diff --git a/src/__tests__/helpers/session-secret.ts b/src/__tests__/helpers/session-secret.ts
new file mode 100644
index 000000000..d1db58feb
--- /dev/null
+++ b/src/__tests__/helpers/session-secret.ts
@@ -0,0 +1,11 @@
+/**
+ * A session cookie secret of the exact shape `createSession` mints (`hls_`
+ * and 32 random bytes in hex). The session lookup refuses any other shape
+ * before it asks the database, so a fixture that invents a readable secret
+ * tests the refusal, not the lookup.
+ */
+import { randomBytes } from "node:crypto";
+
+export function fixtureSessionSecret(): string {
+  return `hls_${randomBytes(32).toString("hex")}`;
+}
diff --git a/src/app/api/auth/native/complete/__tests__/route.test.ts b/src/app/api/auth/native/complete/__tests__/route.test.ts
index 540b12105..e82eeed96 100644
--- a/src/app/api/auth/native/complete/__tests__/route.test.ts
+++ b/src/app/api/auth/native/complete/__tests__/route.test.ts
@@ -45,6 +45,7 @@ import { GET } from "../route";
 import { checkAuthSurfaceRateLimit } from "@/lib/rate-limit";
 import { mintNativeHandoff } from "@/lib/auth/native-handoff";
 import { prisma } from "@/lib/db";
+import { fixtureSessionSecret } from "@/__tests__/helpers/session-secret";
 import {
   NATIVE_HANDOFF_STATE_COOKIE,
   encodeNativeHandoffState,
@@ -71,7 +72,7 @@ function makeRequest(opts: {
     );
   }
   if (opts.withSession !== false) {
-    r.cookies.set("healthlog_session", "hls_sessionsecret");
+    r.cookies.set("healthlog_session", fixtureSessionSecret());
   }
   return r;
 }
diff --git a/src/app/api/auth/oidc/callback/__tests__/route.test.ts b/src/app/api/auth/oidc/callback/__tests__/route.test.ts
index 370884fba..a01fa9212 100644
--- a/src/app/api/auth/oidc/callback/__tests__/route.test.ts
+++ b/src/app/api/auth/oidc/callback/__tests__/route.test.ts
@@ -101,6 +101,7 @@ import {
   verifyIdToken,
 } from "@/lib/auth/oidc";
 import { mintNativeHandoff } from "@/lib/auth/oidc-native-handoff";
+import { fixtureSessionSecret } from "@/__tests__/helpers/session-secret";
 
 const CONFIG = {
   issuerUrl: "https://idp.example.com",
@@ -616,7 +617,8 @@ describe("GET /api/auth/oidc/callback — duplicate-callback idempotency", () =>
     const req = new NextRequest(
       "http://localhost/api/auth/oidc/callback?code=abc&state=STATE",
     );
-    if (withSession) req.cookies.set("healthlog_session", "hls_valid_secret");
+    if (withSession)
+      req.cookies.set("healthlog_session", fixtureSessionSecret());
     return req;
   }
 
diff --git a/tests/integration/session-time-zone.test.ts b/tests/integration/session-time-zone.test.ts
index 3743de7e8..4a58deab4 100644
--- a/tests/integration/session-time-zone.test.ts
+++ b/tests/integration/session-time-zone.test.ts
@@ -23,6 +23,7 @@ import {
 } from "vitest";
 
 import { PrismaClient } from "@/generated/prisma/client";
+import { fixtureSessionSecret } from "@/__tests__/helpers/session-secret";
 import { getPrismaClient, truncateAllTables } from "./setup";
 
 const ZONE = "Europe/Berlin";
@@ -167,17 +168,19 @@ describe("on a database whose default zone is Europe/Berlin", () => {
     const { validateSessionFromCookieValue } =
       await import("@/lib/auth/session");
     const { hashToken } = await import("@/lib/auth/hmac");
+    const liveSecret = fixtureSessionSecret();
+    const expiredSecret = fixtureSessionSecret();
     const live = await getPrismaClient().session.create({
       data: {
         userId: USER_ID,
-        tokenHash: hashToken("hls_live-secret"),
+        tokenHash: hashToken(liveSecret),
         expiresAt: new Date(Date.now() + 60 * 60_000),
       },
     });
     await getPrismaClient().session.create({
       data: {
         userId: USER_ID,
-        tokenHash: hashToken("hls_expired-secret"),
+        tokenHash: hashToken(expiredSecret),
         expiresAt: new Date(Date.now() - 60_000),
       },
     });
@@ -185,10 +188,10 @@ describe("on a database whose default zone is Europe/Berlin", () => {
       60_000,
     );
     await expect(
-      validateSessionFromCookieValue("hls_live-secret"),
+      validateSessionFromCookieValue(liveSecret),
     ).resolves.not.toBeNull();
     await expect(
-      validateSessionFromCookieValue("hls_expired-secret"),
+      validateSessionFromCookieValue(expiredSecret),
     ).resolves.toBeNull();
   });