diff --git a/apps/server/.env b/apps/server/.env
index 3d8b381ee..716d12b75 100644
--- a/apps/server/.env
+++ b/apps/server/.env
@@ -1,16 +1,18 @@
 DATABASE_URL="postgresql://postgres:example-PAssw0rd-xHjDYR.b7N@db:5432/postgres"
 REDIS_URL="redis://localhost:6379"
 
-AUTH_GOOGLE_CLIENT_ID="change-me"
-AUTH_GOOGLE_CLIENT_SECRET="change-me"
+BETTER_AUTH_SECRET=""
 
-AUTH_GITHUB_CLIENT_ID="change-me"
-AUTH_GITHUB_CLIENT_SECRET="change-me"
+AUTH_GOOGLE_CLIENT_ID=""
+AUTH_GOOGLE_CLIENT_SECRET=""
 
-STRIPE_SECRET_KEY="change-me"
-STRIPE_WEBHOOK_SECRET="change-me"
+AUTH_GITHUB_CLIENT_ID=""
+AUTH_GITHUB_CLIENT_SECRET=""
 
-CLIENT_URL="change-me"
-API_SERVER_URL="change-me"
+STRIPE_SECRET_KEY=""
+STRIPE_WEBHOOK_SECRET=""
+
+CLIENT_URL=""
+API_SERVER_URL=""
 
 # OTEL_EXPORTER_OTLP_ENDPOINT="http://localhost:4318"
diff --git a/apps/server/src/libs/auth.ts b/apps/server/src/libs/auth.ts
index 3284696fe..b71472aa7 100644
--- a/apps/server/src/libs/auth.ts
+++ b/apps/server/src/libs/auth.ts
@@ -296,6 +296,8 @@ export async function seedTrustedClients(db: Database, env: Env): Promise<void>
 
 export function createAuth(db: Database, env: Env, metrics?: AuthMetrics | null) {
   return betterAuth({
+    secret: env.BETTER_AUTH_SECRET,
+
     database: drizzleAdapter(db, {
       provider: 'pg',
       schema: {
diff --git a/apps/server/src/libs/env.test.ts b/apps/server/src/libs/env.test.ts
index a04b3de48..edd641c60 100644
--- a/apps/server/src/libs/env.test.ts
+++ b/apps/server/src/libs/env.test.ts
@@ -7,6 +7,7 @@ describe('parseEnv', () => {
     const env = parseEnv({
       DATABASE_URL: 'postgres://example',
       REDIS_URL: 'redis://example',
+      BETTER_AUTH_SECRET: 'test-secret-at-least-32-characters-long',
       AUTH_GOOGLE_CLIENT_ID: 'google-client',
       AUTH_GOOGLE_CLIENT_SECRET: 'google-secret',
       AUTH_GITHUB_CLIENT_ID: 'github-client',
diff --git a/apps/server/src/libs/env.ts b/apps/server/src/libs/env.ts
index 2d827154e..5749fbff0 100644
--- a/apps/server/src/libs/env.ts
+++ b/apps/server/src/libs/env.ts
@@ -43,6 +43,11 @@ const EnvSchema = object({
   DATABASE_URL: pipe(string(), nonEmpty('DATABASE_URL is required')),
   REDIS_URL: pipe(string(), nonEmpty('REDIS_URL is required')),
 
+  // Required: signs session cookies and encrypts JWKS private keys in DB.
+  // Must be stable across deploys/instances, otherwise every redeploy invalidates
+  // all existing sessions and forces users to re-login.
+  BETTER_AUTH_SECRET: pipe(string(), nonEmpty('BETTER_AUTH_SECRET is required')),
+
   AUTH_GOOGLE_CLIENT_ID: pipe(string(), nonEmpty('AUTH_GOOGLE_CLIENT_ID is required')),
   AUTH_GOOGLE_CLIENT_SECRET: pipe(string(), nonEmpty('AUTH_GOOGLE_CLIENT_SECRET is required')),
   AUTH_GITHUB_CLIENT_ID: pipe(string(), nonEmpty('AUTH_GITHUB_CLIENT_ID is required')),