Self-Hosting a Next.js 15 AI SaaS, Architecture and Cost

Serverless is a good default until the product is an AI agent. Then duration caps, cold starts, and stream egress start to set the architecture, not your ORM.

The free containerized baseline for this architecture is open-source: Agentic-Packs/agenticpacks-oss. Keep it open while you read the cost table.

The serverless AI hangover

Vercel Pro and AWS Lambda are billed on execution time and outbound data. An agent that calls a model, tools, and a second model is not a 200ms handler.

You still pay the model vendor. Serverless adds a second meter on the glue: function GB-seconds, Fast Data Transfer, concurrent executions. At low traffic that meter is noise. At 10k MAU with streaming chat it is the line item you notice. For the reverse-proxy side of streaming egress, see Configuring Caddy for Next.js 15 SSE Streaming.

The alternative: about 4.50 euro per month

A Hetzner CX22 / CX23 (or CAX11 ARM) running Docker Compose:

• Next.js 15 standalone on port 3000, unpublished
• PostgreSQL 16 on 5432, unpublished
• Caddy on 80/443 with automatic TLS

That is the topology in the Docker + Caddy guide. AgenticPacks OSS is Compose plus Postgres locally. Pro adds Caddy, Terraform, and the Hetzner TLS path.

You trade autoscaling for a known monthly invoice and a process that can sit on a stream for minutes.

Cost at 1k, 10k, and 50k MAU

Figures are order-of-magnitude, USD per month, infra only (not OpenAI / Anthropic tokens). Serverless assumes Vercel Pro, a hosted Postgres (Neon or Supabase Pro), and streaming chat as the hot path. VPS assumes one CX23, Caddy, self-hosted Postgres, backups to object storage.

The VPS column stays flat until CPU or RAM is saturated. The serverless column tracks stream-minutes. If each active user does a few long generations, 10k MAU is not 10x of 1k. It can be 20x.

When you outgrow one node, you still understand the bill: another CX, managed Postgres, or object storage. You do not discover a 4-digit Fast Data Transfer invoice after a launch week of SSE.

Production architecture

                     browsers / Stripe / GitHub
                                |
                         Caddy :443 TLS
                                |
                    Docker bridge (unpublished)
              +-----------------+------------------+
              |                                    |
     Next.js 15 App Router                  PostgreSQL 16
     Auth.js v5 (Node, not Edge-only)       Prisma
     Route Handlers:                        pgvector (Pro)
       /api/chat  SSE                       users, sessions
       /api/webhooks/stripe                 messages, usage
       /api/agent  LangGraph (Pro)          Stripe customer ids
     background: cron / worker              |
     (report jobs, RAG ingest)              |
              |                             |
              +-------- Stripe API ---------+

Auth.js v5 session strategy on a VPS is JWT or a database session in Postgres. Skip Edge middleware that assumes Vercel. Keep auth checks in Node Route Handlers and server layouts.

Stripe: checkout session or payment link from a server action, webhook on a dedicated route, raw body for signature verify, then write entitlements before you ACK. The webhook must be fast. Do agent work after the 200.

AgenticPacks OSS ships Auth.js, Prisma, Postgres, streaming chat, Compose, and Free + Pro $19 Stripe checkout with webhooks and a credit counter. Pro adds Caddy, 3-tier Stripe with proration and team credit pools, pgvector RAG, LangGraph, Terraform, Redis.

Build vs buy

Wiring this from a blank create-next-app is not a weekend:

That is before product UI. The 40+ hour number is the floor if you have done it before. First time through Caddy plus Stripe signatures, add a buffer.

Forty hours of Stripe, Caddy, and Terraform is not the product.

Clone the free baseline: Agentic-Packs/agenticpacks-oss.

Or skip the wiring with the Hetzner cost stack in Pro.

A starter is worth it when those hours are not the product. It is not worth it if you only need a marketing page.

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Where to Go From Here

You do not have to assemble this from a blank create-next-app. Pick a path based on where the project is.

Option A: Free open-source baseline (MIT)

• GitHub: github.com/Agentic-Packs/agenticpacks-oss
• What is included: standalone Dockerfile, Compose, PostgreSQL 16, Auth.js v5, single-plan ($19/mo) Stripe checkout with webhooks and a credit counter, streaming chat
• Cost: free, MIT. Star the repo if this guide saved you a debugging session.

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Option B: Ship with AgenticPacks Pro ($149 one-time)

If you would rather not spend 40+ hours on Terraform, Caddy, Stripe webhooks, and agent loops:

• Automated Hetzner provisioning: terraform apply for the VPS, firewall, Caddy auto-SSL, and Compose.
• Production multi-agent core: LangGraph, pgvector RAG, Redis.
• Usage & Unit Economics: Stripe subscriptions, webhooks, and real-time token-to-dollar cost ledger.