Peven

Peven is Python authoring for structured LLM environments backed by a Julia Petri-net runtime.

If PydanticAI makes it easy to build agents, Peven makes it easy to build the environment around them: places, transitions, joins, guards, retries, and the topology you want to evaluate.

Interactive examples

A MiniGrid DoorKey rollout with the Petri net execution trace is shown here:

View the Peven MiniGrid demo

Why use it

• Author environments in Python, next to the agents and tools you already write.
• Make topology explicit instead of hiding it inside one giant agent loop.
• Run the hard state-machine part on a Julia engine built for Petri nets and concurrent firing.
• Compare workflows: single-shot, judge loops, keyed joins, guarded retries, branch-and-merge topologies.

When not to use it

Peven is probably overkill for a single prompt, a linear chain, or an agent loop that is easier to read as ordinary Python. It starts to pay for itself when the environment has real topology: branching, joins, guards, retries, traces, or reproducible state you want to inspect and compare.

Install

First install the Python package:

uv add peven

or

pip install peven

Peven also needs a Julia runtime. peven.install_runtime() provisions that layer through juliapkg, including:

• Julia itself if it is not already available
• PevenPy.jl
• Peven.jl

Recommended: do that immediately after install so the one-time Julia download, package resolution, and precompile work does not happen on your first real run.

uv run peven-install

or

uv run peven install-runtime --verbose

To check the Python-to-Julia runtime wiring, run:

uv run peven doctor

A healthy install prints the resolved Julia executable, Julia project, Julia version, and doctor_ok. If setup fails, rerun with peven install-runtime --verbose and include that output in the bug report.

If you skip explicit setup, the first Env.run() will do the same provisioning automatically.

Quickstart

from pydantic_ai import Agent
from pydantic_ai.models.openai import OpenAIChatModel
from pydantic_ai.output import NativeOutput
from pydantic_ai.providers.ollama import OllamaProvider

import peven


agent = Agent(
    OpenAIChatModel(
        "qwen3.5:9b",
        provider=OllamaProvider(base_url="http://127.0.0.1:11434/v1"),
    ),
    output_type=NativeOutput(str),
)


@peven.executor("answer")
async def answer(ctx, prompt):
    question = prompt.payload["question"]
    result = await agent.run(question)
    return ctx.token(
        {
            "question": question,
            "answer": result.output.strip().lower(),
        }
    )


@peven.env("single_question")
class SingleQuestionEnv(peven.Env):
    prompt = peven.place(schema={"kind": "question"})
    report = peven.place(terminal=True)

    def initial_marking(self) -> peven.Marking:
        return peven.marking(
            prompt=[{"question": "What planet is known as the red planet?"}]
        )

    solve = peven.transition(
        inputs=["prompt"],
        outputs=["report"],
        executor="answer",
    )


result = SingleQuestionEnv().run()
print(result.status)
print(result.final_marking["report"][0].payload)

That same pattern scales to richer topologies:

• tee one prompt into multiple branches
• join outputs back together by key
• gate transitions with guards
• retry transitions without rewriting control flow
• cap the run with fuse

Mark a final place with terminal=True when a token there means the run is complete. The Julia engine still reports the real Petri-net condition; Python normalizes no_enabled_transition into a completed RunResult only when a terminal place contains a token.

Use peven.input("place", optional=True) when a transition should fire from its required inputs and receive None when that optional place is absent. This maps directly to ArcFrom(...; optional=true) in Peven.jl. Optional arcs must not be used on keyed joins, and a transition still needs at least one required input.

Tracing runs

Peven can stream the run as it happens. RichSink renders a live terminal trace of run start/finish events, transition firings, retries, failures, guard errors, timings, and final marking. JSONLSink writes the same event stream as one JSON record per line for later inspection.

sink = peven.CompositeSink(
    peven.RichSink(show_payloads=True),
    peven.JSONLSink("runs/trace.jsonl"),
)

result = SingleQuestionEnv().run(sink=sink)

Executors can add local trace records with ctx.trace(...). When using PydanticAI, pass peven.integrations.pydantic_ai.event_stream_handler(ctx, ...) to an agent run to include model/tool stream events in the same sink. See examples/trace.py for a small tool-and-judge trace with fuse support.

Why Julia

The Julia side is not there for novelty. It keeps the engine closer to the real Petri-net model.

Python is a great place to author agents and executors, but it pushes engine code toward shims, wrappers, and dynamic glue. Julia is a better fit for the symbolic runtime: markings, firing rules, joins, guards, retries, and termination stay explicit instead of dissolving into spaghetti soup.

Architecture

Peven has three layers:

• peven — Python authoring, executors, integrations, sinks, and runtime ownership.
• PevenPy.jl — the narrow Julia adapter boundary.
• Peven.jl — the execution engine.

Technically, Python authors the env and owns transition callbacks. PevenPy.jl lowers the authored env into Julia runtime structures, runs the net, and streams runtime events back. Peven.jl owns the actual Petri-net execution semantics.

Examples

The repo examples are intentionally small but representative:

• examples/trace.py — PydanticAI trace integration, fuse, and rich run output
• examples/guarded_batch.py — guarded retries around a batch step
• examples/keyed_join.py — branch, answer in parallel, and keyed-join the results
• examples/minigrid/ — MiniGrid DoorKey with a mover, planner, fog memory, and terminal scoring

Release notes

0.2.2

• Added optional input arcs via peven.input(..., optional=True).
• Updated the MiniGrid DoorKey example so planner advice is an optional token, not a sentinel {"advice": "none"} token.
• Updated the packaged Julia runtime pins for optional-arc support.
• Added adapter parity coverage for optional inputs, optional-only rejection, and optional keyed-join rejection.

0.2.1

• Added peven.place(terminal=True) for Python-side completion normalization.
• Updated Rich output to hide no_enabled_transition for completed terminal-place runs.
• Added the MiniGrid DoorKey example under the examples dependency group.
• Added gymnasium and minigrid to the optional examples dependencies.

Inspiration

Peven is inspired by a couple different things. For starters the name is taken from Patricia A. McKillip's Riddle-Master trilogy. Peven of Aum is a king, a ghost, and a master riddler who has only ever lost once. In the Riddle-Master trilogy, riddles are made up of three parts: questions, answers, and strictures. My hope for Peven is that it can help you explore evaluations by providing a runtime where you can ask a question, iterate based on the stricture, and, eventually, get to an answer. "Beware the unanswered Riddle."

My second point of inspiration comes from my time working at The LLM Data Company, where I had the chance to learn and experiment to my heart's content. A lot of my work centered around environments and benchmarks. I often wished I had a reusable framework or package to support my work here, something like a pydantic (which I love) but for evaluations.

Most of the architectural decisions I made regarding the engine are because I thought the math was cool. Peven should give you a pretty clear sense of (1) how I think about evaluations and (2) what types of evaluations I'm interested in.