diff --git a/src/api/endpoints/sync-progress.ts b/src/api/endpoints/sync-progress.ts
index 4caf985..7678fdf 100644
--- a/src/api/endpoints/sync-progress.ts
+++ b/src/api/endpoints/sync-progress.ts
@@ -74,7 +74,9 @@ export const syncProgressHandler: RouteHandler<typeof syncProgressRoute> =
         processedBlocks: processed,
         historicalBlocksFetchedPct: pct,
         isRealtime: (isRealtime.get(chain) ?? 0) === 1,
-        isComplete: (isComplete.get(chain) ?? 0) === 1,
+        // ponder_sync_is_complete never reaches 1 for chains with live block handlers
+        // (C1–C5 run indefinitely). Derive locally: synced means realtime + all blocks processed.
+        isComplete: (isRealtime.get(chain) ?? 0) === 1 && pct >= 100,
       };
     }
 
diff --git a/tests/api/sync-progress.test.ts b/tests/api/sync-progress.test.ts
index b39e401..97e0727 100644
--- a/tests/api/sync-progress.test.ts
+++ b/tests/api/sync-progress.test.ts
@@ -95,14 +95,38 @@ describe("GET /api/sync-progress", () => {
     expect(body["gnosis"]!.historicalBlocksFetchedPct).toBe(14.1);
   });
 
-  it("sets isRealtime and isComplete from metrics flags", async () => {
+  it("sets isRealtime from ponder_sync_is_realtime metric", async () => {
     mockFetch(SAMPLE_METRICS);
     const app = buildApp();
     const res = await app.request("http://localhost/api/sync-progress");
     const body = (await res.json()) as Record<string, ChainProgress>;
     expect(body["mainnet"]!.isRealtime).toBe(false);
-    expect(body["mainnet"]!.isComplete).toBe(false);
     expect(body["gnosis"]!.isRealtime).toBe(true);
+  });
+
+  it("isComplete requires both isRealtime=true and pct>=100 (ignores ponder_sync_is_complete)", async () => {
+    // gnosis is realtime but only 14.1% processed — must NOT be complete
+    mockFetch(SAMPLE_METRICS);
+    const app = buildApp();
+    const res = await app.request("http://localhost/api/sync-progress");
+    const body = (await res.json()) as Record<string, ChainProgress>;
+    expect(body["gnosis"]!.isRealtime).toBe(true);
+    expect(body["gnosis"]!.isComplete).toBe(false);
+  });
+
+  it("isComplete is true when isRealtime=true and all blocks processed", async () => {
+    // Regression: ponder_sync_is_complete=0 must not block isComplete when realtime+100%
+    const fullSyncMetrics = `
+ponder_historical_total_blocks{chain="gnosis"} 1000
+ponder_historical_completed_blocks{chain="gnosis"} 600
+ponder_historical_cached_blocks{chain="gnosis"} 400
+ponder_sync_is_realtime{chain="gnosis"} 1
+ponder_sync_is_complete{chain="gnosis"} 0
+`.trim();
+    mockFetch(fullSyncMetrics);
+    const app = buildApp();
+    const res = await app.request("http://localhost/api/sync-progress");
+    const body = (await res.json()) as Record<string, ChainProgress>;
     expect(body["gnosis"]!.isComplete).toBe(true);
   });