Quickstart — FastAPI

Last updated on Oct 17, 2026

Zero to a billed charge in 5 steps.

1. Install

bash

pip install "mythos-sdk[fastapi,llm]" "fastapi[standard]"
npx @mythos-work/sdk init

2. Environment

bash

# .env
MYTHOS_LISTING_ID=your-listing-id
# openssl rand -base64 32  (≥ 32 chars)
MYTHOS_SESSION_SECRET=
# Mythos dashboard → Settings → Mythos key
MYTHOS_KEY=

uvicorn does not read .env on its own — step 5 starts it with --env-file. In production, set these in your host's environment instead.

3. Server

Missing MYTHOS_KEY is soft in 0.5.0; static listing launches remain silent, while dynamic registration, handshake and callbacks warn once per process. It will be required in 0.6.0. Update deployment env after key regeneration. See the binding matrix.

Troubleshooting code	Fix
PRODUCER_MISMATCH	The MYTHOS_KEY in this deployment belongs to a different Producer, or was regenerated. Copy the current key from the dashboard and redeploy.

init created mythos_setup.py (mythos = create_mythos()); import it in main.py:

python

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from mythos_sdk import MythosError
from mythos_setup import mythos

app = FastAPI()
app.include_router(mythos.router)

@app.post("/api/calculate")
async def calculate(request: Request):
    body = await request.json()
    try:
        await mythos.charge(request, credits=1, reason="calculate")
        return {"success": True, "data": {"result": body["a"] + body["b"]}}
    except MythosError as err:
        return JSONResponse({"success": False, "error": str(err), "code": err.code}, status_code=err.http_status)

Defining mythos = create_mythos() directly in main.py is equivalent.

4. Page

html

<button onclick="calculate()">Calculate (1 credit)</button>
<script src="https://cdn.jsdelivr.net/npm/@mythos-work/sdk@0.5.0/dist/mythos-client.global.js"></script>
<script>
  const m = Mythos.initMythos();
  m.ready.then((s) => { document.body.dataset.mythos = s.status; });
  async function calculate() {
    const { approved } = await m.confirmCharge({ credits: 1, reason: '1 + 2' });
    if (approved) await m.fetch('/api/calculate', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ a: 1, b: 2 }) });
  }
</script>

5. Run and check it

bash

uvicorn main:app --env-file .env
python -m mythos_sdk doctor

Then launch your listing from the Mythos dashboard.