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Migrating from TRL v0 to v1

This guide covers the breaking changes introduced in TRL v1 and how to update your code. Most structural changes (trainers moved to experimental, removed model classes, etc.) already shipped in v0.29 — if you're already on v0.29, this migration is minimal.

Changed defaults

Config Parameter v0 default v1 default Action needed
GRPOConfig vllm_mode "server" "colocate" If you use use_vllm=True without specifying vllm_mode, vLLM will now run in the same process instead of connecting to a separate server. Set vllm_mode="server" explicitly if you rely on server mode.
RLOOConfig vllm_mode "server" "colocate" Same as above.

Renamed options

Config Parameter v0 value v1 value Action needed
SFTConfig packing "bfd-requeue" "bfd_split" Replace packing="bfd-requeue" with packing="bfd_split". The old value will still be accepted for a few versions but will be removed in a future release.

Removed automatic None stripping from trainer preprocessing

TRL trainers (SFT, DPO, Reward) no longer automatically strip None values from dataset examples during preprocessing. Previously, each trainer applied remove_none_values via dataset.with_transform to work around tabular backends (Arrow/Parquet) inserting None for missing keys in nested structures.

This affects datasets that contain None values because they were:

  • Created before datasets v4.7.0, which introduced the Json dtype that preserves nested structures without inserting None.
  • Created with datasets v4.7.0 or later, but saved without using the Json feature.

Action needed: If your dataset falls into one of the above categories and contains None values in nested columns, apply the fix manually before training:

from trl.trainer.utils import remove_none_values

dataset = dataset.with_transform(remove_none_values)
trainer = SFTTrainer(..., train_dataset=dataset)

Datasets created or re-saved with datasets v4.7.0+ using the Json dtype are unaffected.

Migrating from an earlier version

Depending on which version you're migrating from, refer to the release notes for v0.29 and earlier for version-specific changes.