Stable Baselines3

Overview

Stable Baselines3 (SB3) is a PyTorch-based library providing reliable implementations of reinforcement learning algorithms. This skill provides comprehensive guidance for training RL agents, creating custom environments, implementing callbacks, and optimizing training workflows using SB3's unified API.

Current upstream: SB3 2.9.0 (June 15, 2026). Docs: stable-baselines3.readthedocs.io.

Installation

Tested against stable-baselines3 2.9.0. Requires Python 3.10+ (3.9 dropped in 2.8.0) and PyTorch >= 2.8.

# Basic installation
uv pip install "stable-baselines3==2.9.0"

# With extra dependencies (TensorBoard, ale-py for Atari, etc.)
uv pip install "stable-baselines3[extra]==2.9.0"

The 2.9.0 release supports Gymnasium >=0.29.1,<2.0; this review exercised Gymnasium 1.3.0, PyTorch 2.14.1 and Python 3.13 on CPU. pandas/matplotlib are now optional extras. Installation requires network unless packages are cached; local RL training needs no credentials or service endpoints.

On zsh, quote brackets: uv pip install 'stable-baselines3[extra]==2.9.0'.

For MuJoCo continuous-control benchmarks:

uv pip install "gymnasium[mujoco]"

Check your version:

import stable_baselines3
print(stable_baselines3.__version__)

Related Projects

• SB3-Contrib: experimental algorithms (MaskablePPO, CrossQ, QR-DQN, RecurrentPPO) — separate sb3-contrib package
• RL Baselines3 Zoo: pre-trained agents, hyperparameters, training scripts
• SBX: SB3 + JAX implementations for users who prefer JAX over PyTorch

Core Capabilities

1. Training RL Agents

Basic Training Pattern:

import gymnasium as gym
from stable_baselines3 import PPO

# Create environment
env = gym.make("CartPole-v1")

# Initialize agent (device="cpu" is often faster for MlpPolicy on small envs)
model = PPO("MlpPolicy", env, verbose=1, device="cpu", seed=0)

# Train the agent
model.learn(total_timesteps=10000)

# Save the model
model.save("ppo_cartpole")

# Load the model (without prior instantiation)
model = PPO.load("ppo_cartpole", env=env, device="cpu")
env.close()

Important Notes:

• total_timesteps is a lower bound; actual training may exceed this due to batch collection
• Call the class method PPO.load(...) and keep the returned new model
• The replay buffer is NOT saved with the model to save space

Algorithm Selection: Use references/algorithms.md for detailed algorithm characteristics and selection guidance. Quick reference:

• PPO/A2C: General-purpose, supports Box, Discrete, flat MultiDiscrete and MultiBinary actions, good for multiprocessing
• SAC/TD3: Continuous control, off-policy, sample-efficient
• DQN: Discrete actions, off-policy
• HER: Replay-buffer strategy for goal-conditioned off-policy tasks

See scripts/train_rl_agent.py for a complete training template with best practices.

2. Custom Environments

Requirements: Custom environments must inherit from gymnasium.Env and implement:

• __init__(): Define action_space and observation_space
• reset(seed, options): Return initial observation and info dict
• step(action): Return observation, reward, terminated, truncated, info
• render(): Visualization (optional)
• close(): Cleanup resources

Key Constraints:

• Default CNN image preprocessing expects np.uint8 in range [0, 255]
• Use channel-first format when possible (channels, height, width)
• SB3 normalizes images automatically by dividing by 255
• For pre-normalized float images, use channel-first layout and policy_kwargs={"normalize_images": False}
• SB3 does NOT support Discrete or MultiDiscrete spaces with start!=0

Validation:

from stable_baselines3.common.env_checker import check_env

check_env(env, warn=True)

See the template and environment guide. The template now observes both agent and random goal coordinates, shape (4,); old shape (2,) checkpoints require retraining. gym.make("CustomEnv-v0") adds the 100-step time limit; direct CustomEnv() does not. check_env checks API consistency, not Markov sufficiency, reward correctness or learnability.

3. Vectorized Environments

Purpose: Vectorized environments run multiple environment instances in parallel, accelerating training and enabling certain wrappers (frame-stacking, normalization).

Types:

• DummyVecEnv: Sequential execution on current process (for lightweight environments)
• SubprocVecEnv: Parallel execution across processes (for compute-heavy environments)

Quick Setup:

from stable_baselines3 import PPO
from stable_baselines3.common.env_util import make_vec_env

# DummyVecEnv batches 4 lightweight environments sequentially.
env = make_vec_env("CartPole-v1", n_envs=4, seed=0)
try:
    model = PPO("MlpPolicy", env, verbose=1, device="cpu")
    model.learn(total_timesteps=25000)
finally:
    env.close()

Off-Policy Optimization: With step-based train_freq, gradient_steps=-1 matches gradient updates to collected transitions (train_freq * n_envs) after warmup. This changes compute and reuse of data; benchmark it rather than assuming it is always faster. SubprocVecEnv creation belongs under a main guard in a Python file.

API Differences:

• reset() returns only observations (info available in vec_env.reset_infos)
• step() returns 4-tuple: (obs, rewards, dones, infos) not 5-tuple
• Environments auto-reset after episodes
• Terminal observations available via infos[env_idx]["terminal_observation"]

See references/vectorized_envs.md for detailed information on wrappers and advanced usage.

4. Callbacks for Monitoring and Control

Purpose: Callbacks enable monitoring metrics, saving checkpoints, implementing early stopping, and custom training logic without modifying core algorithms.

Common Callbacks:

• EvalCallback: Evaluate periodically and save best model
• CheckpointCallback: Save model checkpoints at intervals
• StopTrainingOnRewardThreshold: Stop when target reward reached
• ProgressBarCallback: Display training progress with timing

Custom Callback Structure:

from stable_baselines3.common.callbacks import BaseCallback

class CustomCallback(BaseCallback):
    def _on_training_start(self):
        # Called before first rollout
        pass

    def _on_step(self):
        # Called after each environment step
        # Return False to stop training
        return True

    def _on_rollout_end(self):
        # Called at end of rollout
        pass

Available Attributes:

• self.model: The RL algorithm instance
• self.num_timesteps: Total environment steps
• self.training_env: The training environment

Chaining Callbacks:

from stable_baselines3.common.callbacks import CallbackList

callback = CallbackList([eval_callback, checkpoint_callback, custom_callback])
model.learn(total_timesteps=10000, callback=callback)

See references/callbacks.md for comprehensive callback documentation.

5. Model Persistence and Inspection

Saving and Loading:

from stable_baselines3.common.vec_env import VecNormalize

# After training with VecNormalize, save a matching pair:
model.save("model_name")
model.get_vec_normalize_env().save("vec_normalize.pkl")

# Build the same underlying environment and wrappers before loading:
vec_env = make_vec_env("Pendulum-v1", n_envs=1, seed=20000)
vec_env = VecNormalize.load("vec_normalize.pkl", vec_env)
vec_env.training = False
vec_env.norm_reward = False
model = PPO.load("model_name", env=vec_env, device="cpu")

This is a continuation fragment for a PPO/Pendulum run with normalization. Load only trusted model/statistics files. For off-policy training continuation, save_replay_buffer() / load_replay_buffer() are separate from save() / load(). Resume with a live environment and learn(..., reset_num_timesteps=False).

Parameter Access:

# Get parameters
params = model.get_parameters()

# Set parameters
model.set_parameters(params)

# Access PyTorch state dict
state_dict = model.policy.state_dict()

6. Evaluation and Recording

Evaluation: When training uses VecNormalize, load its saved training statistics into a separate evaluation environment with the same observation wrappers. Set training=False to freeze those statistics and norm_reward=False to report rewards in the original units; do not fit normalization on evaluation episodes. Save the normalization state alongside the model checkpoint.

from stable_baselines3.common.evaluation import evaluate_policy

mean_reward, std_reward = evaluate_policy(
    model,
    eval_env,  # Separate Monitor-wrapped environment with held-out seeds
    n_eval_episodes=10,
    deterministic=True
)

Video Recording:

from stable_baselines3.common.vec_env import VecVideoRecorder

# Requires moviepy, an FFmpeg encoder and the environment rendering dependency.
env = make_vec_env("CartPole-v1", n_envs=1, env_kwargs={"render_mode": "rgb_array"})
# Wrap before stepping, and close after recording to flush the clip.
env = VecVideoRecorder(
    env,
    "videos/",
    record_video_trigger=lambda x: x % 2000 == 0,
    video_length=200
)

Use evaluate_agent.py, passing algorithm=SAC etc. for the training algorithm and the normalization file from that exact checkpoint. The helper records one bounded clip. It raises on a missing requested statistics file. MaskablePPO requires the specialized contrib evaluator.

Evaluate whole episodes on a separate Monitor-wrapped environment. Report the number of episodes, seeds, reward units, wrapper stack and deterministic/stochastic action choice. Episode SD is not a confidence interval across training runs. Use multiple independently trained seeds and a final held-out test after checkpoint selection; a short smoke run proves mechanics, not a good policy.

7. Advanced Features

Learning Rate Schedules:

def linear_schedule(initial_value):
    def func(progress_remaining):
        # progress_remaining goes from 1 to 0
        return progress_remaining * initial_value
    return func

model = PPO("MlpPolicy", env, learning_rate=linear_schedule(0.001))

Multi-Input Policies (Dict Observations):

model = PPO("MultiInputPolicy", env, verbose=1)

Use when observations are dictionaries (e.g., combining images with sensor data).

Hindsight Experience Replay (illustrative; requires a goal environment):

from stable_baselines3 import SAC, HerReplayBuffer

# env must expose observation/achieved_goal/desired_goal and vectorized compute_reward.
model = SAC(
    "MultiInputPolicy",
    env,
    replay_buffer_class=HerReplayBuffer,
    replay_buffer_kwargs=dict(
        n_sampled_goal=4,
        goal_selection_strategy="future",
    ),
)

TensorBoard Integration:

model = PPO("MlpPolicy", env, tensorboard_log="./tensorboard/")
model.learn(total_timesteps=10000)

The scripts and bounded CPU fixtures are executed in the repository suite. Long training budgets, HER/CNN/Atari/MuJoCo and unexecuted reference fragments are illustrative; retain the task-specific wrappers and validation described there.

Workflow Guidance

Starting a New RL Project:

1. Define the problem: Identify observation space, action space, and reward structure
2. Choose algorithm: Use references/algorithms.md for selection guidance
3. Create/adapt environment: Use scripts/custom_env_template.py if needed
4. Validate environment: Always run check_env() before training
5. Set up training: Use scripts/train_rl_agent.py as starting template
6. Add monitoring: Implement callbacks for evaluation and checkpointing
7. Optimize performance: Consider vectorized environments for speed
8. Evaluate and iterate: Use scripts/evaluate_agent.py for assessment

Common Issues:

• Memory errors: Reduce buffer_size for off-policy algorithms or use fewer parallel environments
• Slow training: Consider SubprocVecEnv for parallel environments
• Unstable training: Try different algorithms, tune hyperparameters, or check reward scaling
• Import errors: Ensure stable_baselines3 is installed: uv pip install 'stable-baselines3[extra]==2.9.0'

Resources

scripts/

• train_rl_agent.py: Complete training script template with best practices
• evaluate_agent.py: Agent evaluation and video recording template
• custom_env_template.py: Custom Gym environment template

references/

• algorithms.md: Detailed algorithm comparison and selection guide
• custom_environments.md: Comprehensive custom environment creation guide
• callbacks.md: Complete callback system reference
• vectorized_envs.md: Vectorized environment usage and wrappers