diff --git a/actions/setup/js/check_daily_aic_workflow_guardrail.cjs b/actions/setup/js/check_daily_aic_workflow_guardrail.cjs
index cc9bbe2b8a8..c16bde0d6ef 100644
--- a/actions/setup/js/check_daily_aic_workflow_guardrail.cjs
+++ b/actions/setup/js/check_daily_aic_workflow_guardrail.cjs
@@ -13,6 +13,8 @@ const { createRateLimitAwareGithub, fetchAndLogRateLimit } = require("./github_r
 
 const PRIMARY_GUARDRAIL_ARTIFACT_NAMES = ["usage"];
 const DAILY_WORKFLOW_WINDOW_MS = 24 * 60 * 60 * 1000;
+/** Cache entries older than this threshold (in ms) are skipped when loading. */
+const CACHE_RETENTION_MS = 48 * 60 * 60 * 1000;
 const MAX_WORKFLOW_RUN_PAGES = 10;
 const RATE_LIMIT_RESERVE = 100;
 const REQUEST_OVERHEAD_BUDGET = MAX_WORKFLOW_RUN_PAGES + 4;
@@ -94,7 +96,11 @@ function shouldSkipDailyAICGuardrail() {
 
 /**
  * Loads the per-workflow usage cache from the JSONL file restored by the activation job's
- * cache-restore step.  Each line is a JSON object `{ run_id: number, aic: number }`.
+ * cache-restore step.  Each line is a JSON object `{ run_id: number, aic: number, timestamp?: string }`.
+ *
+ * Entries with a `timestamp` older than {@link CACHE_RETENTION_MS} (48 h) are skipped so that
+ * stale data cannot inflate the daily-AIC total.  Entries without a `timestamp` (written by an
+ * older version of the write script) are kept for backward compatibility.
  *
  * Returns a `Map<runId, aic>` so that callers can check whether a prior run's AIC is already
  * known without downloading the run's artifact from the GitHub API.
@@ -112,7 +118,10 @@ function loadAICUsageCache(filePath) {
       return cache;
     }
     const content = fs.readFileSync(cachePath, "utf8");
+    const now = Date.now();
+    const cutoff = now - CACHE_RETENTION_MS;
     let loaded = 0;
+    let skippedStale = 0;
     for (const rawLine of content.split("\n")) {
       const line = rawLine.trim();
       if (!line || !line.startsWith("{")) {
@@ -120,6 +129,14 @@ function loadAICUsageCache(filePath) {
       }
       try {
         const entry = JSON.parse(line);
+        // Skip entries that have a timestamp and are older than the retention window.
+        if (typeof entry?.timestamp === "string") {
+          const ts = Date.parse(entry.timestamp);
+          if (Number.isFinite(ts) && ts < cutoff) {
+            skippedStale++;
+            continue;
+          }
+        }
         const runId = Number(entry?.run_id);
         const rawAic = entry?.aic;
         const aic = typeof rawAic === "number" ? rawAic : NaN;
@@ -131,7 +148,7 @@ function loadAICUsageCache(filePath) {
         // Ignore malformed lines.
       }
     }
-    logDailyGuardrail("Loaded usage cache", { path: cachePath, entriesLoaded: loaded });
+    logDailyGuardrail("Loaded usage cache", { path: cachePath, entriesLoaded: loaded, skippedStale });
   } catch (err) {
     logDailyGuardrail("Failed to load usage cache; proceeding without it", {
       path: cachePath,
diff --git a/actions/setup/js/check_daily_aic_workflow_guardrail.test.cjs b/actions/setup/js/check_daily_aic_workflow_guardrail.test.cjs
index 12d3fb73f54..61ab16c94ee 100644
--- a/actions/setup/js/check_daily_aic_workflow_guardrail.test.cjs
+++ b/actions/setup/js/check_daily_aic_workflow_guardrail.test.cjs
@@ -612,5 +612,30 @@ describe("check_daily_aic_workflow_guardrail", () => {
       expect(cache.has(603)).toBe(false);
       expect(cache.get(604)).toBe(4.2);
     });
+
+    it("loads entries that have a recent timestamp (within 48 h)", () => {
+      const recentTimestamp = new Date(Date.now() - 60 * 60 * 1000).toISOString(); // 1 hour ago
+      fs.writeFileSync(cacheFile, JSON.stringify({ run_id: 701, aic: 8.0, timestamp: recentTimestamp }) + "\n", "utf8");
+      const cache = exports.loadAICUsageCache(cacheFile);
+      expect(cache.has(701)).toBe(true);
+      expect(cache.get(701)).toBe(8.0);
+    });
+
+    it("skips entries whose timestamp is older than 48 h", () => {
+      const staleTimestamp = new Date(Date.now() - 49 * 60 * 60 * 1000).toISOString(); // 49 hours ago
+      const recentTimestamp = new Date(Date.now() - 30 * 60 * 1000).toISOString(); // 30 minutes ago
+      fs.writeFileSync(cacheFile, [JSON.stringify({ run_id: 801, aic: 3.0, timestamp: staleTimestamp }), JSON.stringify({ run_id: 802, aic: 5.0, timestamp: recentTimestamp })].join("\n") + "\n", "utf8");
+      const cache = exports.loadAICUsageCache(cacheFile);
+      expect(cache.has(801)).toBe(false);
+      expect(cache.has(802)).toBe(true);
+      expect(cache.get(802)).toBe(5.0);
+    });
+
+    it("keeps entries without a timestamp (backward compatibility)", () => {
+      fs.writeFileSync(cacheFile, JSON.stringify({ run_id: 901, aic: 2.5 }) + "\n", "utf8");
+      const cache = exports.loadAICUsageCache(cacheFile);
+      expect(cache.has(901)).toBe(true);
+      expect(cache.get(901)).toBe(2.5);
+    });
   });
 });
diff --git a/actions/setup/js/write_daily_aic_usage_cache.cjs b/actions/setup/js/write_daily_aic_usage_cache.cjs
index a33aeb56a19..bb9f074b8fc 100644
--- a/actions/setup/js/write_daily_aic_usage_cache.cjs
+++ b/actions/setup/js/write_daily_aic_usage_cache.cjs
@@ -20,6 +20,9 @@ const { getErrorMessage } = require("./error_helpers.cjs");
 /** Path where the restored (and updated) usage cache lives on the runner. */
 const CACHE_FILE_PATH = "/tmp/gh-aw/agentic-workflow-usage-cache.jsonl";
 
+/** Entries older than this threshold (in ms) are pruned when rewriting the cache. */
+const CACHE_RETENTION_MS = 48 * 60 * 60 * 1000;
+
 /**
  * Directory prepared by the "Collect usage artifact files" step in the conclusion job.
  * Contains agent_usage.jsonl and agent/token_usage.jsonl which mirror the contents of
@@ -47,12 +50,18 @@ function logCache(message, details) {
 }
 
 /**
- * Appends a `{run_id, aic}` JSONL entry to the cache file, preserving any existing entries
- * that were restored from the previous cache snapshot.
+ * Appends a `{run_id, aic, timestamp}` JSONL entry to the cache file, preserving any existing
+ * entries that were restored from the previous cache snapshot and are within the 48-hour
+ * retention window.  Entries older than {@link CACHE_RETENTION_MS} are pruned to keep the
+ * cache file bounded.
  *
+ * @param {string} [cacheFilePath] Override the cache file path (defaults to {@link CACHE_FILE_PATH}; useful in tests).
+ * @param {string} [usageDir] Override the usage directory (defaults to {@link USAGE_DIR}; useful in tests).
  * @returns {Promise<void>}
  */
-async function main() {
+async function mainWithPaths(cacheFilePath, usageDir) {
+  const cachePath = cacheFilePath || CACHE_FILE_PATH;
+  const usageDirPath = usageDir || USAGE_DIR;
   try {
     const runId = Number(process.env.GITHUB_RUN_ID || 0);
     if (!runId) {
@@ -61,8 +70,8 @@ async function main() {
     }
 
     // Compute AIC from the usage JSONL files prepared by buildUsageArtifactUploadSteps.
-    const usageFiles = findJSONLFiles(USAGE_DIR);
-    logCache("Scanning usage JSONL files", { dir: USAGE_DIR, count: usageFiles.length, files: usageFiles });
+    const usageFiles = findJSONLFiles(usageDirPath);
+    logCache("Scanning usage JSONL files", { dir: usageDirPath, count: usageFiles.length, files: usageFiles });
     const aic = sumAICFromUsageJSONLFiles(usageFiles);
     logCache("Computed AIC for current run", { runId, aic });
 
@@ -76,32 +85,67 @@ async function main() {
     }
 
     // Read existing cache content (restored from the previous run's cache snapshot, if any).
-    let existingLines = "";
+    // Entries with a `timestamp` older than CACHE_RETENTION_MS are pruned to keep the file
+    // bounded.  Entries without a `timestamp` (written by an older version of this script)
+    // are preserved for backward compatibility.
+    /** @type {string[]} */
+    let keptLines = [];
     try {
-      if (fs.existsSync(CACHE_FILE_PATH)) {
-        existingLines = fs.readFileSync(CACHE_FILE_PATH, "utf8").trimEnd();
-        const lineCount = existingLines ? existingLines.split("\n").length : 0;
-        logCache("Loaded existing cache entries", { path: CACHE_FILE_PATH, lineCount });
+      if (fs.existsSync(cachePath)) {
+        const raw = fs.readFileSync(cachePath, "utf8").trimEnd();
+        const now = Date.now();
+        const cutoff = now - CACHE_RETENTION_MS;
+        let total = 0;
+        let pruned = 0;
+        for (const rawLine of raw.split("\n")) {
+          const line = rawLine.trim();
+          if (!line) continue;
+          total++;
+          try {
+            const entry = JSON.parse(line);
+            if (typeof entry?.timestamp === "string") {
+              const ts = Date.parse(entry.timestamp);
+              if (Number.isFinite(ts) && ts < cutoff) {
+                pruned++;
+                continue;
+              }
+            }
+            keptLines.push(line);
+          } catch {
+            // Preserve lines that cannot be parsed (defensive: avoids data loss).
+            keptLines.push(line);
+          }
+        }
+        logCache("Loaded existing cache entries", { path: cachePath, total, kept: keptLines.length, pruned });
       } else {
-        logCache("No existing cache file found; starting fresh", { path: CACHE_FILE_PATH });
+        logCache("No existing cache file found; starting fresh", { path: cachePath });
       }
     } catch (readErr) {
       core.warning(`[daily-aic-cache] Could not read existing cache file: ${getErrorMessage(readErr)}`);
     }
 
     // Build the updated JSONL content.
-    const newEntry = JSON.stringify({ run_id: runId, aic });
-    const updatedContent = existingLines ? `${existingLines}\n${newEntry}\n` : `${newEntry}\n`;
+    const newEntry = JSON.stringify({ run_id: runId, aic, timestamp: new Date().toISOString() });
+    const updatedContent = keptLines.length > 0 ? `${keptLines.join("\n")}\n${newEntry}\n` : `${newEntry}\n`;
 
     // Ensure the directory exists and write the updated file.
-    const dir = path.dirname(CACHE_FILE_PATH);
+    const dir = path.dirname(cachePath);
     fs.mkdirSync(dir, { recursive: true });
-    fs.writeFileSync(CACHE_FILE_PATH, updatedContent, "utf8");
-    logCache("Wrote cache entry", { runId, aic, path: CACHE_FILE_PATH });
+    fs.writeFileSync(cachePath, updatedContent, "utf8");
+    logCache("Wrote cache entry", { runId, aic, path: cachePath });
   } catch (error) {
     // Non-fatal: a cache write failure should never block the conclusion job.
     core.warning(`[daily-aic-cache] Failed to write usage cache: ${getErrorMessage(error)}`);
   }
 }
 
-module.exports = { main };
+/**
+ * Entry point called from the GitHub Actions step.
+ *
+ * @returns {Promise<void>}
+ */
+async function main() {
+  return mainWithPaths();
+}
+
+module.exports = { main, mainWithPaths };
diff --git a/actions/setup/js/write_daily_aic_usage_cache.test.cjs b/actions/setup/js/write_daily_aic_usage_cache.test.cjs
new file mode 100644
index 00000000000..2f3f85ffc60
--- /dev/null
+++ b/actions/setup/js/write_daily_aic_usage_cache.test.cjs
@@ -0,0 +1,123 @@
+// @ts-check
+import fs from "fs";
+import os from "os";
+import path from "path";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+let exports;
+
+describe("write_daily_aic_usage_cache", () => {
+  let tmpDir;
+  let cacheFile;
+  let usageDir;
+
+  beforeEach(async () => {
+    vi.resetModules();
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "write-aic-cache-test-"));
+    cacheFile = path.join(tmpDir, "agentic-workflow-usage-cache.jsonl");
+    usageDir = path.join(tmpDir, "usage");
+    fs.mkdirSync(usageDir, { recursive: true });
+
+    global.core = { info: vi.fn(), warning: vi.fn(), error: vi.fn(), setFailed: vi.fn() };
+    process.env.GITHUB_RUN_ID = "12345";
+
+    const mod = await import("./write_daily_aic_usage_cache.cjs");
+    exports = mod.default || mod;
+  });
+
+  afterEach(() => {
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+    delete global.core;
+    delete process.env.GITHUB_RUN_ID;
+  });
+
+  /**
+   * Writes a usage JSONL file in the temp usage directory so that sumAICFromUsageJSONLFiles
+   * can pick it up. The entry uses the `ai_credits` field for simplicity.
+   *
+   * @param {number} aiCredits
+   */
+  function writeUsageFile(aiCredits) {
+    fs.writeFileSync(path.join(usageDir, "agent_usage.jsonl"), JSON.stringify({ ai_credits: aiCredits }) + "\n", "utf8");
+  }
+
+  /**
+   * Invoke the main() function after patching the module-level paths to point to the
+   * temp directory. We call the function directly on the in-process module instance.
+   *
+   * @returns {Promise<void>}
+   */
+  async function runMain() {
+    // Patch the module-level path constants by calling main() on the already-imported
+    // module; to make the paths configurable we call the exported helper that accepts
+    // explicit path arguments (defined below), falling back to swapping env vars.
+    await exports.mainWithPaths(cacheFile, usageDir);
+  }
+
+  it("writes a new entry with run_id, aic, and a timestamp when no cache file exists", async () => {
+    writeUsageFile(7.5);
+    await runMain();
+
+    const content = fs.readFileSync(cacheFile, "utf8").trim();
+    const entry = JSON.parse(content);
+    expect(entry.run_id).toBe(12345);
+    expect(entry.aic).toBe(7.5);
+    expect(typeof entry.timestamp).toBe("string");
+    // Timestamp should be a valid ISO 8601 date within the last minute.
+    const ts = Date.parse(entry.timestamp);
+    expect(Number.isFinite(ts)).toBe(true);
+    expect(ts).toBeLessThanOrEqual(Date.now());
+    expect(ts).toBeGreaterThan(Date.now() - 60_000);
+  });
+
+  it("appends to an existing cache file and preserves entries within 48 h", async () => {
+    const recentTs = new Date(Date.now() - 2 * 60 * 60 * 1000).toISOString(); // 2 hours ago
+    fs.writeFileSync(cacheFile, JSON.stringify({ run_id: 9999, aic: 3.0, timestamp: recentTs }) + "\n", "utf8");
+
+    writeUsageFile(5.0);
+    await runMain();
+
+    const lines = fs.readFileSync(cacheFile, "utf8").trim().split("\n");
+    expect(lines).toHaveLength(2);
+    const first = JSON.parse(lines[0]);
+    expect(first.run_id).toBe(9999);
+    const second = JSON.parse(lines[1]);
+    expect(second.run_id).toBe(12345);
+    expect(second.aic).toBe(5.0);
+  });
+
+  it("prunes existing entries whose timestamp is older than 48 h", async () => {
+    const staleTs = new Date(Date.now() - 50 * 60 * 60 * 1000).toISOString(); // 50 hours ago
+    const recentTs = new Date(Date.now() - 1 * 60 * 60 * 1000).toISOString(); // 1 hour ago
+    fs.writeFileSync(cacheFile, [JSON.stringify({ run_id: 1001, aic: 1.0, timestamp: staleTs }), JSON.stringify({ run_id: 1002, aic: 2.0, timestamp: recentTs })].join("\n") + "\n", "utf8");
+
+    writeUsageFile(4.0);
+    await runMain();
+
+    const lines = fs.readFileSync(cacheFile, "utf8").trim().split("\n");
+    const runIds = lines.map(line => JSON.parse(line).run_id);
+    // Stale entry 1001 must be pruned; recent 1002 and new 12345 kept.
+    expect(runIds).not.toContain(1001);
+    expect(runIds).toContain(1002);
+    expect(runIds).toContain(12345);
+  });
+
+  it("preserves entries without a timestamp (backward compatibility)", async () => {
+    fs.writeFileSync(cacheFile, JSON.stringify({ run_id: 2001, aic: 8.0 }) + "\n", "utf8");
+
+    writeUsageFile(1.0);
+    await runMain();
+
+    const lines = fs.readFileSync(cacheFile, "utf8").trim().split("\n");
+    const runIds = lines.map(line => JSON.parse(line).run_id);
+    expect(runIds).toContain(2001);
+    expect(runIds).toContain(12345);
+  });
+
+  it("skips writing when GITHUB_RUN_ID is not set", async () => {
+    delete process.env.GITHUB_RUN_ID;
+    writeUsageFile(5.0);
+    await runMain();
+    expect(fs.existsSync(cacheFile)).toBe(false);
+  });
+});