fix: accept HTTP 204 No Content in SendNotification

HTTP 204 is a valid success status code per RFC 7231, commonly returned
by servers for notification endpoints where no response body is expected.
The client previously only accepted 200 and 202, causing initialization
to fail when connecting to servers that return 204 for the initialized
notification.

Fixes #700

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: satish-karri-glean

client/transport/streamable_http.go

@@ -573,7 +573,7 @@ func (c *StreamableHTTP) SendNotification(ctx context.Context, notification mcp.
 	}
 	defer resp.Body.Close()
 
-	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusAccepted {
+	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusAccepted && resp.StatusCode != http.StatusNoContent {
 		// Handle unauthorized error
 		if resp.StatusCode == http.StatusUnauthorized {
 			if c.oauthHandler != nil {

client/transport/streamable_http_test.go

@@ -988,6 +988,36 @@ func TestStreamableHTTP_SendNotification_Unauthorized_StaticToken(t *testing.T)
 	}
 }
 
+// TestStreamableHTTP_SendNotification_Accepts204NoContent verifies that SendNotification
+// treats HTTP 204 No Content as a success response per RFC 7231.
+// See: https://github.com/mark3labs/mcp-go/issues/700
+func TestStreamableHTTP_SendNotification_Accepts204NoContent(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNoContent)
+	}))
+	defer server.Close()
+
+	transport, err := NewStreamableHTTP(server.URL)
+	if err != nil {
+		t.Fatalf("Failed to create StreamableHTTP: %v", err)
+	}
+
+	if err := transport.Start(context.Background()); err != nil {
+		t.Fatalf("Failed to start transport: %v", err)
+	}
+
+	err = transport.SendNotification(context.Background(), mcp.JSONRPCNotification{
+		JSONRPC: "2.0",
+		Notification: mcp.Notification{
+			Method: "notifications/initialized",
+		},
+	})
+
+	if err != nil {
+		t.Fatalf("SendNotification should accept 204 No Content, got error: %v", err)
+	}
+}
+
 // TestStreamableHTTPHostOverride tests Host header override for StreamableHTTP transport
 func TestStreamableHTTPHostOverride(t *testing.T) {
 	// Create a test server that captures the Host header