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mcp: avoid double-copy of payload in writeEvent (#1106)

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`writeEvent` built the SSE frame with `fmt.Fprintf(&b, "data: %s\n\n",
string(evt.Data))`, which copies the payload into a fresh string and
then again into the buffer, growing it via repeated `bytes.growSlice`
doublings. For large MCP responses (e.g. ~162KB `tools/list`) under a
burst of concurrent writes this ~3x amplifies allocations and peak heap.

Write the payload directly into a pre-grown buffer instead. Output
framing is byte-identical. Benchmarks (162KB payload): 517KB->172KB per
op, 9->4 allocs/op, ~3x faster.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Yaroslav <yarolegovich@gmail.com>
This commit is contained in:
George McCarron 2026-07-24 09:07:06 +01:00 • committed by GitHub
commit e761950d97
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1 changed files with 6 additions and 1 deletions
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7
mcp/event.go
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@ -52,7 +52,12 @@ func writeEvent(w http.ResponseWriter, evt Event) (int, error) {
if evt.Retry != "" {
fmt.Fprintf(&b, "retry: %s\n", evt.Retry)
}
fmt.Fprintf(&b, "data: %s\n\n", string(evt.Data))
// Write the payload directly into a pre-grown buffer to avoid the extra
// copy from string(evt.Data) and repeated buffer regrowths.
b.Grow(len("data: \n\n") + len(evt.Data))
b.WriteString("data: ")
b.Write(evt.Data)
b.WriteString("\n\n")
n, err := w.Write(b.Bytes())
rc := http.NewResponseController(w)
// Ignore returned error as flushing is best-effort.