Verifiers

Environments for LLM Reinforcement Learning

Overview

Verifiers is a library of modular components for creating RL environments and training LLM agents. Verifiers includes an async GRPO implementation built around the transformers Trainer, is supported by prime-rl for large-scale FSDP training, and can easily be integrated into any RL framework which exposes an OpenAI-compatible inference client. In addition to RL training, Verifiers can be used directly for building LLM evaluations, creating synthetic data pipelines, and implementing agent harnesses.

Setup

For local (CPU) development and evaluation with API models, do:

uv add verifiers # uv add 'verifiers[dev]' for Jupyter + testing support

For training on GPUs with vf.GRPOTrainer, do:

uv add 'verifiers[all]' && uv pip install flash-attn --no-build-isolation

To use the latest main branch, do:

uv add verifiers @ git+https://github.com/willccbb/verifiers.git

To use with prime-rl, see here.

To install verifiers from source for core library development, do:

git clone https://github.com/willccbb/verifiers.git
cd verifiers
uv sync --all-extras && uv pip install flash-attn --no-build-isolation
uv run pre-commit install

In general, we recommend that you build and train Environments with verifiers, not in verifiers. If you find yourself needing to clone and modify the core library in order to implement key functionality for your project, we'd love for you to open an issue so that we can try and streamline the development experience. Our aim is for verifiers to be a reliable toolkit to build on top of, and to minimize the "fork proliferation" which often pervades the RL infrastructure ecosystem.

Environments

Environments in Verifiers are installable Python modules which can specify dependencies in a pyproject.toml, and which expose a load_environment function for instantiation by downstream applications (e.g. trainers). See environments/ for examples.

To initialize a blank Environment module template, do:

vf-init my-new-environment # -p /path/to/environments (defaults to "./environments")

To an install an Environment module into your project, do:

vf-install my-new-environment # -p /path/to/environments (defaults to "./environments") 

To install an Environment module from this repo's environments folder, do:

vf-install math-python --from-repo # -b branch_or_commit (defaults to "main")

Once an Environment module is installed, you can create an instance of the Environment using load_environment, passing any necessary args:

import verifiers as vf
vf_env = vf.load_environment("my-new-environment", **env_args)

To run a quick evaluation of your Environment with an API-based model, do:

vf-eval my-new-environment # vf-eval -h for config options; defaults to gpt-4.1-mini, 5 prompts, 3 rollouts for each

The core elements of Environments in are:

• Datasets: a Hugging Face Dataset with a prompt column for inputs, and either answer (str) or info (dict) columns for evaluation
• Rollout logic: interactions between models and the environment (e.g. env_response + is_completed for any MultiTurnEnv)
• Rubrics: an encapsulation for one or more reward functions
• Parsers: optional; an encapsulation for reusable parsing logic

We support both /v1/chat/completions-style and /v1/completions-style inference via OpenAI clients, though we generally recommend /v1/chat/completions-style inference for the vast majority of applications. Both the included GRPOTrainer as well as prime-rl support the full set of SamplingParams exposed by vLLM (via their OpenAI-compatible server interface), and leveraging this will often be the appropriate way to implement rollout strategies requiring finer-grained control, such as interrupting and resuming generations for interleaved tool use, or enforcing reasoning budgets.

The primary constraint we impose on rollout logic is that token sequences must be increasing, i.e. once a token has been added to a model's context in a rollout, it must remain as the rollout progresses. Note that this causes issues with some popular reasoning models such as the Qwen3 and DeepSeek-R1-Distill series; see Footguns for guidance on adapting these models to support multi-turn rollouts.

SingleTurnEnv

For tasks requiring only a single response from a model for each prompt, you can use SingleTurnEnv directly by specifying a Dataset and a Rubric.

from datasets import load_dataset
import verifiers as vf

dataset = load_dataset("my-account/my-dataset", split="train")

def reward_A(prompt, completion, info) -> float:
	# reward fn, e.g. correctness
	...
def reward_B(parser, completion) -> float:
	# auxiliary reward fn, e.g. format
	...
def metric(completion) -> float:
	# non-reward metric, e.g. proper noun count
	...

rubric = vf.Rubric(funcs=[reward_A, reward_B, metric], weights=[1.0, 0.5, 0.0])

vf_env = SingleTurnEnv(
	dataset=dataset,
	rubric=rubric
)
results = vf_env.evaluate(client=OpenAI(), model="gpt-4.1-mini", num_examples=100, rollouts_per_example=1)
vf_env.make_dataset(results, push_to_hub=True, hub_name="my-new-environment-eval-results") # save results to HF hub

Datasets should be formatted with columns for:

• 'prompt' (List[ChatMessage]) OR 'question' (str) fields
  • ChatMessage = e.g. {'role': 'user', 'content': '...'}
  • if question is set instead of prompt, you can also pass system_prompt (str) and/or few_shot (List[ChatMessage])
• answer (str) AND/OR info (dict)
• task (str): optional, used by EnvGroup and RubricGroup for orchestrating composition of Environments and Rubrics

The following named attributes available for use by reward functions in your Rubric:

• prompt: sequence of input messages
• completion: sequence of messages generated during rollout by model and Environment
• answer: primary answer column, optional if info is used
• state: can be modified during rollout to accumulate any metadata (state['responses'] includes full OpenAI response objects by default)
• info: auxiliary info needed for reward computation (e.g. test cases), optional if answer is used
• task: tag for task type (used by EnvGroup and RubricGroup)
• parser: the parser object declared, defaults to vf.Parser()

For tasks involving LLM judges, you may wish to use vf.JudgeRubric() for managing requests to auxiliary models.

ToolEnv

For many applications involving tool use, you can use ToolEnv to leverage models' native tool/function-calling capabilities in an agentic loop. Tools can be specified as generic Python functions (with type hints and docstrings), which will then be passed in JSON schema form to each inference request.

import verifiers as vf
vf_env = vf.ToolEnv(
	dataset= ... # HF Dataset with 'prompt'/'question' + 'answer'/'info' columns
	rubric= ... # Rubric object; vf.ToolRubric() can be optionally used for counting tool invocations in each rollout
	tools=[search_tool, read_article_tool, python_tool], # python functions with type hints + docstrings
	max_turns=10
)

In cases where your tools require heavy computational resources, we recommend hosting your tools as standalone servers (e.g. MCP servers) and creating lightweight wrapper functions to pass to ToolEnv. Parallel tool call support is enabled by default.

For training, or self-hosted endpoints, you'll want to enable auto tool choice in vLLM with the appropriate parser. If your model does not support native tool calling, you may find the XMLParser abstraction useful for rolling your own tool call parsing on top of MultiTurnEnv; see environments/xml_tool_env for an example.

MultiTurnEnv

Both SingleTurnEnv and ToolEnv are instances of MultiTurnEnv, which exposes an interface for writing custom Environment interaction protocols. The two methods you must override are

from typing import Tuple
import verifiers as vf
from verifiers.types import Messages, State
class YourMultiTurnEnv(vf.MultiTurnEnv):
    def __init__(self,
                 dataset: Dataset,
                 rubric: Rubric,
				 max_turns: int,
                 **kwargs):
	
  def is_completed(self, messages: Messages, state: State, **kwargs) -> bool:
    # return whether or not a rollout is completed

  def env_response(self, messages: Messages, state: State, **kwargs) -> Tuple[Messages, State]:
    # return new environment message(s) + updated state

If your application requires more fine-grained control than is allowed by MultiTurnEnv, you may want to inherit from the base Environment functionality directly and override the rollout method.

Training

GRPOTrainer

The included trainer (vf.GRPOTrainer) supports running GRPO-style RL training via Accelerate/DeepSpeed, and uses vLLM for inference. It supports both full-parameter finetuning, and is optimized for efficiently training dense transformer models on 2-16 GPUs.

# install environment
vf-install wordle (-p /path/to/environments | --from-repo)

# quick eval
vf-eval wordle -m (model_name in endpoints.py)

# inference
CUDA_VISIBLE_DEVICES=0,1,2,3,4,5 vf-vllm --model willcb/Qwen3-1.7B-Wordle \
    --data-parallel-size 7 --enforce-eager --disable-log-requests

# training
CUDA_VISIBLE_DEVICES=6,7 accelerate launch --num-processes 1 \
    --config-file configs/zero3.yaml examples/grpo/train_wordle.py --size 1.7B

Troubleshooting

• Ensure your wandb and huggingface-cli logins are set up (or set report_to=None in training_args). You should also have something set as your OPENAI_API_KEY in your environment (can be a dummy key for vLLM).
• If using high max concurrency, increase the number of allowed open sockets (e.g. ulimit -n 4096)
• On some setups, inter-GPU communication can hang or crash during vLLM weight syncing. This can usually be alleviated by setting (or unsetting) NCCL_P2P_DISABLE=1 in your environment. Try this as your first step if you experience NCCL-related issues.
• If problems persist, please open an issue.

Resource Requirements

GRPOTrainer is optimized for setups with at least 2 GPUs, scaling up to multiple nodes. 2-GPU setups with sufficient memory to enable small-scale experimentation can be rented for <$1/hr.

PRIME-RL

If you do not require LoRA support, you may want to use the prime-rl trainer, which natively supports Environments created using verifiers, is more optimized for performance and scalability via FSDP, includes a broader set of configuration options and user experience features, and has more battle-tested defaults. Both trainers support asynchronous rollouts, and use a one-step off-policy delay by default for overlapping training and inference. See the prime-rl docs for usage instructions.

Further Documentation

See the full docs for more info, including:

• Dataset configuration options (system prompts, few-shot examples, eval datasets)
• Parsers (e.g. ThinkParser, XMLParser)
• Advanced Rubric patterns
• Composing Environments (EnvGroup) and Rubrics (RubricGroup)
• Creating and saving rollout datasets using Environments
• More Environment example walkthroughs
• Hardware considerations
• SFT warmup for improving small-model training efficiency
• RL + GRPO best practices
• Common footguns

Footguns

Non-Increasing Chat Templates: The Qwen3 and DeepSeek-R1 model series both remove <think> sections from messages when processing inputs, which violates the increasing context requirement for multi-turn GRPO-style training. We provide versions of many of these models with modified chat templates here.

Contributions

Verifiers warmly welcomes community contributions! Please open an issue or PR if you encounter bugs or other pain points during your development, or start a discussion for more open-ended questions.

Please note that the core verifiers/ library is intended to be a relatively lightweight set of reusable components rather than an exhaustive catalog of RL environments. For applications of verifiers (e.g. "an Environment for XYZ task"), you are welcome to submit a PR for a self-contained module that lives within environments/ if it serves as a canonical example of a new pattern. Stay tuned for more info shortly about our plans for supporting community Environment contributions 🙂

Citation

If you use this code in your research, please cite:

@article{brown2025verifiers,
  title={Verifiers: Reinforcement Learning with LLMs in Verifiable Environments},
  author={Brown, William},
  year={2025}
}

Roadmap

• A community Environments hub for crowdsourcing, sharing, and discovering new RL environments built with verifiers
• Default patterns for hosted resources such as code sandboxes, auxiliary models, and MCP servers
• Multimodal input support
• Non-increasing token sequences via REINFORCE