mcp: avoid double-copy of payload in writeEvent#1106
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yarolegovich merged 2 commits intoJul 24, 2026
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July 22, 2026 11:26
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>
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the change makes sense, thanks for the contribution.
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writeEventbuilt the SSE frame withfmt.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 repeatedbytes.growSlicedoublings. For large MCP responses (e.g. ~162KBtools/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.