Razio05/Google-GenAI-Pydantic-v2-Structured-JSON-FastAPI

⚡ FastAPI + Google Gemini 2026 Structured JSON Blueprint (Python 3.13 Ready)

Stop fighting unstructured LLM markdown and Python 3.13 bcrypt crashes. This reference blueprint shows how to wire FastAPI, Pydantic v2, and the official google-genai SDK with native response_mime_type="application/json" and 3-model automatic failover.

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1. Enforcing Strict Pydantic v2 JSON with 3-Model Gemini Failover

import os, json
from google import genai
from pydantic import BaseModel
from typing import List

client = genai.Client(api_key=os.getenv("GEMINI_API_KEY"))

DEFAULT_MODELS = [
    os.getenv("GEMINI_MODEL", "gemini-3.5-flash"),
    "gemini-flash-lite-latest",
    "gemini-3.8-flash",
]

class AIStructuredResponse(BaseModel):
    status: str
    summary: str
    key_points: List[str]
    action_items: List[str]

def generate_structured_json(prompt: str, system_instruction: str) -> AIStructuredResponse:
    for model_name in DEFAULT_MODELS:
        try:
            res = client.models.generate_content(
                model=model_name,
                contents=prompt,
                config={
                    "response_mime_type": "application/json",
                    "system_instruction": system_instruction,
                },
            )
            return AIStructuredResponse(**json.loads(res.text))
        except Exception as err:
            if any(code in str(err) for code in ("404", "503", "UNAVAILABLE", "NOT_FOUND")):
                continue
            raise

2. Fixing the Python 3.12 / 3.13 passlib[bcrypt] 72-Byte Startup Crash

Pin bcrypt==4.0.1 alongside passlib[bcrypt] in requirements.txt to avoid the ValueError: password cannot be longer than 72 bytes exception during container boot:

fastapi
uvicorn
google-genai
pydantic[email]
passlib[bcrypt]
bcrypt==4.0.1
python-jose[cryptography]

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