SSEStreamingApi.writeSSE does not handle \r correctly

[LarryWu1]

What version of Hono are you using?

4.7.4

What runtime/platform is your app running on? (with version if possible)

Node v22.4.1

What steps can reproduce the bug?

1. Return the following SSE stream from a GET endpoint:

async function testStream(c: Context) {
  return streamSSE(c, async (stream) => {
    await stream.writeSSE({
      data: `Left\r\rdata: Right`,
      event: 'time-update',
    })
  })
}

app.get('/test', testStream)

2. Listen on the client via EventSource and log the messages:

const evtSource = new EventSource("http://localhost:8080/test")
evtSource.onmessage = ((e) => {
  console.log('Message:', e)
})
evtSource.addEventListener('time-update', (e) => {
  console.log('Time-Update:', e)
})

What is the expected behavior?

It should log:

Time-Update: Left\r\rdata: Right

What do you see instead?

Time-Update: Left
Message: Right

Additional information

If \r is replaced with \n, things work correctly.

E.g

await stream.writeSSE({
  data: `Left\n\ndata: Right`,
  event: 'time-update',
})

logs:

Time-Update: Left

data: Right

[LarryWu1]

The following code listens to an event via a stream reader:

const res = await fetch('http://localhost:8080/test')

if (!res.body) {
  return
}
const reader = res.body.pipeThrough(new TextDecoderStream()).getReader()
while (true) {
  const { value, done } = await reader.read();
  if (done) break;
  console.log(value);
}

await reader.closed

Using it to listen to Left\n\ndata: Right logs:

event: time-update
data: Left
data: 
data: data: Right

So it seems that hono adds the data: prefix for every line ending with \n to prevent double \n from ending an event. It doesn't do the same with \r:

Left\r\rdata: Right logs:

event: time-update
data: Leftdata: Right

Lines can end with \r according to the SSE spec https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation

[yusukebe]

Hi @sor4chi @watany-dev

I'll work on it if I have time, but can you handle this if you can?