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Policies and Model Contracts

Research snapshot: 2026-07-23. Policy APIs changed substantially between published PufferLib 3.0.0 and current 4.0 source.

Published 3.0.0

PufferLib 3.0 policies are ordinary torch.nn.Module objects. The environment exposes single_observation_space and single_action_space; size heads from those single-agent spaces, not from the batched spaces.

Minimal feed-forward policy

Build an nn.Module with an encoder sized from env.single_observation_space.shape, an action head sized from env.single_action_space, and a one-value critic head. The official stable example defines a rollout method named forward_eval(observations, state=None) and makes the normal forward method use the same contract. Here, forward_eval is a PufferLib/PyTorch method name; it does not invoke Python's dangerous eval() builtin.

For a discrete action space, the first output contains action logits and the second is the value estimate. Preserve the leading agent-batch dimension.

Recurrent composition

The stable pufferlib.models.LSTMWrapper expects a base policy with:

def encode_observations(self, observations, state=None):
    ...

def decode_actions(self, hidden):
    ...

The wrapper uses an LSTMCell during rollout inference and an LSTM over time-batched data during training. Do not manually reshape recurrent state without checking the source's batch/time convention. Reset hidden state on actual terminations and truncations according to the trainer's mask behavior.

Structured observations

Stable emulation flattens Dict and Tuple spaces into a homogeneous array. The byte layout is described by env.emulated. In policy setup:

native_dtype = pufferlib.pytorch.nativize_dtype(env.emulated)

In the forward pass:

structured = pufferlib.pytorch.nativize_tensor(observations, native_dtype)

Keep the original flattened dtype. Constructing a new float tensor before unflattening can destroy the packed representation. Validate every recovered leaf shape and dtype before training.

Action spaces

The 3.0 source handles:

• Discrete: one categorical logits tensor.
• MultiDiscrete: one logits tensor per action branch.
• Box: a Normal distribution path for continuous actions.

Do not infer support from the 2024 paper's limitations section; that paper describes an earlier release. Test clipping/scaling against the environment's actual Box.low, Box.high, shape, and dtype. A tanh output is not a general substitute for affine mapping to arbitrary bounds.

Stable model utilities

Useful 3.0 symbols include:

• pufferlib.pytorch.layer_init
• pufferlib.pytorch.nativize_dtype
• pufferlib.pytorch.nativize_tensor
• pufferlib.models.Default
• pufferlib.models.LSTMWrapper
• pufferlib.models.Convolutional
• pufferlib.models.ProcgenResnet

Inspect the exact 3.0 source before copying signatures. Do not use top-level from pufferlib import PuffeRL; the trainer is pufferlib.pufferl.PuffeRL.

Current 4.0 source

The current PyTorch fallback composes a policy from three modules:

policy = pufferlib.models.Policy(
    encoder=encoder,
    decoder=decoder,
    network=network,
)

The source contract is:

• Policy.initial_state(batch_size, device)
• Policy.forward_eval(x, state) for rollout inference
• Policy.forward(x) for time-batched training
• encoder maps observations to hidden vectors
• recurrent/network module maps hidden vectors and state
• decoder maps hidden vectors to action logits and values

Current built-ins include DefaultEncoder, DefaultDecoder, MLP, MinGRU, LSTM, GRU, NatureEncoder, and ImpalaEncoder. INI config selects the Torch fallback components:

[torch]
network = MinGRU
encoder = DefaultEncoder
decoder = DefaultDecoder

[policy]
hidden_size = 128
num_layers = 4

The default 4.0 backend is the native implementation, not this Torch fallback. The CLI flag --slowly selects the fallback.

Shape and numerical checks

Run these checks before a long job:

1. Reset the reviewed environment and record observation shape/dtype/range.
2. Run one policy inference under torch.no_grad().
3. For discrete actions, require logits shape (agent_batch, action_space.n).
4. Require values to represent one scalar per active agent.
5. For MultiDiscrete, verify branch count and each branch width.
6. For recurrent policies, verify state batch matches active agent rows and that masks reset state at episode boundaries.
7. Reject NaN/Infinity in observations, logits, values, losses, and gradients.
8. Confirm inactive/padded multi-agent rows do not contribute to loss.
9. Run backward once and verify finite, non-missing gradients.
10. Compare eager and compiled outputs before enabling compilation.

torch.compile and reduced precision can alter performance and numerical behavior. Record PyTorch, CUDA, compiler mode, precision, and deterministic settings. Do not claim determinism solely because seeds are fixed.

Checkpoint-safe policy workflow

• Save weights/state dictionaries, architecture config, environment revision, package lock, seed, and checksum separately.
• Do not serialize arbitrary policy objects.
• Never call torch.load on an untrusted file. PufferLib 3.0 and the 4.0 Torch fallback use torch.load for model paths; provenance review is therefore a precondition, not an optional cleanup.
• Inspect metadata first with scripts/inspect_checkpoint.py; it never imports Torch or deserializes.
• Verify an expected SHA-256 and license before loading.
• If business requirements force inspection of an untrusted model, isolate the operation in a disposable sandbox with no credentials, network, host mounts, or sensitive data. PyTorch warns that models are programs and that even inspection tools may execute model code.

Sources

• PufferLib 3.0 policy example — stable example; accessed 2026-07-23.
• PufferLib 3.0 PyTorch utilities — stable implementation; accessed 2026-07-23.
• PufferLib 3.0 models — stable model classes; accessed 2026-07-23.
• PufferLib 4.0 models — current source model contract; accessed 2026-07-23.
• PufferLib 4.0 Torch trainer — current fallback and checkpoint loading; accessed 2026-07-23.
• PyTorch security policy — untrusted-model guidance; accessed 2026-07-23.
• PyTorch torch.load documentation — deserialization warning; accessed 2026-07-23.