diff --git a/packages/dashboard/src/__tests__/routes-agent-prompt-sizes-integration.test.ts b/packages/dashboard/src/__tests__/routes-agent-prompt-sizes-integration.test.ts
new file mode 100644
index 000000000..f3876bfd2
--- /dev/null
+++ b/packages/dashboard/src/__tests__/routes-agent-prompt-sizes-integration.test.ts
@@ -0,0 +1,58 @@
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { mkdtempSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { AgentStore, TaskStore } from "@fusion/core";
+import { createServer } from "../server.js";
+import { get } from "../test-request.js";
+
+describe("GET /api/agents/:id/prompt-sizes integration", () => {
+  let rootDir: string;
+  let store: TaskStore;
+  let agentId: string;
+
+  beforeEach(async () => {
+    rootDir = mkdtempSync(join(tmpdir(), "fn-4928-prompt-sizes-"));
+    store = new TaskStore(rootDir, join(rootDir, ".fusion-global-settings"));
+    await store.init();
+
+    const agentStore = new AgentStore({ rootDir: store.getFusionDir() });
+    await agentStore.init();
+
+    const agent = await agentStore.createAgent({
+      name: "Prompt Sizes Agent",
+      role: "executor",
+      metadata: {},
+    });
+    agentId = agent.id;
+
+    await agentStore.saveRun({
+      id: "run-prompt-size-1",
+      agentId,
+      startedAt: "2026-05-17T12:00:00.000Z",
+      endedAt: "2026-05-17T12:00:01.000Z",
+      status: "completed",
+      systemPrompt: "sys prompt",
+      executionPrompt: "execute now",
+    });
+  });
+
+  afterEach(() => {
+    rmSync(rootDir, { recursive: true, force: true });
+  });
+
+  it("returns prompt-size rows derived from startedAt and run JSON", async () => {
+    const app = createServer(store);
+    const okRes = await get(app, `/api/agents/${agentId}/prompt-sizes`);
+    expect(okRes.status).toBe(200);
+    expect(okRes.body).toEqual([
+      {
+        runId: "run-prompt-size-1",
+        createdAt: "2026-05-17T12:00:00.000Z",
+        systemChars: "sys prompt".length,
+        execChars: "execute now".length,
+        totalChars: "sys prompt".length + "execute now".length,
+      },
+    ]);
+  });
+});
diff --git a/packages/dashboard/src/routes/register-agent-runtime-routes.ts b/packages/dashboard/src/routes/register-agent-runtime-routes.ts
index f8df0d1ce..bc2a3f62f 100644
--- a/packages/dashboard/src/routes/register-agent-runtime-routes.ts
+++ b/packages/dashboard/src/routes/register-agent-runtime-routes.ts
@@ -659,14 +659,14 @@ export function registerAgentRuntimeRoutes(ctx: ApiRoutesContext, deps: AgentRun
       const rows = scopedStore.getDatabase().prepare(`
         SELECT
           id AS runId,
-          createdAt,
+          startedAt AS createdAt,
           COALESCE(length(json_extract(data, '$.systemPrompt')), 0) AS systemChars,
           COALESCE(length(json_extract(data, '$.executionPrompt')), 0) AS execChars,
           COALESCE(length(json_extract(data, '$.systemPrompt')), 0)
             + COALESCE(length(json_extract(data, '$.executionPrompt')), 0) AS totalChars
         FROM agentRuns
         WHERE json_extract(data, '$.agentId') = ?
-        ORDER BY createdAt DESC
+        ORDER BY startedAt DESC
         LIMIT ?
       `).all(req.params.id, limit) as Array<{
         runId: string;