diff --git a/lib/data/gantt-api.ts b/lib/data/gantt-api.ts
index df37ebc..683c389 100644
--- a/lib/data/gantt-api.ts
+++ b/lib/data/gantt-api.ts
@@ -21,7 +21,13 @@ function deriveApiBaseUrlFromGanttBoardUrl(): string | null {
   try {
     const parsed = new URL(ganttBoardUrl);
     const pathname = parsed.pathname.replace(/\/+$/, "");
-    parsed.pathname = `${pathname}/api`;
+    if (!pathname || pathname === "/") {
+      parsed.pathname = "/api";
+    } else if (pathname.endsWith("/api")) {
+      parsed.pathname = pathname;
+    } else {
+      parsed.pathname = `${pathname}/api`;
+    }
     parsed.search = "";
     parsed.hash = "";
     return normalizeBaseUrl(parsed.toString());
@@ -107,11 +113,11 @@ export async function fetchGanttApi<T>(endpoint: string): Promise<T> {
         payload?.error ||
         payload?.message ||
         (nonJsonBody ? nonJsonBody.replace(/\s+/g, " ").slice(0, 200) : response.statusText);
-      throw new Error(`gantt-board API request failed (${response.status} ${endpoint}): ${details}`);
+      throw new Error(`gantt-board API request failed (${response.status} ${cacheKey}): ${details}`);
     }
 
     if (!isJson) {
-      throw new Error(`gantt-board API request failed (${response.status} ${endpoint}): expected JSON response`);
+      throw new Error(`gantt-board API request failed (${response.status} ${cacheKey}): expected JSON response`);
     }
 
     return payload as T;