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Prefect with RunVouch

A Prefect deployment with a schedule creates flow runs; a worker on the right work pool executes them. When the worker is gone, runs pile up as Late and Scheduled, and the flow's own failure hooks never fire because nothing failed.

How it runs on Prefect

Around the flow: wrap the process that runs the flow (rv run ... -- python flow.py or prefect deployment run), or use the Python client inside the flow function. Prefect's own retries and logging stay untouched.

Store the key

A Prefect Secret block (Secret.load("runvouch-key")) or an environment variable on the work pool's job template.

Wrap the job

from prefect import flow, task
from prefect.blocks.system import Secret
import os, runvouch

@flow(name="nightly-report")
def nightly_report():
    os.environ["RUNVOUCH_KEY"] = Secret.load("runvouch-key").get()
    with runvouch.vouch("nightly-report", evidence=lambda: {"report": os.path.exists("out/report.html")}) as run:
        rows = extract()
        run.tool("llm.summarise", {"rows": len(rows)}, cost=0.03)
        write_report(rows)

# prefect deploy ... --cron "0 2 * * *"

rv fails open: if RunVouch is unreachable the job still runs and you get one warning line.

Register the cadence and caps

rv agent nightly-report --cadence 24h --grace 30m --max-runtime 1h --evidence --cap-run-cost 2

Register the agent once, from anywhere with the key. Cadence is what turns a schedule that stopped into an alert; --evidence makes a run without evidence a failure; the caps pause the agent when it overspends.

What goes silent on Prefect

• Late runs are visible in the UI and in automations you have to create; RunVouch's MISSED needs neither.
• A flow that completes with an empty result is Completed; evidence separates it from a good run.
• Prefect task retries multiply LLM calls; the retry-storm detector sees identical tool inputs across retries.

What RunVouch detects

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