diff --git a/app/components/BillingCreditsViewer.vue b/app/components/BillingCreditsViewer.vue
index 2bdb783d..f925020b 100644
--- a/app/components/BillingCreditsViewer.vue
+++ b/app/components/BillingCreditsViewer.vue
@@ -219,6 +219,42 @@
               are hidden until GitHub starts returning attributed data.
             </div>
           </v-alert>
+          <v-alert
+            v-if="unmatchedBillingUsernamesList.length > 0"
+            type="warning"
+            variant="tonal"
+            density="comfortable"
+            class="mx-3 mt-3 mb-2"
+            icon="mdi-account-question-outline"
+          >
+            <div class="font-weight-medium mb-1">
+              {{ unmatchedBillingUsernamesList.length }} billing username<span v-if="unmatchedBillingUsernamesList.length !== 1">s</span>
+              with spend did not match the loaded Metrics API logins
+            </div>
+            <div class="text-body-2 mb-2">
+              This can happen on EMU enterprises when GitHub's metrics and billing feeds use
+              different handles for the same person. Configure <code>NUXT_BILLING_USER_ALIASES</code>
+              to map billing usernames to metrics logins.
+            </div>
+            <v-expansion-panels variant="accordion">
+              <v-expansion-panel>
+                <v-expansion-panel-title class="text-body-2">
+                  Show unmatched billing usernames
+                </v-expansion-panel-title>
+                <v-expansion-panel-text>
+                  <v-chip
+                    v-for="username in unmatchedBillingUsernamesList"
+                    :key="username"
+                    size="small"
+                    variant="tonal"
+                    class="ma-1"
+                  >
+                    {{ username }}
+                  </v-chip>
+                </v-expansion-panel-text>
+              </v-expansion-panel>
+            </v-expansion-panels>
+          </v-alert>
           <v-row v-if="perUserRows.length > 0 && !noPerUserAttribution" dense class="px-3 mt-2">
             <v-col cols="12" md="6">
               <v-card variant="outlined">
@@ -516,6 +552,7 @@ export default defineComponent({
     interface BillingAgg { credits: number; grossAmount: number; netAmount: number; models: Set<string>; display: string }
     const billingByLogin = reactive(new Map<string, BillingAgg>());
     const loadedLogins = reactive(new Set<string>());
+    const unmatchedBillingUsernames = reactive(new Set<string>());
     const perUserLoading = ref(false);
 
     // When the user switches month or toggles month view, drop cached
@@ -523,6 +560,7 @@ export default defineComponent({
     watch([selectedMonth, monthView, () => props.queryParams.since, () => props.queryParams.until], () => {
       billingByLogin.clear();
       loadedLogins.clear();
+      unmatchedBillingUsernames.clear();
     });
 
     async function loadBillingForLogins(logins: string[]): Promise<void> {
@@ -552,10 +590,15 @@ export default defineComponent({
           const qp: Record<string, string> = { ...parent, logins: chunk.join(',') };
           try {
             const resp = await $fetch<BillingCreditsResponse>('/api/billing-credits-by-user', { query: qp });
+            for (const username of resp.unmatchedBillingUsernames ?? []) {
+              const trimmed = username.trim();
+              if (trimmed) unmatchedBillingUsernames.add(trimmed);
+            }
             for (const it of resp.usageItems ?? []) {
               const u = (it.user || '').trim();
               if (!u) continue;
               const key = u.toLowerCase();
+              unmatchedBillingUsernames.delete(u);
               const prev = billingByLogin.get(key) || { credits: 0, grossAmount: 0, netAmount: 0, models: new Set<string>(), display: u };
               prev.credits += Number.isFinite(it.netQuantity) ? it.netQuantity : 0;
               prev.credits += Number.isFinite(it.discountQuantity) ? it.discountQuantity : 0;
@@ -646,6 +689,9 @@ export default defineComponent({
       });
     });
     const loadedLoginsCount = computed(() => loadedLogins.size);
+    const unmatchedBillingUsernamesList = computed(() =>
+      [...unmatchedBillingUsernames].sort((a, b) => a.localeCompare(b))
+    );
 
     // True when we've loaded at least one page of users AND the aggregate
     // totals show non-zero spend AND zero per-user attribution has come back.
@@ -683,6 +729,10 @@ export default defineComponent({
         billingByLogin.set(key, prev);
         loadedLogins.add(key);
       }
+      for (const username of data.value?.unmatchedBillingUsernames ?? []) {
+        const trimmed = username.trim();
+        if (trimmed) unmatchedBillingUsernames.add(trimmed);
+      }
     });
 
     // Distinguish "our admin gate" 403 from "GitHub billing API" 403 from
@@ -832,6 +882,7 @@ export default defineComponent({
       errorReason, headers,
       perUserRows, perUserHeaders,
       perUserLoading, loadedLoginsCount, onTableOptions, noPerUserAttribution,
+      unmatchedBillingUsernamesList,
       topSpendersChartData, topSpendersChartOptions,
       topTokensChartData, topTokensChartOptions,
       dataSourceBadge,
diff --git a/server/api/billing-credits-by-user.get.ts b/server/api/billing-credits-by-user.get.ts
index 089c9108..fea5f605 100644
--- a/server/api/billing-credits-by-user.get.ts
+++ b/server/api/billing-credits-by-user.get.ts
@@ -35,6 +35,10 @@ import {
   resolveWindow,
   aggregateForBillingByUser,
 } from '../services/billing-credit-reader';
+import {
+  billingUsernamesForMetricsLogins,
+  parseBillingUserAliases,
+} from '../../shared/utils/billing-user-identity';
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 import mockBilling from '../../public/mock-data/billing-credits.json';
 
@@ -189,13 +193,14 @@ export default defineEventHandler(async (event): Promise<BillingCreditsResponse>
   });
 
   const tagged: BillingUsageItem[] = [];
+  const aliases = parseBillingUserAliases(process.env.NUXT_BILLING_USER_ALIASES);
   let timePeriod: BillingCreditsResponse['timePeriod'] = { year: 0, month: 0 };
   let orgSlug: string | undefined;
   let entSlug: string | undefined;
   let failures = 0;
 
-  async function fetchOne(login: string): Promise<void> {
-    const params = new URLSearchParams({ ...forwardParams, user: login });
+  async function fetchOne(metricsLogin: string, billingUsername: string): Promise<void> {
+    const params = new URLSearchParams({ ...forwardParams, user: billingUsername });
     const url = `${apiUrl}?${params.toString()}`;
     try {
       const resp = await $fetch<BillingCreditsResponse>(url, { headers: billingHeaders });
@@ -203,23 +208,27 @@ export default defineEventHandler(async (event): Promise<BillingCreditsResponse>
       orgSlug = resp.organization || orgSlug;
       entSlug = resp.enterprise || entSlug;
       for (const it of resp.usageItems ?? []) {
-        tagged.push({ ...it, user: login });
+        tagged.push({ ...it, user: metricsLogin });
       }
     } catch (err) {
       failures++;
-      logger.warn(`billing-credits-by-user: ${login} fan-out failed`, err);
+      logger.warn(`billing-credits-by-user: ${billingUsername} fan-out failed`, err);
     }
   }
 
+  const fetchTargets = requestedLogins.flatMap(login =>
+    billingUsernamesForMetricsLogins([login], aliases).map(billingUsername => ({ login, billingUsername }))
+  );
+
   let cursor = 0;
   async function worker(): Promise<void> {
-    while (cursor < requestedLogins.length) {
+    while (cursor < fetchTargets.length) {
       const i = cursor++;
-      const login = requestedLogins[i];
-      if (login) await fetchOne(login);
+      const target = fetchTargets[i];
+      if (target) await fetchOne(target.login, target.billingUsername);
     }
   }
-  await Promise.all(Array.from({ length: Math.min(CONCURRENCY, requestedLogins.length) }, () => worker()));
+  await Promise.all(Array.from({ length: Math.min(CONCURRENCY, fetchTargets.length) }, () => worker()));
 
   if (failures > 0 && tagged.length === 0) {
     throw createError({
diff --git a/server/api/billing-credits.get.ts b/server/api/billing-credits.get.ts
index 68b3d6c1..16bb8dbd 100644
--- a/server/api/billing-credits.get.ts
+++ b/server/api/billing-credits.get.ts
@@ -64,6 +64,7 @@ export interface BillingCreditsResponse {
   organization?: string;
   enterprise?: string;
   user?: string;
+  unmatchedBillingUsernames?: string[];
   usageItems: BillingUsageItem[];
 }
 
diff --git a/server/services/billing-credit-reader.ts b/server/services/billing-credit-reader.ts
index 15b41295..61a815d4 100644
--- a/server/services/billing-credit-reader.ts
+++ b/server/services/billing-credit-reader.ts
@@ -30,6 +30,11 @@
 
 import { getPool } from '../storage/db';
 import type { BillingCreditsResponse, BillingUsageItem } from '../api/billing-credits.get';
+import {
+  billingUsernamesForMetricsLogins,
+  normalizeBillingUsername,
+  parseBillingUserAliases,
+} from '../../shared/utils/billing-user-identity';
 
 export interface CoverageDecision {
   source: 'db' | 'live';
@@ -284,6 +289,8 @@ export async function aggregateForBillingByUser(
   }
 
   const pool = getPool();
+  const aliases = parseBillingUserAliases(process.env.NUXT_BILLING_USER_ALIASES);
+  const matchedBillingUsernames = billingUsernamesForMetricsLogins(logins, aliases);
 
   const conds: string[] = [
     'enterprise = $1',
@@ -294,7 +301,7 @@ export async function aggregateForBillingByUser(
     enterprise,
     window.startDate,
     window.endDate,
-    logins.map(l => l.toLowerCase()),
+    matchedBillingUsernames,
   ];
   const push = (col: string, val: string | undefined) => {
     if (val === undefined || val === '') return;
@@ -327,12 +334,30 @@ export async function aggregateForBillingByUser(
   const { rows } = await pool.query(sql, params);
   const usageItems: BillingUsageItem[] = rows.map(r => ({
     ...mapAggregateRowToItem(r),
-    user: r.username || undefined,
+    user: r.username ? normalizeBillingUsername(r.username, aliases) : undefined,
   }));
 
+  const unmatchedConds = [...conds];
+  const unmatchedParams = [...params];
+  unmatchedConds[2] = 'LOWER(username) <> ALL($4::text[])';
+  unmatchedConds.push('(quantity <> 0 OR gross_amount <> 0 OR net_amount <> 0)');
+  const unmatchedSql = `
+    SELECT DISTINCT username
+    FROM billing_credit_usage
+    WHERE ${unmatchedConds.join(' AND ')}
+      AND username IS NOT NULL
+      AND username <> ''
+    ORDER BY username
+  `;
+  const unmatchedResult = await pool.query(unmatchedSql, unmatchedParams);
+  const unmatchedBillingUsernames = (unmatchedResult?.rows ?? [])
+    .map((r: { username?: string }) => (r.username || '').trim())
+    .filter(Boolean);
+
   return {
     timePeriod: window.timePeriod,
     enterprise,
+    ...(unmatchedBillingUsernames.length ? { unmatchedBillingUsernames } : {}),
     usageItems,
   };
 }
diff --git a/shared/utils/billing-user-identity.ts b/shared/utils/billing-user-identity.ts
new file mode 100644
index 00000000..a2f74d19
--- /dev/null
+++ b/shared/utils/billing-user-identity.ts
@@ -0,0 +1,39 @@
+export type BillingUserAliases = Record<string, string>;
+
+function canonicalUserKey(value: string): string {
+  return value.trim().toLowerCase();
+}
+
+export function parseBillingUserAliases(raw: string | undefined): BillingUserAliases {
+  if (!raw?.trim()) return {};
+  try {
+    const parsed = JSON.parse(raw) as Record<string, unknown>;
+    const aliases: BillingUserAliases = {};
+    for (const [billingUsername, metricsLogin] of Object.entries(parsed)) {
+      if (typeof metricsLogin !== 'string') continue;
+      const billingKey = canonicalUserKey(billingUsername);
+      const metricsKey = canonicalUserKey(metricsLogin);
+      if (billingKey && metricsKey) aliases[billingKey] = metricsKey;
+    }
+    return aliases;
+  } catch {
+    return {};
+  }
+}
+
+export function normalizeBillingUsername(username: string, aliases: BillingUserAliases): string {
+  const key = canonicalUserKey(username);
+  return aliases[key] || key;
+}
+
+export function billingUsernamesForMetricsLogins(
+  logins: string[],
+  aliases: BillingUserAliases,
+): string[] {
+  const metricsKeys = new Set(logins.map(canonicalUserKey).filter(Boolean));
+  const usernames = new Set(metricsKeys);
+  for (const [billingUsername, metricsLogin] of Object.entries(aliases)) {
+    if (metricsKeys.has(metricsLogin)) usernames.add(billingUsername);
+  }
+  return [...usernames];
+}
diff --git a/tests/billing-credit-reader.spec.ts b/tests/billing-credit-reader.spec.ts
index 15cca240..60ecbcbb 100644
--- a/tests/billing-credit-reader.spec.ts
+++ b/tests/billing-credit-reader.spec.ts
@@ -28,6 +28,7 @@ import {
 
 beforeEach(() => {
   mockQuery.mockReset();
+  delete process.env.NUXT_BILLING_USER_ALIASES;
 });
 
 describe('resolveWindow', () => {
@@ -259,6 +260,51 @@ describe('aggregateForBillingByUser', () => {
     expect(mockQuery).not.toHaveBeenCalled();
   });
 
+  it('includes configured EMU billing aliases in the DB username filter and normalizes returned users', async () => {
+    process.env.NUXT_BILLING_USER_ALIASES = JSON.stringify({
+      readable_emu: 'opaquehash_emu',
+    });
+    mockQuery
+      .mockResolvedValueOnce({
+        rows: [{
+          username: 'readable_emu',
+          product: 'copilot',
+          sku: 'copilot_ai_credit',
+          model: 'gpt-4o',
+          unit_type: 'credits',
+          price_per_unit: 0.01,
+          gross_quantity: 100,
+          gross_amount: 1,
+          discount_amount: 0,
+          net_amount: 1,
+        }],
+      })
+      .mockResolvedValueOnce({ rows: [] });
+
+    const resp = await aggregateForBillingByUser('ent', {
+      startDate: '2026-06-01', endDate: '2026-06-30', timePeriod: { year: 2026, month: 6 },
+    }, ['opaquehash_emu']);
+
+    expect(mockQuery.mock.calls[0]![1][3]).toEqual(['opaquehash_emu', 'readable_emu']);
+    expect(resp.usageItems[0]!.user).toBe('opaquehash_emu');
+  });
+
+  it('surfaces billing usernames with spend that are not matched by requested metrics logins or aliases', async () => {
+    process.env.NUXT_BILLING_USER_ALIASES = JSON.stringify({
+      readable_emu: 'opaquehash_emu',
+    });
+    mockQuery
+      .mockResolvedValueOnce({ rows: [] })
+      .mockResolvedValueOnce({ rows: [{ username: 'unmapped_emu' }] });
+
+    const resp = await aggregateForBillingByUser('ent', {
+      startDate: '2026-06-01', endDate: '2026-06-30', timePeriod: { year: 2026, month: 6 },
+    }, ['opaquehash_emu']);
+
+    expect(resp.unmatchedBillingUsernames).toEqual(['unmapped_emu']);
+    expect(mockQuery.mock.calls[1]![0]).toMatch(/LOWER\(username\) <> ALL/);
+  });
+
   it('groups by username and tags each item with the user field (case-insensitive match)', async () => {
     mockQuery.mockResolvedValueOnce({
       rows: [