v1.18.0 breaks Server-Sent Events (SSE) real-time streaming - first chunk delayed by pre-read logic

[hcnode]

Bug Description:

Summary

@hono/node-server v1.18.0 introduced a regression that breaks real-time Server-Sent Events (SSE) streaming. The new pre-read logic delays the first chunk delivery, causing browsers to show "no response" and breaking the real-time nature of SSE connections.

Affected Versions

• Working: v1.17.1 and earlier
• Broken: v1.18.0 and later

Root Cause

The v1.18.0 release introduced a change described as "feat: always respond res.body" which added pre-read logic that attempts to read the first 2 chunks of a ReadableStream before sending headers and data to the client.

Problematic code in src/listener.ts (lines ~138-175):

if (res.body) {
  const reader = res.body.getReader()
  const values: Uint8Array[] = []
  
  // This pre-read logic breaks real-time SSE streaming
  for (let i = 0; i < 2; i++) {
    currentReadPromise = reader.read()
    const chunk = await readWithoutBlocking(currentReadPromise)
    // ... waits to read chunks before sending anything
  }
  
  // Headers/data only sent after pre-reading
  outgoing.writeHead(res.status, resHeaderRecord)
}

Impact

1. Browser DevTools shows "no response": SSE connections appear stuck/pending
2. Delayed first events: Users don't see SSE data until multiple chunks are buffered
3. Breaks real-time applications: Chat apps, live updates, progress indicators, etc.
4. Production issue: Affects any application using SSE with async data generation

Reproduction

// SSE endpoint that generates data asynchronously
app.get('/events', (c) => {
  const stream = new ReadableStream({
    async start(controller) {
      controller.enqueue('data: first event\n\n')
      
      // Any async operation (DB query, API call, etc.)
      await new Promise(resolve => setTimeout(resolve, 100))
      controller.enqueue('data: second event\n\n')
      controller.close()
    }
  })
  
  c.header('Content-Type', 'text/event-stream')
  return c.body(stream)
})

Expected: First event appears immediately in browser
Actual (v1.18.0): Browser shows "no response" until second event is ready

Suggested Fix

Add a fast-path for SSE responses that bypasses the pre-read logic:

// Fast-path for Server-Sent Events
const contentTypeHeader = resHeaderRecord["content-type"] as string | undefined
if (contentTypeHeader && /^text\/event-stream\b/i.test(contentTypeHeader) && res.body) {
  // Stream immediately without pre-reading
  outgoing.writeHead(res.status, resHeaderRecord)
  flushHeaders(outgoing)
  await writeFromReadableStream(res.body, outgoing)
  return
}

Environment

• Node.js: Any version
• Browser: All browsers (Chrome DevTools most visible)
• Use case: Any SSE implementation with async data generation

This regression significantly impacts real-time web applications and should be prioritized for the next patch release.

[usualoma]

Hi @hcnode, thank you for your feedback.

In my environment, regardless of the Node.js version, the first chunk is delivered without delay in the following project.

https://github.com/usualoma/hono-se-266

Are there any other conditions necessary for reproduction?

CleanShot.2025-08-01.at.11.54.42.mp4

[usualoma]

Hi @hcnode

Thanks for the report yesterday. Sorry, I understand.

#265 was causing the problem. I'll fix it right away.

[usualoma]

Workaround

The problem does not occur if the application includes Transfer-Encoding: chunked.

[hcnode]

@usualoma
Thank you for the quick resolution! The workaround using Transfer-Encoding: chunked works for me.

Sorry if the issue I initially described was not the root cause. Interestingly, if I remove app.use('*', cors()); from the code, SSE works correctly without needing Transfer-Encoding: chunked.