}

Stream to client

Return streaming output to users via Server-Sent Events:

import { getSandbox } from "@cloudflare/sandbox";

export { Sandbox } from "@cloudflare/sandbox";

export default {
	async fetch(request, env) {
		const sandbox = getSandbox(env.Sandbox, "builder");

		const stream = await sandbox.execStream("npm run build");

		return new Response(stream, {
			headers: {
				"Content-Type": "text/event-stream",
				"Cache-Control": "no-cache",
			},
		});
	},
};

import { getSandbox } from '@cloudflare/sandbox';

export { Sandbox } from '@cloudflare/sandbox';

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const sandbox = getSandbox(env.Sandbox, 'builder');

    const stream = await sandbox.execStream('npm run build');

    return new Response(stream, {
      headers: {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache'
      }
    });
  }
};

Client-side consumption:

// Browser JavaScript
const eventSource = new EventSource("/build");

eventSource.addEventListener("stdout", (event) => {
	const data = JSON.parse(event.data);
	console.log(data.data);
});

eventSource.addEventListener("complete", (event) => {
	const data = JSON.parse(event.data);
	console.log("Exit code:", data.exitCode);
	eventSource.close();
});

// Browser JavaScript
const eventSource = new EventSource('/build');

eventSource.addEventListener('stdout', (event) => {
  const data = JSON.parse(event.data);
  console.log(data.data);
});

eventSource.addEventListener('complete', (event) => {
  const data = JSON.parse(event.data);
  console.log('Exit code:', data.exitCode);
  eventSource.close();
});

Stream process logs

Monitor background process output:

import { parseSSEStream } from "@cloudflare/sandbox";

const process = await sandbox.startProcess("node server.js");

const logStream = await sandbox.streamProcessLogs(process.id);

for await (const log of parseSSEStream(logStream)) {
	console.log(log.data);

	if (log.data.includes("Server listening")) {
		console.log("Server is ready");
		break;
	}
}

import { parseSSEStream, type LogEvent } from '@cloudflare/sandbox';

const process = await sandbox.startProcess('node server.js');

const logStream = await sandbox.streamProcessLogs(process.id);

for await (const log of parseSSEStream<LogEvent>(logStream)) {
  console.log(log.data);

  if (log.data.includes('Server listening')) {
    console.log('Server is ready');
    break;
  }
}

Handle errors

Check exit codes and handle stream errors:

const stream = await sandbox.execStream("npm run build");

for await (const event of parseSSEStream(stream)) {
	switch (event.type) {
		case "stdout":
			console.log(event.data);
			break;

		case "error":
			throw new Error(`Build failed: ${event.error}`);

		case "complete":
			if (event.exitCode !== 0) {
				throw new Error(`Build failed with exit code ${event.exitCode}`);
			}
			break;
	}
}

const stream = await sandbox.execStream('npm run build');

for await (const event of parseSSEStream<ExecEvent>(stream)) {
  switch (event.type) {
    case 'stdout':
      console.log(event.data);
      break;

    case 'error':
      throw new Error(`Build failed: ${event.error}`);

    case 'complete':
      if (event.exitCode !== 0) {
        throw new Error(`Build failed with exit code ${event.exitCode}`);
      }
      break;
  }
}

Best practices

Always consume streams - Don't let streams hang unconsumed
Handle all event types - Process stdout, stderr, complete, and error events
Check exit codes - Non-zero exit codes indicate failure
Provide feedback - Show progress to users for long operations

Commands API reference - Complete streaming API
Execute commands guide - Command execution patterns
Background processes guide - Process log streaming
Code Interpreter guide - Stream code execution output

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