Firefox does not hand a streaming `fetch()` response to JS until the
first *body* byte arrives; Chromium resolves the promise as soon as the
headers do. Both SSE endpoints flushed headers immediately and then
stayed silent until there was something to report, which deadlocked
`/api/mcp/events`: `RemoteClientTransport.openEventStream()` awaits that
fetch *before* the MCP client sends `initialize`, so no `initialize` ->
no event to report -> no body byte -> the fetch never resolves. The web
UI hung on "Connecting..." forever with nothing in the UI, the console,
or the node logs.
Write an inert `:` SSE comment the instant each stream opens. Conforming
parsers ignore a comment line, so this unblocks the read without
inventing a wire event and needs no client-side protocol change.
`X-Content-Type-Options: nosniff` was also proposed on the issue but does
not fix it -- verified against Firefox 153, the fetch still never
resolves with the header set and no body byte. The delay is not MIME
sniffing.
The priming write is deliberately ordered *after* each handler registers
its consumer (the session event consumer; the file-watch subscriber plus
`ensureWatcher()`). Callers treat the arrival of the stream's first bytes
as proof they are subscribed, so priming first would hand out that proof
across an `await` and drop an edit made in the gap.
`/api/servers/events` gets the same treatment: its fetch was equally
stuck on Firefox until the first real change event, which for an unedited
`mcp.json` never comes. Its client-side reader counted any `\n\n` as a
change, so the inert frame would have fired a spurious background
re-fetch on every connection -- it now skips frames carrying no
`event:`/`data:` field. The transport's own `parseSSE` already drops
comment frames correctly and needed no change.
Verified end to end in real Firefox against a live streamable-HTTP test
server: before, stuck on "Connecting..." past 25s; after, initialize ->
notifications/initialized -> tools/list, connected.
Closes#1858
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YAt8rqxysNbhYWLhoRm3fU
9644ac7fe8