Cloudflare transport doesn't consume fetch response bodies

[igoforth]

Is there an existing issue for this?

• I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
• I have reviewed the documentation https://docs.sentry.io/
• I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases

How do you use Sentry?

Sentry Saas (sentry.io)

Which SDK are you using?

@sentry/cloudflare

SDK Version

10.30.0

Framework Version

Cloudflare Workers (Durable Objects)

Link to Sentry event

No events are being sent - that's the issue

Reproduction Example/SDK Setup

// packages/signal/src/index.ts
import * as Sentry from '@sentry/cloudflare';

Sentry.init({
  dsn: 'https://xxx@xxx.ingest.us.sentry.io/xxx',
  tracesSampleRate: 1.0,
  environment: 'production',
});

export default instrumentDurableObjectWithSentry(TradingAgent, {
  dsn: 'https://xxx@xxx.ingest.us.sentry.io/xxx',
  tracesSampleRate: 1.0,
});

Steps to Reproduce

1. Set up @sentry/cloudflare in a Cloudflare Worker or Durable Object
2. Configure Sentry with valid DSN
3. Trigger any Sentry event (error, message, etc.)
4. Monitor Worker logs with wrangler tail
5. Observe warnings: "(warn) A stalled HTTP response was canceled to prevent deadlock..."

Expected Result

• Sentry events should be successfully transmitted to Sentry servers
• No "stalled HTTP response" warnings in logs
• Events should appear in Sentry dashboard

Actual Result

Console logs work correctly (visible in wrangler tail), but Sentry events never reach the server.

Logs show repeated warnings:

(warn) A stalled HTTP response was canceled to prevent deadlock. This can happen when a Worker calls fetch() or cache.match() several times without reading the bodies of the returned Response objects.

Root Cause

The Cloudflare transport implementation at packages/cloudflare/src/transport.ts performs a fetch() to send events but never consumes the response body.

Cloudflare Workers runtime requires ALL fetch response bodies to be either:

1. Read via response.text(), response.json(), response.arrayBuffer(), etc.
2. Explicitly canceled via response.body?.cancel()

Otherwise, the runtime cancels stalled HTTP connections to prevent deadlock.

Problematic code pattern:

const response = await (options.fetch ?? fetch)(options.url, requestOptions);

return {
  statusCode: response.status,
  headers: {
    'x-sentry-rate-limits': response.headers.get('X-Sentry-Rate-Limits'),
    'retry-after': response.headers.get('Retry-After'),
  },
};
// ❌ response.body is never consumed!

Suggested Fix

Add response body consumption after the fetch:

const response = await (options.fetch ?? fetch)(options.url, requestOptions);

// Consume the response body to prevent Cloudflare from canceling the request
// We don't need the body content, but we must read or cancel it
await response.text().catch(() => {}); // or response.body?.cancel()

return {
  statusCode: response.status,
  headers: {
    'x-sentry-rate-limits': response.headers.get('X-Sentry-Rate-Limits'),
    'retry-after': response.headers.get('Retry-After'),
  },
};

Additional Context

This issue affects all Cloudflare Workers deployments using @sentry/cloudflare, including:

• Cloudflare Workers
• Cloudflare Durable Objects
• Cloudflare Pages Functions

The problem is specific to the Cloudflare Workers runtime, which enforces stricter fetch response handling than browser or Node.js environments.

Workaround: Currently using wrangler tail for live logging instead of Sentry event transmission.

Related Cloudflare Documentation:

• Workers fetch API best practices
• Cloudflare explicitly warns about unconsumed response bodies causing connection cancellations

Priority

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[linear]

JS-1319

[JPeer264]

@igoforth thanks for your issue - I'll have a look. Have you had the problems only with the deployed state or also locally?

About the reproduction - I wonder if that worked, since we don't expose init in @sentry/cloudflare and the instrumentDurableObjectWithSentry takes the DurableObject as second parameter based on our documentation. Is that the code you are running on your end?

[JPeer264]

Ok so a little update. I could reproduce it and can confirm that these warnings are there in wrangler tail. However, all events were being sent to Sentry - do you have any reproduction where the events where not sent properly?

In the meantime I'll work on a fix with tests.

[igoforth]

Ok so a little update. I could reproduce it and can confirm that these warnings are there in wrangler tail. However, all events were being sent to Sentry - do you have any reproduction where the events where not sent properly?

In the meantime I'll work on a fix with tests.

Hey thanks for identifying. I was mistaken in how to set up logging. I had thought all it took was enableLogs: true, but I also needed to add Sentry.consoleLoggingIntegration() to properly wrap console.