diff --git a/src/app/api/send/route.ts b/src/app/api/send/route.ts
index e07ba2971..7d6f64ec1 100644
--- a/src/app/api/send/route.ts
+++ b/src/app/api/send/route.ts
@@ -10,6 +10,7 @@ import { fetchWebsite } from '@/lib/load';
 import { parseRequest } from '@/lib/request';
 import { badRequest, forbidden, json, serverError } from '@/lib/response';
 import { anyObjectParam, urlOrPathParam } from '@/lib/schema';
+import { getSessionId } from '@/lib/session';
 import { safeDecodeURI, safeDecodeURIComponent } from '@/lib/url';
 import { createSession, saveEvent, saveSessionData, saveSessionLink } from '@/queries/sql';
 
@@ -155,7 +156,7 @@ export async function POST(request: Request) {
     const sessionSalt = getSalt(saltRotation, createdAt);
     const visitSalt = hash(startOfHour(createdAt).toUTCString());
 
-    const sessionId = id ? uuid(sourceId, id) : uuid(sourceId, ip, userAgent, sessionSalt);
+    const sessionId = getSessionId(sourceId, ip, userAgent, sessionSalt, id);
 
     // Create a session if not found
     if (!clickhouse.enabled && !cache?.sessionId) {
@@ -284,7 +285,7 @@ export async function POST(request: Request) {
       if (websiteId && id) {
         // Persist both memberships so stitched activity can be resolved from
         // either the anonymous session or the identified session.
-        const anonymousSessionId = uuid(sourceId, ip, userAgent, sessionSalt);
+        const anonymousSessionId = getSessionId(sourceId, ip, userAgent, sessionSalt);
         const linkedSessionIds = [...new Set([anonymousSessionId, sessionId])];
         const newLinkId = hash(...linkedSessionIds, id);
 
diff --git a/src/lib/session.test.ts b/src/lib/session.test.ts
new file mode 100644
index 000000000..412faa498
--- /dev/null
+++ b/src/lib/session.test.ts
@@ -0,0 +1,56 @@
+import { expect, test } from 'vitest';
+import { uuid } from './crypto';
+import { getSessionId } from './session';
+
+const sourceId = '2e2fc91f-77a7-4e5d-a7b1-18a74c67b4e0';
+const sessionSalt = 'monthly-session-salt';
+
+test('keeps identified sessions stable on the same device', () => {
+  const args = [sourceId, '192.0.2.1', 'Desktop Browser', sessionSalt, 'user-123'] as const;
+
+  expect(getSessionId(...args)).toBe(getSessionId(...args));
+});
+
+test('creates separate sessions for the same distinct ID on different devices', () => {
+  const desktopSessionId = getSessionId(
+    sourceId,
+    '192.0.2.1',
+    'Desktop Browser',
+    sessionSalt,
+    'user-123',
+  );
+  const mobileSessionId = getSessionId(
+    sourceId,
+    '198.51.100.2',
+    'Mobile Browser',
+    sessionSalt,
+    'user-123',
+  );
+
+  expect(desktopSessionId).not.toBe(mobileSessionId);
+});
+
+test('does not merge different identified users on the same device', () => {
+  const firstUserSessionId = getSessionId(
+    sourceId,
+    '192.0.2.1',
+    'Desktop Browser',
+    sessionSalt,
+    'user-123',
+  );
+  const secondUserSessionId = getSessionId(
+    sourceId,
+    '192.0.2.1',
+    'Desktop Browser',
+    sessionSalt,
+    'user-456',
+  );
+
+  expect(firstUserSessionId).not.toBe(secondUserSessionId);
+});
+
+test('preserves anonymous session IDs', () => {
+  expect(getSessionId(sourceId, '192.0.2.1', 'Desktop Browser', sessionSalt)).toBe(
+    uuid(sourceId, '192.0.2.1', 'Desktop Browser', sessionSalt),
+  );
+});
diff --git a/src/lib/session.ts b/src/lib/session.ts
new file mode 100644
index 000000000..fdfe44631
--- /dev/null
+++ b/src/lib/session.ts
@@ -0,0 +1,13 @@
+import { uuid } from './crypto';
+
+export function getSessionId(
+  sourceId: string,
+  ip: string,
+  userAgent: string,
+  sessionSalt: string,
+  distinctId?: string,
+) {
+  return distinctId
+    ? uuid(sourceId, ip, userAgent, sessionSalt, distinctId)
+    : uuid(sourceId, ip, userAgent, sessionSalt);
+}