mcp: relax SSE connection response handling in non-strict mode

Signed-off-by: Xie Zhihao <[email protected]>

Author: zhxie

mcp/streamable.go

@@ -1377,13 +1377,14 @@ func (c *streamableClientConn) connectStandaloneSSE() {
 		resp.Body.Close()
 		return
 	}
-	if resp.StatusCode == http.StatusNotFound && !c.strict {
-		// modelcontextprotocol/gosdk#393: some servers return NotFound instead
-		// of MethodNotAllowed for the standalone SSE stream.
+	if resp.StatusCode >= 400 && resp.StatusCode < 500 && !c.strict {
+		// modelcontextprotocol/go-sdk#393,#610: some servers return NotFound or
+		// other status codes instead of MethodNotAllowed for the standalone SSE
+		// stream.
 		//
 		// Treat this like MethodNotAllowed in non-strict mode.
 		if c.logger != nil {
-			c.logger.Warn("got 404 instead of 405 for standalone SSE stream")
+			c.logger.Warn(fmt.Sprintf("got %d instead of 405 for standalone SSE stream", resp.StatusCode))
 		}
 		resp.Body.Close()
 		return

mcp/streamable_client_test.go

@@ -305,7 +305,8 @@ func TestStreamableClientStrictness(t *testing.T) {
 		{"strict initialized", true, http.StatusOK, http.StatusMethodNotAllowed, true},
 		{"unstrict initialized", false, http.StatusOK, http.StatusMethodNotAllowed, false},
 		{"strict GET", true, http.StatusAccepted, http.StatusNotFound, true},
-		{"unstrict GET", false, http.StatusOK, http.StatusNotFound, false},
+		{"unstrict GET on StatusNotFound", false, http.StatusOK, http.StatusNotFound, false},
+		{"unstrict GET on StatusBadRequest", false, http.StatusOK, http.StatusBadRequest, false},
 	}
 	for _, test := range tests {
 		t.Run(test.label, func(t *testing.T) {