Marvin (PrefectHQ)

Common errors

Warnings

Install compatibility stale last tested: 2026-05-12

Imports

• marvin.extract
  wrong

  from marvin import settings
  settings.openai.api_key = 'sk-...'
  result = marvin.extract(...)

  correct

  import marvin
  
  result = marvin.extract(
      'i found $30 on the ground and bought 5 bagels for $10',
      int,
      instructions='only USD amounts'
  )
  print(result)  # [30, 10]

  marvin.settings.openai.api_key pattern from v2 is gone in v3. Set OPENAI_API_KEY env var or configure via pydantic-ai settings.

• marvin.fn decorator
  wrong

  from marvin import ai_fn
  
  @ai_fn
  def sentiment(text: str) -> float:
      ...

  correct

  import marvin
  
  @marvin.fn
  def sentiment(text: str) -> float:
      """Returns sentiment score from -1.0 (negative) to 1.0 (positive)."""
  
  score = sentiment('I love this product!')
  print(score)  # ~0.9

  ai_fn decorator from v1/old docs renamed to marvin.fn in v2+. Both may exist but marvin.fn is canonical.

• marvin.Thread (v3)
  wrong

  from controlflow import flow
  
  @flow
  def my_flow():
      ...

  correct

  import marvin
  
  with marvin.Thread() as thread:
      marvin.run('What is quantum computing?')
      marvin.run('How does that relate to AI?')

  ControlFlow's @flow decorator removed in v3. Use marvin.Thread() context manager instead. ControlFlow is now merged into marvin.

Quickstart draft last tested: 2026-04-23

Core marvin 3.x structured output functions.

# pip install marvin
import marvin
import os

# Set OPENAI_API_KEY env var — marvin uses OpenAI by default

# Structured extraction
numbers = marvin.extract(
    'I paid $45 for lunch and $12 for coffee',
    int,
    instructions='USD amounts only'
)
print(numbers)  # [45, 12]

# Classification
category = marvin.classify(
    'The new iPhone has great camera features',
    labels=['tech', 'sports', 'politics']
)
print(category)  # 'tech'

# AI function
@marvin.fn
def translate(text: str, language: str) -> str:
    """Translates text to the given language."""

print(translate('Hello world', 'Spanish'))  # 'Hola mundo'