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mcp: flush headers immediately for the hanging GET

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Flush headers immediately for the persistent hanging GET of the
streamable transport; otherwise, clients may time out.

Fixes #410
This commit is contained in:
Rob Findley 2025-09-04 14:00:15 +00:00 • committed by Robert Findley
commit 07b9cee8cc
1 changed files with 96 additions and 15 deletions
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37
mcp/streamable.go
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@ -743,17 +743,28 @@ func (c *streamableServerConn) respondJSON(stream *stream, w http.ResponseWriter
// lastIndex is the index of the last seen event if resuming, else -1.
func (c *streamableServerConn) respondSSE(stream *stream, w http.ResponseWriter, req *http.Request, lastIndex int, persistent bool) {
writes := 0
// Accept checked in [StreamableHTTPHandler]
// Accept was checked in [StreamableHTTPHandler]
w.Header().Set("Cache-Control", "no-cache, no-transform")
w.Header().Set("Content-Type", "text/event-stream") // Accept checked in [StreamableHTTPHandler]
w.Header().Set("Connection", "keep-alive")
if c.sessionID != "" {
w.Header().Set(sessionIDHeader, c.sessionID)
}
if persistent {
// Issue #410: the hanging GET is likely not to receive messages for a long
// time. Ensure that headers are flushed.
//
// For non-persistent requests, delay the writing of the header in case we
// may want to set an error status.
// (see the TODO: this probably isn't worth it).
w.WriteHeader(http.StatusOK)
if f, ok := w.(http.Flusher); ok {
f.Flush()
}
}
// write one event containing data.
writes := 0
write := func(data []byte) bool {
lastIndex++
e := Event{
@ -770,23 +781,19 @@ func (c *streamableServerConn) respondSSE(stream *stream, w http.ResponseWriter,
return true
}
errorf := func(code int, format string, args ...any) {
if writes == 0 {
http.Error(w, fmt.Sprintf(format, args...), code)
} else {
// TODO(#170): log when we add server-side logging
}
}
// Repeatedly collect pending outgoing events and send them.
ctx := req.Context()
for msg, err := range c.messages(ctx, stream, persistent, lastIndex) {
if err != nil {
if ctx.Err() != nil && writes == 0 {
// This probably doesn't matter, but respond with NoContent if the client disconnected.
w.WriteHeader(http.StatusNoContent)
if ctx.Err() == nil && writes == 0 && !persistent {
// If we haven't yet written the header, we have an opportunity to
// promote an error to an HTTP error.
//
// TODO: This may not matter in practice, in which case we should
// simplify.
http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
} else {
errorf(http.StatusInternalServerError, http.StatusText(http.StatusInternalServerError))
// TODO(#170): log when we add server-side logging
}
return
}