fix: surface JSON-RPC error responses in mcpcurl

readJSONRPCResponse now checks for an "error" field in responses
and returns a descriptive error instead of silently passing it through.

Author: ra-n-dom

cmd/mcpcurl/main.go

@@ -503,6 +503,9 @@ func readJSONRPCResponse(scanner *bufio.Scanner) (string, error) {
 			return "", fmt.Errorf("failed to parse JSON-RPC message: %w", err)
 		}
 		if _, hasID := msg["id"]; hasID {
+			if errField, hasErr := msg["error"]; hasErr {
+				return "", fmt.Errorf("server returned error: %s", string(errField))
+			}
 			return line, nil
 		}
 		// No "id" — this is a notification, skip it

cmd/mcpcurl/main_test.go

@@ -88,6 +88,23 @@ func TestReadJSONRPCResponse_InvalidJSON(t *testing.T) {
 	}
 }
 
+func TestReadJSONRPCResponse_ServerError(t *testing.T) {
+	t.Parallel()
+	input := `{"jsonrpc":"2.0","id":1,"error":{"code":-32601,"message":"method not found"}}` + "\n"
+	scanner := bufio.NewScanner(strings.NewReader(input))
+
+	_, err := readJSONRPCResponse(scanner)
+	if err == nil {
+		t.Fatal("expected error for server error response, got nil")
+	}
+	if !strings.Contains(err.Error(), "server returned error") {
+		t.Fatalf("expected 'server returned error', got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "method not found") {
+		t.Fatalf("expected error to contain server message, got: %v", err)
+	}
+}
+
 func TestBuildInitializeRequest(t *testing.T) {
 	t.Parallel()
 	got, err := buildInitializeRequest()