mcp: parse SSE field values per the SSE spec

mcp/event.go

@@ -158,15 +158,18 @@ func scanEventsLimited(r io.Reader, maxEventSize int) iter.Seq2[Event, error] {
 			if b, a, found := bytes.Cut(line, []byte{':'}); found {
 				before, after = b, a
 			}
+			// A field value is everything after the colon, with at most one
+			// leading U+0020 removed; any other whitespace is part of the value.
+			value := bytes.TrimPrefix(after, []byte{' '})
 			switch {
 			case bytes.Equal(before, eventKey):
-				evt.Name = strings.TrimSpace(string(after))
+				evt.Name = string(value)
 			case bytes.Equal(before, idKey):
-				evt.ID = strings.TrimSpace(string(after))
+				evt.ID = string(value)
 			case bytes.Equal(before, retryKey):
-				evt.Retry = strings.TrimSpace(string(after))
+				evt.Retry = string(value)
 			case bytes.Equal(before, dataKey):
-				data := bytes.TrimSpace(after)
+				data := value
 				if dataBuf == nil {
 					dataBuf = new(bytes.Buffer)
 				} else {

mcp/event_test.go

@@ -134,6 +134,29 @@ func TestScanEvents(t *testing.T) {
 				{Data: []byte("hello\n")},
 			},
 		},
+		{
+			// Per the SSE parsing rules, only a single leading space is
+			// removed from a field value, and no trailing whitespace.
+			name:  "data values keep all but one leading space",
+			input: "data: {\"a\":\ndata:  1}\n\n",
+			want: []Event{
+				{Data: []byte("{\"a\":\n 1}")},
+			},
+		},
+		{
+			name:  "data value keeps trailing whitespace",
+			input: "data: hello  \n\n",
+			want: []Event{
+				{Data: []byte("hello  ")},
+			},
+		},
+		{
+			name:  "field values keep all but one leading space",
+			input: "event:  message\nid: 1 \nretry:  500\ndata: x\n\n",
+			want: []Event{
+				{Name: " message", ID: "1 ", Retry: " 500", Data: []byte("x")},
+			},
+		},
 	}
 
 	for _, tt := range tests {