Issue evidence · Run · huggingface/trl

log_completions=True crashes GRPO training with tool calls: ArrowInvalid writing the completions parquet (regression in 1.15.0)

GitHub issue: https://github.com/huggingface/trl/issues/7674

Reproduced, workaround proven

With log_completions=True, GRPOTrainer crashes at the first logging step whose rollouts include a tool call, a regression in TRL 1.15.0.

What Badgr ran

Ran the reporter's repro.py unchanged on an RTX 4090 through Badgr (pytorch 2.8.0 cu126 image, transformers 5.19.0, pyarrow 26.0.0), both modes (content_blocks, arg_types), on TRL 1.14.2 and then 1.15.0.

What came back

  • TRL 1.14.2: exit 0 in both modes.
  • TRL 1.15.0: exit 1 in both modes with pyarrow.lib.ArrowInvalid: ('cannot mix list and non-list, non-null values', 'Conversion failed for column completion with type object').

Pinning trl==1.14.2 is the workaround that passed here. main was not tested. The same script hit unrelated errors on a Mac CPU, so only the GPU run is reported.

Checked 2026-10-14 in Badgr’s local development environment.

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