Claude Opus 5.5: 40% Cheaper and 27.5% Less Code — But Sonar's Independent Benchmark Found a Concurrency Trap

Wed, 23 Sep 2026 00:00:00 +0000

Anthropic shipped Claude Opus 5.5 on September 22, 2026 — the first member of the new Claude 5.5 family, and its first release since the company publicly called for pacing the frontier. The pitch is simple and appealing: Fable 5.1-class capability at 40% lower cost, with input tokens at $4 and output at $20 per million.

The efficiency claim holds up. An independent evaluation run by Sonar on a pre-release build confirms that Opus 5.5 writes dramatically less code for the same tasks and burns far fewer tokens doing it. But that same evaluation surfaces something the launch post does not emphasize: bug density per line went up 11.8%, and concurrency findings jumped 44%.

API Migration on SoloSoft

Claude Opus 5.5: 40% Cheaper and 27.5% Less Code — But Sonar's Independent Benchmark Found a Concurrency Trap