Free AI App Builder with Backend: FastAPI Microservice Guide

If you need a free AI app builder with backend to get a FastAPI microservice running today, you can do it with a handful of platforms that bundle hosting, a database, and auth for zero cost. The catch is that the free tiers have hard limits, and they expose the same failure modes you’ll hit in production if you’re not careful. Below I walk through the exact steps, show the code that works, compare the popular builders, and explain how to transition to a production-grade stack when the free tier starts to choke.

What free AI app builder platforms include backend services?

The short answer is: Cursor, Bolt, and Lovable all ship with a “one-click deploy” that creates a container, wires up a PostgreSQL instance, and adds optional OAuth. They are marketed as “no-code AI app builders,” but you can drop in any Dockerfile – including one that runs FastAPI – and they’ll handle the rest.

All three let you push a Git repo and they rebuild automatically. That’s the “free AI app builder with backend” you’re after – you get a place to run your FastAPI code without paying for a VM.

How do I build a FastAPI AI microservice and deploy it with a free builder?

The first thing most builders break on is the cold-start latency of a Python container that pulls a large model at import time. I’ve been bitten by this on Cursor: the first request took 30 seconds, then timed out because the free tier caps request time at 15 seconds. The fix is to load the model lazily or move it to a separate worker.

Below is a minimal FastAPI app that calls Claude via the anthropic SDK. The code fits in a 30-line file and works on any of the three platforms.

# main.py
from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
import os
import anthropic

app = FastAPI()
client = anthropic.Anthropic(api_key=os.getenv("ANTHROPIC_API_KEY"))

class Prompt(BaseModel):
    text: str

@app.post("/generate")
async def generate(prompt: Prompt):
    try:
        resp = client.completions.create(
            model="claude-2.1",
            max_tokens=256,
            temperature=0.7,
            prompt=prompt.text,
        )
        return {"completion": resp.completion}
    except Exception as e:
        raise HTTPException(status_code=500, detail=str(e))

Dockerfile for the free builder

# Use a slim Python base to stay within free RAM limits
FROM python:3.11-slim

WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY main.py .

ENV PORT 8080
EXPOSE 8080
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8080"]

requirements.txt

fastapi
uvicorn[standard]
pydantic
anthropic

Push this repo to GitHub, then connect the repo in the builder’s UI. Set the environment variable ANTHROPIC_API_KEY in the dashboard – that’s the only secret you need.

Why this works on the free tier

• The image stays under ~150 MB, well inside the 500 MB RAM limit of Cursor.
• No background workers are started, so the container never exceeds the 1 CPU quota.
• The endpoint is stateless; the builder can spin up multiple instances if you hit the request cap.

Which free builder should I pick? (hosting, database, auth, scaling limits)

I’ve tried each platform on a real-world AI chatbot prototype. Here’s how they compare when you’re building a FastAPI microservice.

Hosting

• Cursor gives you a persistent container that restarts on each deploy. It also provides a built-in health check endpoint (/health).
• Bolt restarts containers on every push, which can cause a brief downtime if you have a long-running model load.
• Lovable bundles a load balancer even on the free tier, but the balancer adds a 100 ms latency overhead.

Database

• Cursor ships with PostgreSQL 13 out of the box. Good for relational data, migrations via Alembic work without extra config.
• Bolt defaults to SQLite – fine for prototypes, but you’ll hit file-size limits quickly. You can enable a managed Postgres add-on for $5/mo.
• Lovable gives MySQL; if you’re used to Postgres you’ll need to adjust your ORM settings.

Authentication

All three support OAuth, but the implementations differ:

If you need a quick email-password flow for a small user base, Bolt or Lovable are easier.

Scaling limits

Free tiers cap request counts per month (see the table above). They also limit concurrent connections to 10–20. If your AI endpoint takes >2 seconds, you’ll quickly hit the “max request time” timeout and see 504 errors. The usual pattern is:

1. Cold start → request > 15 s → 504.
2. Rate limit → 429 Too Many Requests after the monthly cap.

The way around this without paying is to pre-warm the container by hitting a /ping endpoint every few minutes (a cheap cron job on GitHub Actions). That keeps the model in memory and avoids the first-request penalty.

What are the common pitfalls of free tiers and how do I avoid them?

1. Unexpected timeouts

Free builders enforce a hard request timeout (usually 15 s). If you load a 300 MB model at import, the first request will exceed the limit. Load the model lazily:

# lazy_load.py
_model = None

def get_model():
    global _model
    if _model is None:
        from anthropic import Anthropic
        _model = Anthropic(api_key=os.getenv("ANTHROPIC_API_KEY"))
    return _model

Call get_model() inside the endpoint instead of at module import.

2. Database connection limits

Postgres on Cursor’s free tier allows only 20 connections. FastAPI’s default uvicorn workers spawn multiple threads that each open a connection, quickly exhausting the pool. Set workers=1 in the uvicorn command or configure a connection pool with maxsize=5.

CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8080", "--workers", "1"]

3. Secret leakage

The builder UI stores env vars in plain text for the free tier. If you commit a .env file, it will be visible in the repo history. Use the platform’s secret manager (Cursor’s “Secrets” tab) and keep .gitignore up to date.

4. Logging limits

Free plans only retain logs for 24 hours. If you rely on logs for debugging, set up a remote log sink early (e.g., push logs to a free Loggly account). In my experience, missing logs made a memory-leak bug invisible for days.

When should I move off the free builder to production-grade infrastructure?

If any of these conditions are true, start planning the migration:

• Monthly requests > 80 k – you’ll breach the free cap and start paying per-request fees.
• Latency > 500 ms on steady traffic – the shared containers can’t guarantee CPU.
• Need for compliance (PCI, GDPR) – free builders don’t offer audit logs or VPC isolation.
• Complex data schema – you outgrow the simple managed DB and need custom tuning.

A typical path is:

1. Clone the repo to a private GitHub (or GitLab) workspace.
2. Replace the builder-specific Dockerfile with a multi-stage build that compiles a static binary (e.g., python -m nuitka). This reduces RAM usage and cold-start time.
3. Provision a managed Kubernetes cluster (e.g., DigitalOcean Apps, Fly.io) that offers a free tier with 1 vCPU and 256 MB RAM per container – similar budget but with better scaling controls.
4. Migrate the database to a managed Postgres instance (e.g., Supabase free tier) and update the connection string.
5. Add a reverse proxy (Traefik) for graceful shutdowns and better observability.

If you need a hand with any of those steps, feel free to reach out via the hire page. I’m happy to pair program or run a short audit.

FAQ

Q: Can I use a free AI app builder with backend for a production API?

A: You can for low-traffic internal tools or demos, but the hard limits on requests, CPU, and request time make it unsuitable for a public-facing product that expects consistent latency.

Q: Do these builders support WebSocket connections needed for real-time chat?

A: Cursor and Lovable allow WebSockets, but Bolt’s free tier blocks them. Even when supported, the connection count shares the same concurrency limit as HTTP requests.

Q: How do I store large AI model files (e.g., 1 GB) without blowing the container size?

A: Store the model in an external object store (S3, Wasabi) and download it on first request, caching it to /tmp. The free tier gives you about 1 GB of temporary storage.

Q: What happens to my data if the free tier is discontinued?

A: Most platforms export a SQL dump on request. Schedule a weekly backup to a personal S3 bucket to avoid data loss.

Key Takeaways

• A free AI app builder with backend can host a FastAPI microservice, but you must respect RAM, CPU, and request-time limits.
• Load heavy AI models lazily and keep the container image small to avoid timeouts.
• Choose the builder whose database and auth match your prototype needs – Cursor for Postgres, Bolt for quick email-less auth, Lovable for MySQL.
• Anticipate the migration: keep the code Docker-friendly, use environment-managed secrets, and plan a move to a managed Kubernetes or VPS when traffic climbs.
• If you hit a wall, a short consulting session can shave days off the migration.

Happy building, and remember: the free tier is a stepping stone, not a permanent home.