Software systems often record important runtime information in logs to help with troubleshooting. Log-based anomaly detection has become a key research area that aims to identify system issues through log data, ultimately enhancing the reliability of software systems. Existing methods often fall short in capturing the semantic information, typically expressed in natural language, or the sequential dependencies inherent in log sequences. In this paper, we propose LogLLM, a framework that enables collaboration between heterogeneous LLMs for log-based anomaly detection. LogLLM exploits the complementary capabilities of different LLM architectures: a Transformer encoder-based LLM is employed to extract fine-grained semantic vectors from individual log messages, while a Transformer decoder-based LLM is utilized to model sequential dependencies and generate anomaly detection decisions. To enable effective collaboration between these heterogeneous LLMs, we introduce a learnable projector to align their vector representation spaces. Furthermore, we design a progressive three-stage training strategy to optimize the collaboration between heterogeneous LLMs by gradually aligning their representations and adapting them to log anomaly detection. Unlike conventional methods that require log parsers to extract templates, LogLLM preprocesses log messages with regular expressions, streamlining the entire process. Experimental results on four public real-world datasets demonstrate that LogLLM outperforms state-of-the-art methods, achieving an average F$_1$-score improvement of 6.6% over the strongest existing approach. Further analyses provide insights into the effectiveness of the key components of the model architecture and progressive three-stage training strategy.

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