diff --git a/src/shadowbox/server/api.yml b/src/shadowbox/server/api.yml
index 2532d51c..4dc44ffb 100644
--- a/src/shadowbox/server/api.yml
+++ b/src/shadowbox/server/api.yml
@@ -208,7 +208,8 @@ paths:
                       type: object
                       properties:
                         accessKeyId:
-                          type: integer
+                          description: Exact access key ID.
+                          type: string
                         tunnelTime:
                           type: object
                           properties:
diff --git a/src/shadowbox/server/manager_metrics.spec.ts b/src/shadowbox/server/manager_metrics.spec.ts
index ed3107ff..78bce70d 100644
--- a/src/shadowbox/server/manager_metrics.spec.ts
+++ b/src/shadowbox/server/manager_metrics.spec.ts
@@ -194,7 +194,7 @@ describe('PrometheusManagerMetrics', () => {
   },
   "accessKeys": [
     {
-      "accessKeyId": 0,
+      "accessKeyId": "0",
       "dataTransferred": {
         "bytes": 1000
       },
@@ -380,7 +380,7 @@ describe('PrometheusManagerMetrics', () => {
   },
   "accessKeys": [
     {
-      "accessKeyId": 1,
+      "accessKeyId": "1",
       "dataTransferred": {
         "bytes": 0
       },
@@ -396,7 +396,7 @@ describe('PrometheusManagerMetrics', () => {
       }
     },
     {
-      "accessKeyId": 0,
+      "accessKeyId": "0",
       "dataTransferred": {
         "bytes": 1000
       },
@@ -427,4 +427,33 @@ describe('PrometheusManagerMetrics', () => {
     expect(bytesTransferredByUserId['access-key-2']).toEqual(10000);
     done();
   });
+
+  it('preserves custom access key IDs in server metrics', async () => {
+    const accessKeyIds = ['custom-id', '001', '9007199254740993'];
+    const prometheusClient = {
+      query: async (query: string): Promise<QueryResultData> => {
+        if (query.includes('shadowsocks_data_bytes{')) {
+          return {
+            resultType: 'vector',
+            result: accessKeyIds.map((accessKey, index) => ({
+              metric: {access_key: accessKey},
+              value: [1, String(index + 1)],
+            })),
+          };
+        }
+        return {resultType: 'vector', result: []};
+      },
+      queryRange: async (query: string): Promise<QueryResultData> => {
+        if (query.includes('shadowsocks_data_bytes_per_location')) {
+          return {resultType: 'matrix', result: [{metric: {}, values: []}]};
+        }
+        return {resultType: 'matrix', result: []};
+      },
+    } as PrometheusClient;
+    const managerMetrics = new PrometheusManagerMetrics(prometheusClient);
+
+    const serverMetrics = await managerMetrics.getServerMetrics({seconds: 0});
+
+    expect(serverMetrics.accessKeys.map((entry) => entry.accessKeyId)).toEqual(accessKeyIds);
+  });
 });
diff --git a/src/shadowbox/server/manager_metrics.ts b/src/shadowbox/server/manager_metrics.ts
index 422f1162..981573f9 100644
--- a/src/shadowbox/server/manager_metrics.ts
+++ b/src/shadowbox/server/manager_metrics.ts
@@ -61,7 +61,7 @@ interface ServerMetricsLocationEntry {
 }
 
 interface ServerMetricsAccessKeyEntry {
-  accessKeyId: number;
+  accessKeyId: string;
   tunnelTime: Duration;
   dataTransferred: Data;
   connection: ConnectionStats;
@@ -88,7 +88,8 @@ export class PrometheusManagerMetrics implements ManagerMetrics {
     const result = await this.prometheusClient.query(
       `sum(increase(shadowsocks_data_bytes{dir=~"c<p|p>t"}[${timeframe.hours}h])) by (access_key)`
     );
-    const usage = {} as {[userId: string]: number};
+    // Custom IDs such as __proto__ must remain ordinary own properties.
+    const usage: {[userId: string]: number} = Object.create(null);
     for (const entry of result.result) {
       const bytes = Math.round(parseFloat(entry.value[1]));
       if (bytes === 0) {
@@ -297,7 +298,7 @@ function getServerMetricsAccessKeyEntry(
   let entry = map.get(accessKey);
   if (entry === undefined) {
     entry = {
-      accessKeyId: parseInt(accessKey),
+      accessKeyId: accessKey,
       dataTransferred: {bytes: 0},
       tunnelTime: {seconds: 0},
       connection: {