fix: send Last-Event-ID header as UTF-8

[OsamaAnsar]

What was wrong

Last-Event-ID was set to the raw event ID string. Header values passed to fetch() are byte strings, so:

• An ID containing a character above U+00FF (eg €) makes fetch() throw a TypeError on every reconnect attempt. #onFetchError retries with the same value forever, so the client never reconnects and readyState stays CONNECTING.
• An ID with characters in U+0080 to U+00FF (eg é) is sent as a single Latin-1 byte rather than as UTF-8.

The HTML specification (server-sent events, "reconnection" step of the processing model) says the Last-Event-ID header is the last event ID string, encoded as UTF-8.

The fix

Encode the ID as UTF-8 and pass those bytes through as a byte string (one character per byte), so fetch() puts exactly the UTF-8 bytes on the wire. This is the only place the ID is used as a header value. A plain loop is used for the conversion rather than spreading the bytes into String.fromCharCode, so long IDs cannot hit argument limits.

Before and after

Reproduction on Node v24.19.0: a local server sends id: café-€1 and closes the response, then logs the Last-Event-ID bytes it receives (decoded as UTF-8 for readability).

Before (main):

server hit 1 raw header bytes (hex) = (none)
open | readyState 1
message "hello" lastEventId "café-€1"
error event: undefined | readyState 0
error event: Cannot convert argument to a ByteString because the character at index 5 has a value of 8364 which is greater than 255. | readyState 0
total error events 14 | server hits 1 | readyState 0

After:

server hit 1 raw header bytes (hex) = (none)
open | readyState 1
message "hello" lastEventId "café-€1"
error event: undefined | readyState 0
server hit 2 raw header bytes (hex) = 636166c3a92de282ac31 | decoded as UTF-8 = "café-€1"
open | readyState 1
total error events 1 | server hits 2 | readyState 1

With an ID in the Latin-1 range only (id: café), the server received the bytes 636166e9 before, and receives 636166c3a9 (UTF-8) after.

Tests

Added a test.each in test/client.test.ts with three IDs: Latin-1 range (café), beyond Latin-1 (café-€1) and outside the BMP (id-😀). It uses a new /echo-last-event-id route in test/helpers/server.ts, which sends the ID on the first connection and echoes the Last-Event-ID header, decoded as UTF-8, on the reconnect.

• I reverted only the change in src/EventSource.ts and confirmed all three new tests fail (two of them with Timeout waiting for onMessage call count of 2), then restored it and confirmed they pass.
• npm test (Node, vitest): 55 passed, 1 skipped. npm run lint (oxlint plus both tsc --noEmit runs) and oxfmt --check pass. npm run test:types, npm run test:bun (55 passed, 1 skipped) and npm run test:happy-dom (52 passed, 4 expected failures, as documented in CONTRIBUTING) pass as well.
• I did not run the browser, iOS, Deno or workerd suites (Playwright browsers, Xcode and Deno are not set up on this machine).

A patch changeset is included, as src/ is touched.

[rexxars]

Thank you for your contribution!

[rexxars]

Mistakenly didn't run the CI workflows on this; this crashes on Safari and workerd. I'm working on finding a solution.

[rexxars]

Could not find a proper workaround for Safari, unfortunately: https://bugs.webkit.org/show_bug.cgi?id=327451
Would encourage the usage of ascii for event IDs.