diff --git a/src/infra/session-cost-usage.test.ts b/src/infra/session-cost-usage.test.ts
index b8efd3d3b074..0a31451efcc3 100644
--- a/src/infra/session-cost-usage.test.ts
+++ b/src/infra/session-cost-usage.test.ts
@@ -33,6 +33,7 @@ import {
   loadSessionUsageTimeSeries as loadSessionUsageTimeSeriesForAgent,
   resolveExistingUsageSessionFile as resolveExistingUsageSessionFileForAgent,
 } from "./session-cost-usage.js";
+import { testing as sessionCostUsageTestApi } from "./session-cost-usage.test-support.js";
 
 type WithOptionalAgentId<T> = T extends (params: infer P) => unknown
   ? Omit<P, "agentId"> & { agentId?: string }
@@ -77,6 +78,13 @@ function waitForFast<T>(
   return vi.waitFor(callback, { interval: 1, ...options });
 }
 
+async function refreshSessionCostUsageForTest(sessionFile: string): Promise<void> {
+  await sessionCostUsageTestApi.usageCostRefreshRuntime.refreshCostUsageCacheForAgent({
+    agentId: "main",
+    sessionFiles: [sessionFile],
+  });
+}
+
 function clearGatewayModelPricingState(): void {
   replaceGatewayModelPricingCache(new Map(), 0);
   clearGatewayModelPricingFailures();
@@ -931,21 +939,20 @@ describe("session cost usage", () => {
       [
         JSON.stringify(assistantEntry(undefined, 1_000)),
         JSON.stringify(assistantEntry("2026-02-05T12:00:00.000Z", 20)),
+        "",
       ].join("\n"),
       "utf-8",
     );
 
     await withStateDir(root, async () => {
       const session = { sessionId: "sess-v8-upgrade", sessionFile };
-      await loadSessionCostSummariesFromCache({ sessions: [session], agentId: "main" });
-      await waitForFast(async () => {
-        const current = await loadSessionCostSummariesFromCache({
-          sessions: [session],
-          agentId: "main",
-          requestRefresh: false,
-        });
-        expect(current.cacheStatus.status).toBe("fresh");
+      await refreshSessionCostUsageForTest(sessionFile);
+      const current = await loadSessionCostSummariesFromCache({
+        sessions: [session],
+        agentId: "main",
+        requestRefresh: false,
       });
+      expect(current.cacheStatus.status).toBe("fresh");
 
       const currentRow = requireValue(
         readSessionCostUsageRollupRows("main").find((row) => row.key === sessionFile),
@@ -968,41 +975,32 @@ describe("session cost usage", () => {
       ).toBe(true);
 
       const rangeEndMs = Date.UTC(2026, 1, 5) + 24 * 60 * 60 * 1000 - 1;
-      await loadSessionCostSummariesFromCache({
+      await refreshSessionCostUsageForTest(sessionFile);
+      const rebuilt = await loadSessionCostSummariesFromCache({
         sessions: [session],
         agentId: "main",
         startMs: Date.UTC(2026, 1, 5),
         endMs: rangeEndMs,
+        requestRefresh: false,
       });
-      await waitForFast(async () => {
-        const rebuilt = await loadSessionCostSummariesFromCache({
-          sessions: [session],
-          agentId: "main",
-          startMs: Date.UTC(2026, 1, 5),
-          endMs: rangeEndMs,
-          requestRefresh: false,
-        });
-        expect(rebuilt.cacheStatus.status).toBe("fresh");
-        expect(rebuilt.summaries[0]?.totalTokens).toBe(20);
-      });
+      expect(rebuilt.cacheStatus.status).toBe("fresh");
+      expect(rebuilt.summaries[0]?.totalTokens).toBe(20);
 
       await fs.appendFile(
         sessionFile,
-        `\n${JSON.stringify(assistantEntry("2026-02-05T13:00:00.000Z", 5))}`,
+        `${JSON.stringify(assistantEntry("2026-02-05T13:00:00.000Z", 5))}\n`,
         "utf-8",
       );
-      await loadSessionCostSummariesFromCache({ sessions: [session], agentId: "main" });
-      await waitForFast(async () => {
-        const appended = await loadSessionCostSummariesFromCache({
-          sessions: [session],
-          agentId: "main",
-          startMs: Date.UTC(2026, 1, 5),
-          endMs: rangeEndMs,
-          requestRefresh: false,
-        });
-        expect(appended.cacheStatus.status).toBe("fresh");
-        expect(appended.summaries[0]?.totalTokens).toBe(25);
+      await refreshSessionCostUsageForTest(sessionFile);
+      const appended = await loadSessionCostSummariesFromCache({
+        sessions: [session],
+        agentId: "main",
+        startMs: Date.UTC(2026, 1, 5),
+        endMs: rangeEndMs,
+        requestRefresh: false,
       });
+      expect(appended.cacheStatus.status).toBe("fresh");
+      expect(appended.summaries[0]?.totalTokens).toBe(25);
 
       const appendedRow = requireValue(
         readSessionCostUsageRollupRows("main").find((row) => row.key === sessionFile),