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Unsloth SFTTrainer: 'int' object has no attribute 'mask_token'

Constructing an Unsloth SFTTrainer (often in Colab) raises AttributeError: 'int' object has no attribute 'mask_token'. The crash comes from API drift between Unsloth's patched trainer __init__ and a newer TRL/transformers SFTTrainer signature, where a positional integer argument lands where a tokenizer was expected.

Quick answer

Constructing an Unsloth SFTTrainer (often in Colab) raises AttributeError: 'int' object has no attribute 'mask_token'.

Data Pipeline#unsloth#sfttrainer#trl#colab#attributeerror#version-mismatch

What this failure is

Unsloth SFTTrainer: 'int' object has no attribute 'mask_token' is a Data Pipeline failure seen during ML training runs. Constructing an Unsloth SFTTrainer (often in Colab) raises AttributeError: 'int' object has no attribute 'mask_token'. The crash comes from API drift between Unsloth's patched trainer __init__ and a newer TRL/transformers SFTTrainer signature, where a positional integer argument lands where a tokenizer was expected. Common tags: Unsloth, Sfttrainer, Trl, Colab.

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Why it happens (the mechanism)

TRL/transformers changed the SFTTrainer argument order/signature, so an integer (e.g. max_seq_length) was bound to a parameter Unsloth's patch treats as a tokenizer. Unsloth's monkey-patched __init__ then calls .mask_token on that int, raising AttributeError. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.

What you'll observe

  • SFTTrainer(...) construction crashes immediately
  • Error references mask_token on an int
  • Fresh Colab notebooks stop working after a dependency bump

Common symptoms and what they mean

SymptomWhy it happens
AttributeError: 'int' object has no attribute 'mask_token'TRL/transformers changed the SFTTrainer argument order/signature, so an integer (e.g. max_seq_length) was bound to a parameter Unsloth's patch treats as a tokenizer
Traceback through unsloth/trainer.py new_init -> original_initUnsloth's monkey-patched __init__ then calls .mask_token on that int, raising AttributeError
Happens at trainer creation, before training startsTRL/transformers changed the SFTTrainer argument order/signature, so an integer (e.g. max_seq_length) was bound to a parameter Unsloth's patch treats as a tokenizer

Which systems are affected

  • Unsloth SFTTrainer on Colab/Kaggle
  • Mismatched unsloth vs TRL/transformers versions
  • Notebooks that pass trainer args positionally

How to confirm this is the problem

Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.

  • ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
  • ✓Verified signal present: AttributeError: 'int' object has no attribute 'mask_token'
  • ✓Verified signal present: Traceback through unsloth/trainer.py new_init -> original_init
  • ✓Verified signal present: Happens at trainer creation, before training starts
  • ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.

The fix and the prevention pattern

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Related failures to investigate next

Augmentation Pipeline Error often co-occurs with this failure because both share underlying assumptions about data pipeline; if you fix this entry but still see drift, validate the related primary cause next. augmentation pipeline error →
Lmdb Corruption often co-occurs with this failure because both share underlying assumptions about data pipeline; if you fix this entry but still see drift, validate the related neighboring cause next. lmdb corruption →
Text Encoding Mismatch often co-occurs with this failure because both share underlying assumptions about data pipeline; if you fix this entry but still see drift, validate the related neighboring cause next. text encoding mismatch →

Root cause

  • TRL/transformers changed the SFTTrainer argument order/signature, so an integer (e.g. max_seq_length) was bound to a parameter Unsloth's patch treats as a tokenizer
  • Unsloth's monkey-patched __init__ then calls .mask_token on that int, raising AttributeError

The fix and how to prevent it

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References

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