Clipped Importance Sampling Policy Optimization (CISPO)

CISPO (Clipped Importance Sampling Policy Optimization) is a GRPO specialization that clips importance weights directly and uses them to scale a log-prob objective.

CISPO uses the same group-based advantage calculation as GRPO, however, the objective function is closer to that of REINFORCE, multiplying the log-probability term of the function by a scaled importance ratio. A stop gradient is applied to the importance ratio, meaning the ratio is treated as a constant that scales each token’s contribution to the overall policy gradient.

In AgileRL, CISPO can be used for single-turn reasoning tasks or multi-turn agentic finetuning. In the multi-turn case, rollouts are still treated as a bandit problem, with environment generated tokens masked and reward signal calculated from cumulative episode reward.

Example

import torch
from transformers import AutoModelForCausalLM, AutoTokenizer
from agilerl.algorithms import CISPO

model = AutoModelForCausalLM.from_pretrained(
    "Qwen/Qwen2.5-3B",
    torch_dtype=torch.bfloat16,
    device_map="auto",
)
tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen2.5-3B")

agent = CISPO(
    actor_network=model,
    pad_token_id=tokenizer.eos_token_id,
    pad_token=tokenizer.eos_token,
    device="cuda" if torch.cuda.is_available() else "cpu",
    batch_size=8,
    group_size=8,
)

Training and Usage

Use CISPO anywhere you would use GRPO in AgileRL training loops, such as train_llm_rollout. Single-turn reasoning is the max_turns=1 case of the same function.

from agilerl.llm_envs import RolloutHarness
from agilerl.training.llm import train_llm_rollout

def reward_fn(completion: str, answer: str, question: str) -> float:
    del question
    return float(answer.lower() in completion.lower())

# 1) Single-turn / reasoning datasets (a prompt dataset is just an env, max_turns=1)
class QADataset:
    """Single-turn dataset env: serve a question on reset, score it on step."""

    def __init__(self, questions, answers, reward_fn, prompt_builder,
                 test_questions=None, test_answers=None):
        self.questions, self.answers = questions, answers
        self.test_questions, self.test_answers = test_questions, test_answers
        self.reward_fn, self.prompt_builder = reward_fn, prompt_builder
        self._cursor, self._split = 0, ""

    @property
    def dataset_size(self) -> int:
        return len(self.questions)

    def reset(self, seed=None, *, row_index=None, evaluation=None):
        if evaluation and self.test_questions is not None:
            qs, ans, split = self.test_questions, self.test_answers, "eval"
        else:
            qs, ans, split = self.questions, self.answers, "train"
        if row_index is None:
            if split != self._split:
                self._cursor, self._split = 0, split
            row_index, self._cursor = self._cursor, self._cursor + 1
        self._q, self._a = qs[row_index % len(qs)], ans[row_index % len(ans)]
        return self.prompt_builder(self._q), {}

    def step(self, action):
        return "", float(self.reward_fn(action, self._a, self._q)), True, False, {}

env_factory = lambda: RolloutHarness.local(
    QADataset(
        questions=["2+2?", "Capital of France?"],
        answers=["4", "Paris"],
        reward_fn=reward_fn,
        prompt_builder=lambda question: f"Q: {question}\nA:",
        test_questions=["3+3?"],
        test_answers=["6"],
    ),
    tokenizer,
    max_turns=1,
    pad_id=tokenizer.eos_token_id,
    apply_chat_template=True,
    max_model_len=1024,
)

trained_pop = train_llm_rollout(
    pop=[agent],
    max_turns=1,
    env_factory=env_factory,
    max_steps=2000,
    evaluation_interval=50,
)

# 2) Multi-turn text environments (each rollout drives its own in-process env)
class ToyRolloutEnv:
    def reset(self, seed=None):
        del seed
        return "Start: What is 2+2?", {}

    def step(self, action: str):
        reward = 1.0 if "4" in action else 0.0
        return "Done.", reward, True, False, {"correct": bool(reward)}

env_factory = lambda: RolloutHarness.local(
    ToyRolloutEnv(),
    tokenizer,
    max_turns=4,
    pad_id=tokenizer.eos_token_id,
    max_model_len=1024,
    max_output_tokens=128,
)

trained_pop = train_llm_rollout(
    pop=[agent],
    max_turns=4,
    env_factory=env_factory,
    max_steps=2000,
    evaluation_interval=50,
)

Saving and Loading Agents

To save an agent, use the save_llm_checkpoint function:

from agilerl.utils.utils import save_llm_checkpoint

save_llm_checkpoint(agent, "path/to/checkpoint")

Parameters