diff --git a/infra/lib/foundation-stack.ts b/infra/lib/foundation-stack.ts
index 16ade07..69ca391 100644
--- a/infra/lib/foundation-stack.ts
+++ b/infra/lib/foundation-stack.ts
@@ -94,8 +94,10 @@ export class FoundationStack extends cdk.Stack {
     // not permitted"). A distinct ID makes it a clean replace instead.
     const dbCluster = new rds.DatabaseCluster(this, 'AuroraCluster', {
       clusterIdentifier: `proposal-system-db${config.stackSuffix}`,
+      // 15.17 = latest available aurora-postgresql 15.x (15.4 was retired by RDS —
+      // "Cannot find version 15.4"). Stays on major 15 for pgvector / ADR 0001 compat.
       engine: rds.DatabaseClusterEngine.auroraPostgres({
-        version: rds.AuroraPostgresEngineVersion.VER_15_4,
+        version: rds.AuroraPostgresEngineVersion.VER_15_17,
       }),
       vpc: this.vpc,
       vpcSubnets: { subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS },