diff --git a/apps/server/src/integrations/environment/trust-proxy.util.spec.ts b/apps/server/src/integrations/environment/trust-proxy.util.spec.ts
new file mode 100644
index 00000000..294c6fba
--- /dev/null
+++ b/apps/server/src/integrations/environment/trust-proxy.util.spec.ts
@@ -0,0 +1,50 @@
+import { resolveTrustProxy } from './trust-proxy.util';
+
+/**
+ * Unit tests for resolveTrustProxy: the helper that turns the TRUST_PROXY env
+ * string into a Fastify trustProxy value. The contract is: empty/undefined
+ * falls back to the safe loopback/linklocal/uniquelocal default (so a public-IP
+ * client cannot spoof X-Forwarded-For); 'true'/'false' become booleans; a
+ * non-negative integer becomes a hop count (number); anything else (CIDR/IP
+ * lists, negative numbers, named keywords) is passed through verbatim as a
+ * trimmed string.
+ */
+describe('resolveTrustProxy', () => {
+  const SAFE_DEFAULT = 'loopback, linklocal, uniquelocal';
+
+  it('returns the safe default for an empty string', () => {
+    expect(resolveTrustProxy('')).toBe(SAFE_DEFAULT);
+  });
+
+  it('returns the safe default for undefined', () => {
+    expect(resolveTrustProxy(undefined)).toBe(SAFE_DEFAULT);
+  });
+
+  it("returns the boolean true for 'true'", () => {
+    expect(resolveTrustProxy('true')).toBe(true);
+  });
+
+  it("returns the boolean false for 'false'", () => {
+    expect(resolveTrustProxy('false')).toBe(false);
+  });
+
+  it("returns the number 2 for '2'", () => {
+    expect(resolveTrustProxy('2')).toBe(2);
+  });
+
+  it("trims surrounding whitespace and returns the number 3 for '  3 '", () => {
+    expect(resolveTrustProxy('  3 ')).toBe(3);
+  });
+
+  it('passes a CIDR string through unchanged', () => {
+    expect(resolveTrustProxy('10.0.0.0/8')).toBe('10.0.0.0/8');
+  });
+
+  it("passes a negative number through as a string ('-1' is not a valid hop count)", () => {
+    expect(resolveTrustProxy('-1')).toBe('-1');
+  });
+
+  it('passes a non-numeric keyword through unchanged', () => {
+    expect(resolveTrustProxy('loopback')).toBe('loopback');
+  });
+});
diff --git a/apps/server/src/integrations/environment/trust-proxy.util.ts b/apps/server/src/integrations/environment/trust-proxy.util.ts
new file mode 100644
index 00000000..176e0654
--- /dev/null
+++ b/apps/server/src/integrations/environment/trust-proxy.util.ts
@@ -0,0 +1,14 @@
+// Trust X-Forwarded-For ONLY from real proxies on private/loopback nets by
+// default, so a public-IP client cannot spoof its IP via X-Forwarded-For.
+// TRUST_PROXY env overrides: 'true'/'false', a hop count (integer), or a
+// CIDR/IP list string passed through to Fastify/proxy-addr.
+export function resolveTrustProxy(
+  rawInput?: string,
+): boolean | number | string {
+  const raw = rawInput?.trim();
+  if (raw == null || raw === '') return 'loopback, linklocal, uniquelocal';
+  if (raw === 'true') return true;
+  if (raw === 'false') return false;
+  const n = Number(raw);
+  return Number.isInteger(n) && n >= 0 ? n : raw;
+}
diff --git a/apps/server/src/main.ts b/apps/server/src/main.ts
index 3588f045..05968d09 100644
--- a/apps/server/src/main.ts
+++ b/apps/server/src/main.ts
@@ -14,19 +14,7 @@ import fastifyIp from 'fastify-ip';
 import { InternalLogFilter } from './common/logger/internal-log-filter';
 import { EnvironmentService } from './integrations/environment/environment.service';
 import { resolveFrameHeader } from './common/helpers';
-
-// Trust X-Forwarded-For ONLY from real proxies on private/loopback nets by
-// default, so a public-IP client cannot spoof its IP via X-Forwarded-For.
-// TRUST_PROXY env overrides: 'true'/'false', a hop count (integer), or a
-// CIDR/IP list string passed through to Fastify/proxy-addr.
-function resolveTrustProxy(rawInput?: string): boolean | number | string {
-  const raw = rawInput?.trim();
-  if (raw == null || raw === '') return 'loopback, linklocal, uniquelocal';
-  if (raw === 'true') return true;
-  if (raw === 'false') return false;
-  const n = Number(raw);
-  return Number.isInteger(n) && n >= 0 ? n : raw;
-}
+import { resolveTrustProxy } from './integrations/environment/trust-proxy.util';
 
 async function bootstrap() {
   const app = await NestFactory.create<NestFastifyApplication>(