diff --git a/scripts/format-scope.json b/scripts/format-scope.json
index eee546dd5..9b82af4b7 100644
--- a/scripts/format-scope.json
+++ b/scripts/format-scope.json
@@ -79,6 +79,8 @@
   "src/data/terrainHeightsProxy.test.mjs",
   "src/data/tomtomTiles.js",
   "src/data/tomtomTiles.test.mjs",
+  "src/data/tileBudget.js",
+  "src/data/tileBudget.test.mjs",
   "src/data/firmsCsv.js",
   "src/data/firmsCsv.test.mjs",
   "src/tooling/environmentProviders.test.mjs",
diff --git a/scripts/package-boundaries.json b/scripts/package-boundaries.json
index 189192f10..748f11d44 100644
--- a/scripts/package-boundaries.json
+++ b/scripts/package-boundaries.json
@@ -113,12 +113,16 @@
   "traffic-provider": {
     "runtime": "node",
     "exports": ["./server/providers/traffic"],
-    "modules": ["server/providers/traffic.js", "src/data/tomtomTiles.js"],
+    "modules": [
+      "server/providers/traffic.js",
+      "src/data/tomtomTiles.js",
+      "src/data/tileBudget.js"
+    ],
     "external": []
   },
   "traffic-source": {
     "exports": ["./sources/traffic"],
-    "modules": ["src/data/tomtomTiles.js"],
+    "modules": ["src/data/tomtomTiles.js", "src/data/tileBudget.js"],
     "external": []
   },
   "firms-csv": {
@@ -1009,6 +1013,7 @@
       "src/data/cameraSensitivity.js",
       "src/data/flowMatch.js",
       "src/data/tomtomTiles.js",
+      "src/data/tileBudget.js",
       "src/data/trafficBounds.js",
       "src/data/trafficFlowStyle.js",
       "src/data/trafficPresetStyle.js",
@@ -1424,6 +1429,7 @@
       "src/data/telegeographySubmarineCables.js",
       "src/data/terrainHeights.js",
       "src/data/tomtomTiles.js",
+      "src/data/tileBudget.js",
       "src/data/tr3bRegistry.js",
       "src/data/trackedCamera.js",
       "src/data/trackedModelRegime.js",
@@ -2071,6 +2077,7 @@
       "src/data/scenePick.js",
       "src/data/spriteOrder.js",
       "src/data/tomtomTiles.js",
+      "src/data/tileBudget.js",
       "src/data/tr3bRegistry.js",
       "src/data/trackedCamera.js",
       "src/data/trackedModelRegime.js",
@@ -2707,6 +2714,7 @@
     "exports": ["./layers/traffic/source"],
     "modules": [
       "src/data/tomtomTiles.js",
+      "src/data/tileBudget.js",
       "src/layers/traffic/flowDecode.js",
       "src/layers/traffic/flowSource.js",
       "src/layers/traffic/source.js",
diff --git a/src/data/tileBudget.js b/src/data/tileBudget.js
new file mode 100644
index 000000000..dcc44a28c
--- /dev/null
+++ b/src/data/tileBudget.js
@@ -0,0 +1,53 @@
+/**
+ * @file Upstream-request accounting shared by the keyed tile proxies.
+ *
+ * A soft cap per UTC period, persisted as `{date, count}` so it survives a
+ * restart. Nothing here is provider-specific: the caller picks the period by
+ * the key it passes, and decides what happens over the cap.
+ *
+ * Zero dependencies and Cesium-free, so providers and browser helpers can
+ * import it and node:test can exercise it directly.
+ *
+ * @module data/tileBudget
+ */
+
+/**
+ * UTC calendar-day key for budget bucketing.
+ *
+ * @param {number} [epochMs=Date.now()] - Timestamp in ms.
+ * @returns {string} 'YYYY-MM-DD' in UTC.
+ */
+export function utcDayKey(epochMs = Date.now()) {
+  return new Date(epochMs).toISOString().slice(0, 10);
+}
+
+/**
+ * Normalize a persisted budget state against today's UTC day key.
+ * Rolls the counter to zero on day change; replaces missing/corrupt state.
+ * Returns the SAME object when it is already valid for `dayKey` (cheap to
+ * call on every request).
+ *
+ * @param {{date:string, count:number}|null|undefined} state - Persisted state.
+ * @param {string} dayKey - Today's UTC day key (from `utcDayKey`).
+ * @returns {{date:string, count:number}} Valid state for `dayKey`.
+ */
+export function normalizeBudget(state, dayKey) {
+  const valid =
+    Boolean(state) &&
+    state.date === dayKey &&
+    Number.isFinite(state.count) &&
+    state.count >= 0;
+  return valid ? state : { date: dayKey, count: 0 };
+}
+
+/**
+ * Whether the daily soft cap has been reached.
+ *
+ * @param {{count:number}} state - Normalized budget state.
+ * @param {number} limit - Daily tile budget; non-positive/invalid never blocks.
+ * @returns {boolean} True when `count >= limit`.
+ */
+export function isOverBudget(state, limit) {
+  if (!Number.isFinite(limit) || limit <= 0) return false;
+  return state.count >= limit;
+}
diff --git a/src/data/tileBudget.test.mjs b/src/data/tileBudget.test.mjs
new file mode 100644
index 000000000..fa48440bb
--- /dev/null
+++ b/src/data/tileBudget.test.mjs
@@ -0,0 +1,26 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { isOverBudget, normalizeBudget, utcDayKey } from './tileBudget.js';
+import * as tomtom from './tomtomTiles.js';
+
+test('tile budget helpers live in one module and TomTom re-exports them', () => {
+  assert.equal(tomtom.utcDayKey, utcDayKey);
+  assert.equal(tomtom.normalizeBudget, normalizeBudget);
+  assert.equal(tomtom.isOverBudget, isOverBudget);
+});
+
+test('normalizeBudget keeps a valid state and resets another period', () => {
+  const state = { date: '2026-09-24', count: 3 };
+  assert.equal(normalizeBudget(state, '2026-09-24'), state);
+  assert.deepEqual(normalizeBudget(state, '2026-09-25'), {
+    date: '2026-09-25',
+    count: 0,
+  });
+  assert.deepEqual(
+    normalizeBudget({ date: '2026-09-24', count: -1 }, '2026-09-24'),
+    {
+      date: '2026-09-24',
+      count: 0,
+    },
+  );
+});
diff --git a/src/data/tomtomTiles.js b/src/data/tomtomTiles.js
index af4fc95b3..a1dbc18cb 100644
--- a/src/data/tomtomTiles.js
+++ b/src/data/tomtomTiles.js
@@ -131,42 +131,7 @@ export function tilesForBounds(bounds, zoom = 12, { maxTiles = 64 } = {}) {
 // ─── Daily budget accounting ───────────────────────────────
 
 /**
- * UTC calendar-day key for budget bucketing.
- *
- * @param {number} [epochMs=Date.now()] - Timestamp in ms.
- * @returns {string} 'YYYY-MM-DD' in UTC.
+ * Re-exported from `./tileBudget.js`, which keyed tile proxies share. The
+ * traffic provider and its tests keep one import for tile math and budget.
  */
-export function utcDayKey(epochMs = Date.now()) {
-  return new Date(epochMs).toISOString().slice(0, 10);
-}
-
-/**
- * Normalize a persisted budget state against today's UTC day key.
- * Rolls the counter to zero on day change; replaces missing/corrupt state.
- * Returns the SAME object when it is already valid for `dayKey` (cheap to
- * call on every request).
- *
- * @param {{date:string, count:number}|null|undefined} state - Persisted state.
- * @param {string} dayKey - Today's UTC day key (from `utcDayKey`).
- * @returns {{date:string, count:number}} Valid state for `dayKey`.
- */
-export function normalizeBudget(state, dayKey) {
-  const valid =
-    Boolean(state) &&
-    state.date === dayKey &&
-    Number.isFinite(state.count) &&
-    state.count >= 0;
-  return valid ? state : { date: dayKey, count: 0 };
-}
-
-/**
- * Whether the daily soft cap has been reached.
- *
- * @param {{count:number}} state - Normalized budget state.
- * @param {number} limit - Daily tile budget; non-positive/invalid never blocks.
- * @returns {boolean} True when `count >= limit`.
- */
-export function isOverBudget(state, limit) {
-  if (!Number.isFinite(limit) || limit <= 0) return false;
-  return state.count >= limit;
-}
+export { utcDayKey, normalizeBudget, isOverBudget } from './tileBudget.js';