diff --git a/changelog.d/fixes/9625-domain-cost-ms.md b/changelog.d/fixes/9625-domain-cost-ms.md
new file mode 100644
index 0000000000..37c3e1e7e6
--- /dev/null
+++ b/changelog.d/fixes/9625-domain-cost-ms.md
@@ -0,0 +1 @@
+- fix(db): align domain_cost_history cleanup cutoff with millisecond column (#9625)
diff --git a/src/lib/db/cleanup.ts b/src/lib/db/cleanup.ts
index 20d0358ce4..94d5995afe 100644
--- a/src/lib/db/cleanup.ts
+++ b/src/lib/db/cleanup.ts
@@ -253,14 +253,15 @@ export async function cleanupMemoryEntries(): Promise<CleanupResult> {
 
 /**
  * Clean up old domain_cost_history based on retention settings. (#6848)
- * Uses unix-epoch `timestamp` column (INTEGER).
+ * The `timestamp` column stores epoch milliseconds (saveCostEntry default
+ * is Date.now()), so the cutoff must be in milliseconds to match. (#9625)
  */
 export async function cleanupDomainCostHistory(): Promise<CleanupResult> {
   const db = getDbInstance();
   const retention = getRetentionSettings();
 
   const retentionDays = retention.domainCostHistory;
-  const cutoffEpoch = Math.floor(Date.now() / 1000) - retentionDays * 86_400;
+  const cutoffEpoch = Date.now() - retentionDays * 86_400_000;
 
   const result: CleanupResult = { deleted: 0, errors: 0 };
 
diff --git a/tests/unit/repro-9625.test.ts b/tests/unit/repro-9625.test.ts
new file mode 100644
index 0000000000..f6ff2f68cf
--- /dev/null
+++ b/tests/unit/repro-9625.test.ts
@@ -0,0 +1,92 @@
+/**
+ * Issue #9625 — domain_cost_history cleanup cutoff unit mismatch.
+ *
+ * cleanupDomainCostHistory() computes the cutoff in epoch seconds
+ * (Math.floor(Date.now() / 1000)) but the timestamp column stores
+ * epoch milliseconds (Date.now()), as inserted by saveCostEntry().
+ *
+ * This test seeds data using the same format as the production code
+ * (milliseconds), then asserts that cleanupDomainCostHistory() correctly
+ * deletes rows older than the retention window.
+ *
+ * Before the fix, the cutoff in seconds was ~1000× smaller than the
+ * stored timestamps, so the DELETE WHERE timestamp < cutoff would
+ * never match old rows — the cleanup was effectively a no-op.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-9625-"));
+process.env.DATA_DIR = TEST_DATA_DIR;
+
+const { cleanupDomainCostHistory } = await import("../../src/lib/db/cleanup.ts");
+const { getDbInstance, resetDbInstance } = await import("../../src/lib/db/core.ts");
+
+test.after(() => {
+  resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+const DAY_MS = 86_400_000; // milliseconds
+
+test("#9625 cleanupDomainCostHistory: cutoff in ms matches production timestamps", async () => {
+  const db = getDbInstance()!;
+  const now = Date.now(); // milliseconds — same as saveCostEntry() default
+
+  const insert = db.prepare(
+    "INSERT INTO domain_cost_history (api_key_id, cost, timestamp) VALUES (?, ?, ?)"
+  );
+
+  // Seed data using millisecond timestamps (production format).
+  // 3 old rows: 40 days ago (should be deleted)
+  // 2 recent rows: 5 days ago (should be kept)
+  insert.run("key1", 1.0, now - 40 * DAY_MS);
+  insert.run("key1", 2.0, now - 40 * DAY_MS);
+  insert.run("key1", 3.0, now - 40 * DAY_MS);
+  insert.run("key1", 4.0, now - 5 * DAY_MS);
+  insert.run("key1", 5.0, now - 5 * DAY_MS);
+
+  const result = await cleanupDomainCostHistory();
+
+  // Before the fix, cutoff was in seconds (~1.7e9) while timestamps
+  // are in milliseconds (~1.7e12). The comparison `WHERE ts < 1.7e9`
+  // would never match rows with ts ~1.7e12, so nothing was deleted.
+  assert.strictEqual(result.deleted, 3, "Should delete 3 old rows (40 days old)");
+  assert.strictEqual(result.errors, 0);
+
+  const remaining = db.prepare("SELECT COUNT(*) as cnt FROM domain_cost_history").get() as {
+    cnt: number;
+  };
+  assert.strictEqual(remaining.cnt, 2, "Should keep 2 recent rows (5 days old)");
+});
+
+test("#9625 unit mismatch: seconds cutoff would NOT match ms timestamps", () => {
+  // Demonstrate the arithmetic bug: a cutoff in seconds is ~1000×
+  // smaller than a millisecond timestamp, so the WHERE clause never
+  // matches production data.
+  const nowMs = Date.now();
+  const nowSec = Math.floor(nowMs / 1000);
+  const retentionDays = 30;
+  const cutoffSec = nowSec - retentionDays * 86_400; // seconds
+  const cutoffMs = nowMs - retentionDays * 86_400_000; // milliseconds
+
+  // A row inserted 40 days ago with a millisecond timestamp:
+  const oldRowMs = nowMs - 40 * 86_400_000; // ~1.7e12
+
+  // With seconds cutoff: oldRowMs (1.7e12) < cutoffSec (1.7e9) is FALSE
+  // because 1.7e12 > 1.7e9 — the row is never matched.
+  assert.ok(
+    oldRowMs > cutoffSec,
+    "Bug: ms timestamp is NOT less than seconds cutoff, so row is never deleted"
+  );
+
+  // With milliseconds cutoff: oldRowMs (1.7e12) < cutoffMs (1.7e12) is TRUE
+  assert.ok(
+    oldRowMs < cutoffMs,
+    "Fix: ms timestamp IS less than ms cutoff, so row is correctly deleted"
+  );
+});
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