Recently, the developer community found that the MiniMax M2 series model exhibited anomalies when outputting the specific name "Ma Jiaqi". MiniMax immediately conducted a full-chain investigation and released a technical report revealing the underlying mechanism behind this phenomenon: low-frequency Token degradation caused by post-training.

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Root Cause: The "Squeezed" Token

The analysis showed that the tokenizer split "Ma Jiaqi" into ['Ma', 'Jiaqi']. Since "Jiaqi" appeared frequently during pre-training, it was merged into an independent Token (ID 190467). However, in the "post-training" stage, which determines the model's conversational ability, there were fewer than five samples containing this Token.

This extremely low frequency caused the Token to remain unoptimized in the vector space, being pushed away by frequently updated high-frequency Tokens (such as code symbols and tool call markers). Eventually, the model retained knowledge about Ma Jiaqi but lost the ability to output the corresponding Token, instead choosing similar-sounding alternatives like "Jiaqi" or "Qiqi."

Chain Reaction: Forgetting Japanese and Junk Words

After scanning a 200,000-word vocabulary list, MiniMax found that approximately 4.9% of the Tokens showed significant degradation. The most severely degraded were Japanese content (degradation rate of 29.7%), which explains why the model occasionally mixes Russian or Korean characters in Japanese conversations — because Japanese Tokens experienced parameter drift, causing confusion with other languages in the space.

In addition, the degradation list included a large number of Internet SEO junk words (such as "private server", "painless abortion", etc.). Since these words almost never appear in conversation data, the model gradually "forgot" them during post-training.

Solution: Establish a "Minimum Generation Frequency"

To address this issue, MiniMax proposed three core repair strategies:

  1. Full Vocabulary Synthetic Data: Construct repetition tasks to ensure each Token has a minimum level of practice frequency during the post-training phase. Currently, the Japanese confusion rate has dropped from 47% to 1%, and the stability of the entire vocabulary list's parameters has significantly improved.

  2. Injecting Pre-training Corpus: Introduce pre-training corpus proportionally into SFT data to use its breadth to alleviate forgetting.

  3. Vocabulary Trimming and Monitoring: Remove redundant Tokens that are never used and include Token coverage in the post-training quality monitoring metrics.

Summary: