mcp: preserve SSE events buffered after the endpoint - #1270
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guglielmo-san merged 1 commit intoSep 15, 2026
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guglielmo-san merged 1 commit into
guglielmo-san merged 1 commit into
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Reuse the event scanner across endpoint discovery and message reading so that events already buffered after the endpoint are retained. Add a regression test for coalesced events and a separate-delivery control.
guglielmo-san
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Sep 15, 2026
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SSEClientTransport.Connect creates one event scanner to read the endpoint event
and a second scanner to read subsequent messages. Each scanner owns a buffered
reader, so bytes read ahead by the first scanner are discarded when it is
abandoned. A message coalesced with the endpoint can therefore disappear while
the connection remains open.
Reuse the event scanner across the endpoint handshake and the message-reading
goroutine. This preserves unread bytes without changing the public API.
The regression test exercises Connect and Read with a controlled HTTP response
body, covering both a message coalesced with the endpoint and a message arriving
after Connect returns. On unmodified main, the separate-message control passes
and the coalesced case times out; both pass with the fix.
Validation on linux/amd64 with Go 1.26.8:
The HTTP response boundary is simulated; no third-party server or proxy was
used. This change concerns the legacy SSE transport. The Streamable HTTP
conformance suite was not run.