Sonar finds concurrency bugs vary 7x across AI code-generation models, with GPT-5.5 producing 170 bugs per million lines of Java code

Sonar's LLM Leaderboard analysis of thousands of Java coding tasks reveals concurrency bug density spans a 7x range across models: GPT-5.5 at 170 bugs per million lines of code, GPT-5.1 High at 241, GPT-5.2 High at 470, Claude Opus 4.5 Thinking at 133, Claude Sonnet 4.5 at 129, and Gemini 3.0 Pro at 69.

Concurrency bugs (failures in thread-safety mechanisms) compile and pass functional tests but fail in production because their correctness depends on thread timing that test frameworks do not control. Three common patterns are broken double-checked locking (unsafe field initialization across threads), synchronizing on value-based classes like Boolean (which the JVM caches, creating unintended shared locks), and holding locks during Thread.sleep() calls (which prevents other threads from acquiring the lock).

Static code analysis can detect these thread-safety defects by analyzing code structure, catching bugs that standard test frameworks cannot reliably trigger because a single test execution exercises only one thread-timing scenario.