Streamable HTTP: a 2026-07-28 POST carrying Last-Event-ID is refused instead of ignored

[ddromanidis]

Describe the bug

For protocol version 2026-07-28 the streamable HTTP server refuses any POST that carries a Last-Event-ID header, where the spec asks it to ignore the header.

mcp/streamable.go, servePOST (v1.7.0 line 1412, main line 1442):

if len(req.Header.Values(lastEventIDHeader)) > 0 {
    http.Error(w, "can't send Last-Event-ID for POST request", http.StatusBadRequest)
    return
}

The 2026-07-28 transport page, "Backward compatibility with older revisions", says a server that receives such traffic from an older client SHOULD respond as follows, and lists: "A Last-Event-ID header: ignore it; streams are not resumable." Resumable streams left with this revision, so the header is a harmless leftover from a client that speaks the previous one, and as written it turns that client into one that cannot call anything: every POST is a text/plain 400.

To Reproduce

Against any NewStreamableHTTPHandler with Stateless: true:

curl -s -X POST http://127.0.0.1:8080/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -H 'Mcp-Protocol-Version: 2026-07-28' \
  -H 'Mcp-Method: server/discover' \
  -H 'Last-Event-ID: stream-1:7' \
  -d '{"jsonrpc":"2.0","id":1,"method":"server/discover","params":{"_meta":{"io.modelcontextprotocol/protocolVersion":"2026-07-28","io.modelcontextprotocol/clientInfo":{"name":"probe","version":"1"},"io.modelcontextprotocol/clientCapabilities":{}}}}'

Observed: 400 can't send Last-Event-ID for POST request.

Expected behavior

The header is ignored and the request is served. Keeping the rejection for sessions negotiated at an older version, where a POST with Last-Event-ID really is malformed, seems fine; the check just should not apply when the request is 2026-07-28.

Additional context

go-sdk v1.7.0; the code is unchanged on main as of today. I am working around it with a handler in front of the SDK's that deletes the header before delegating.