An Evaluation of OpenAI GPT-5.6 Sol & Terra

Sonar’s evaluation of GPT-5.6 Sol and Terra on 4,444 Java tasks found that Sol improved functional correctness over GPT-5.5, achieving an 81.99% pass rate versus 78.66%, while reducing cyclomatic and cognitive complexity per line despite generating 6.8% more code. However, Sol’s bug density rose 44% and vulnerability density nearly tripled, with concurrency and threading becoming its largest bug category and critical security findings increasing sharply, particularly in cryptographic configuration and insecure system-resource handling. Terra generated 12.2% less code than GPT-5.5 and had a 79.96% pass rate, but its more compact output showed higher cognitive complexity, code-smell density, bug density, and vulnerability density per line. Both variants produced substantially more output tokens than GPT-5.5, and the findings suggest that improved code generation does not eliminate verification needs but shifts review and automated-analysis priorities toward concurrency behavior and security configuration.

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