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#7686 TypeError: This ReadableStream did not return bytes when using trpc with SSE

Recommendation:KEEP OPEN
Difficulty:medium
Reasoning:

Valid workerd bug with reproduction. Multiple users confirmed over 1 year. Related to #7767. Workaround exists: use TransformStream instead of ReadableStream with controller.

Suggested Action:

Keep open as workerd runtime issue; link to #7767; document TransformStream workaround

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Analysis Report

Issue Review: cloudflare/workers-sdk#7686

Summary

tRPC SSE subscriptions fail with "TypeError: This ReadableStream did not return bytes" in workerd/Cloudflare Workers.

Findings

  • Created: 2025-01-07
  • Updated: 2025-10-30
  • Version: wrangler 3.99.0, Node 22.12.0 -> wrangler 4.60.0
  • Component: Workers Runtime (workerd)
  • Labels: bug
  • Comments: 3 (multiple users confirming issue)

Key Evidence

  • Reproduction repo provided: https://github.com/dandoen/cloudflare-trpc-subscription
  • Works with standalone HTTP server but fails in Cloudflare Workers
  • Error: "TypeError: This ReadableStream did not return bytes"
  • Multiple users confirming same issue (Jan 2025, Feb 2025, Oct 2025)
  • One user found workaround: use TransformStream instead of ReadableStream with controller
  • Related to issue #7767 (both SSE-related)
  • This is a workerd runtime limitation with how ReadableStream is handled for SSE/streaming
  • No PRs found fixing this

Recommendation

Status: KEEP OPEN

Reasoning: This is a valid workerd runtime bug affecting tRPC SSE subscriptions and similar streaming patterns. Multiple users have confirmed the issue over the past year. A workaround exists (using TransformStream instead of ReadableStream with controller). The issue has 2 emoji reactions and ongoing community engagement.

Action: Keep open as a workerd runtime issue. Link to related #7767 as both involve SSE/streaming issues. Document the TransformStream workaround prominently.

Suggested Comment

This appears to be related to #7767 - both involve SSE/streaming failing in workerd.

Workaround: As @hyusetiawan discovered, using TransformStream instead of creating a ReadableStream with a controller works:

const { readable, writable } = new TransformStream();
const writer = writable.getWriter();
// Write to writer instead of using ReadableStream controller
return new Response(readable, { headers: { 'Content-Type': 'text/event-stream' } });

This is a workerd runtime limitation. Keeping this open for the runtime team to investigate.

@davyzhang - are you still experiencing this with the latest wrangler version (4.60.0)?

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