Runtime cost matters to me now

AI made me reopen a boring question: why am I still paying Fargate rent for V8?

Bun’s Zig-to-Rust AI rewrite got me running the math – 6 days, 960'000 lines, 99.8% of Bun’s existing tests passing, 9 days to merge ($10'000 total AI spent?)¹.

What does that look like for 15'844 LOC of my own microservice on Fargate?

# My setup

Node.js microservice, talks to MongoDB, calls a few neighbors, responds to others, does its small thing. A typical service sits at 0.5 vCPU / 1 GB RAM – $18/month per task. Three replicas minimum for robustness, so $54/month per service.

Most of that bill is provisioned runtime headroom. Node makes that headroom harder to shrink. V8, dynamic dispatch, garbage collection, startup time, memory spikes. The runtime pays rent on Fargate while I’m paying for it.

# One rewrite, run the math

Say $100 in Claude Code tokens to produce a first port to Go or Rust. Not to ship it blindly – review, tests, and rollout still cost human time. But the first draft is no longer the expensive part.

Fargate bills $0.04048 per vCPU-hour and $0.004445 per GB-hour². Minimum task size: 0.25 vCPU / 0.5 GB, about $9/month. A compiled service with a Mongo driver and a couple of HTTP clients belongs at that floor. No V8. No managed runtime heap sitting around waiting for traffic.

New bill for that service: 3 x $9 = $27/month; 2x savings.

Payback on $100 of AI: under four months; for one service in minimal setup.

That is the toy version. My real setup has dev, prod, workload replicas, autotests, and the usual operational mess. Around ten replicas. Payback lands near one month.

# The cluster framing

The language map changes:

• Rust / C / C++ – smallest runtime footprint. Highest migration friction. Ecosystem question still open for my services.
• Go – small runtime tax, boring deployment, strong backend ecosystem. Looks like the new default from the cost side.³
• Node / Python / PHP / Java / C# – still productive defaults, but their runtime footprint is no longer free. Fargate turns it into a line item.

The runtime tax used to hide inside developer productivity. AI lowers the cost of moving code. Cloud exposes the cost of keeping code running. That makes runtime choice visible again.

# Plan B

Or move off AWS to Hetzner and cut the bill by ~10x.