diff --git a/app/middleware/rate-limit-key.js b/app/middleware/rate-limit-key.js
new file mode 100644
index 0000000..71afccd
--- /dev/null
+++ b/app/middleware/rate-limit-key.js
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright 2026 Aaron K. Clark
+"use strict";
+
+/**
+ * Key generator for express-rate-limit.
+ *
+ * Authenticated requests are keyed by a sha256 hash prefix of the
+ * `authKey` header — so two clients sharing an IP (mobile carrier
+ * NAT, corporate proxy, etc.) get independent rate budgets, and a
+ * brute-force attacker rotating source IPs can't stretch a
+ * per-IP budget by switching networks. Anonymous requests fall
+ * back to IP (the brute-force path, where per-IP is the right
+ * granularity).
+ *
+ * The hash prefix (16 hex chars = 64 bits) is plenty unique for
+ * keyspace separation and keeps the raw token out of any
+ * downstream rate-limiter store.
+ *
+ * Exported separately from server.js so unit tests can exercise
+ * the keying directly without spinning up an HTTP server.
+ */
+
+const crypto = require('crypto');
+
+function keyByAuthKeyOrIp(req /*, res */) {
+    const authKey = req.get && req.get('authKey');
+    if (authKey) {
+        return 'k:' + crypto.createHash('sha256').update(authKey).digest('hex').slice(0, 16);
+    }
+    return 'ip:' + (req.ip || (req.connection && req.connection.remoteAddress) || 'unknown');
+}
+
+module.exports = { keyByAuthKeyOrIp };
diff --git a/server.js b/server.js
index 1071083..fcd3645 100644
--- a/server.js
+++ b/server.js
@@ -117,14 +117,25 @@ app.use(express.json({
 }));
 
 // Rate limit the v1 surface to defend against authKey brute-force.
-// Defaults: 100 requests / 15-minute window per IP. Operators can
-// tune via RATE_LIMIT_WINDOW_MS and RATE_LIMIT_MAX. Set
-// RATE_LIMIT_MAX=0 to disable entirely (e.g. for load testing).
-// /healthz is intentionally NOT rate-limited so orchestrator probes
-// never trip it.
+// Defaults: 100 requests / 15-minute window. Operators can tune via
+// RATE_LIMIT_WINDOW_MS and RATE_LIMIT_MAX. Set RATE_LIMIT_MAX=0 to
+// disable entirely (e.g. for load testing). /healthz is intentionally
+// NOT rate-limited so orchestrator probes never trip it.
+//
+// Key derivation:
+//   - Authenticated requests (authKey header present): key by the
+//     hash prefix of that authKey. Mobile-carrier-NAT users sharing
+//     an IP no longer poison each other's budget; brute-force
+//     attempts get cut off per-key regardless of how many IPs the
+//     attacker rotates through.
+//   - Anonymous requests (no header): key by IP, the
+//     express-rate-limit default. This is the brute-force path —
+//     someone trying keys to find a valid one — and per-IP is the
+//     right granularity there.
 const rateLimitMax = parseInt(process.env.RATE_LIMIT_MAX, 10);
 const rateLimitWindowMs = parseInt(process.env.RATE_LIMIT_WINDOW_MS, 10);
 if (rateLimitMax !== 0) {
+    const { keyByAuthKeyOrIp } = require('./app/middleware/rate-limit-key.js');
     const v1Limiter = rateLimit({
         windowMs: Number.isFinite(rateLimitWindowMs) && rateLimitWindowMs > 0
             ? rateLimitWindowMs
@@ -135,6 +146,7 @@ if (rateLimitMax !== 0) {
         standardHeaders: true,   // RateLimit-* headers
         legacyHeaders: false,    // no X-RateLimit-* legacy headers
         message: { message: 'Too many requests — try again later.' },
+        keyGenerator: keyByAuthKeyOrIp,
     });
     app.use('/v1', v1Limiter);
 }
diff --git a/tests/unit/rate-limit-key.test.js b/tests/unit/rate-limit-key.test.js
new file mode 100644
index 0000000..00d27d6
--- /dev/null
+++ b/tests/unit/rate-limit-key.test.js
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright 2026 Aaron K. Clark
+//
+// Unit tests for the express-rate-limit key generator. Verifies:
+//   - authKey-present requests key by a hash-prefixed string
+//   - same authKey → same key (deterministic), regardless of IP
+//   - different authKeys → different keys (independent budgets)
+//   - no authKey → falls back to IP
+//   - raw authKey value NEVER appears in the returned key
+
+import { describe, test, expect } from 'vitest';
+import { keyByAuthKeyOrIp } from '../../app/middleware/rate-limit-key.js';
+
+function fakeReq({ authKey, ip } = {}) {
+    return {
+        get: (h) => (h === 'authKey' ? authKey : undefined),
+        ip,
+    };
+}
+
+describe('keyByAuthKeyOrIp', () => {
+    test('returns a `k:` prefixed hash when authKey is set', () => {
+        const k = keyByAuthKeyOrIp(fakeReq({ authKey: 'live-token-abc' }));
+        expect(k.startsWith('k:')).toBe(true);
+        // 16 hex chars after the prefix.
+        expect(k).toMatch(/^k:[0-9a-f]{16}$/);
+    });
+
+    test('the raw authKey is never in the returned key', () => {
+        const secret = 'super-secret-token-xyz';
+        const k = keyByAuthKeyOrIp(fakeReq({ authKey: secret }));
+        expect(k.includes(secret)).toBe(false);
+    });
+
+    test('same authKey → same key regardless of IP', () => {
+        const a = keyByAuthKeyOrIp(fakeReq({ authKey: 'tok', ip: '1.2.3.4' }));
+        const b = keyByAuthKeyOrIp(fakeReq({ authKey: 'tok', ip: '5.6.7.8' }));
+        expect(a).toBe(b);
+    });
+
+    test('different authKeys → different keys', () => {
+        const a = keyByAuthKeyOrIp(fakeReq({ authKey: 'tok-a' }));
+        const b = keyByAuthKeyOrIp(fakeReq({ authKey: 'tok-b' }));
+        expect(a).not.toBe(b);
+    });
+
+    test('no authKey + present IP → `ip:` prefix with the IP', () => {
+        const k = keyByAuthKeyOrIp(fakeReq({ ip: '203.0.113.42' }));
+        expect(k).toBe('ip:203.0.113.42');
+    });
+
+    test('no authKey + no IP → falls back to "unknown"', () => {
+        const k = keyByAuthKeyOrIp(fakeReq({}));
+        expect(k).toBe('ip:unknown');
+    });
+
+    test('two anonymous requests from the same IP get the same key (per-IP fallback works)', () => {
+        const a = keyByAuthKeyOrIp(fakeReq({ ip: '1.1.1.1' }));
+        const b = keyByAuthKeyOrIp(fakeReq({ ip: '1.1.1.1' }));
+        expect(a).toBe(b);
+    });
+});