Opus 4.7 vs GLM 5.1: is mixing models worth it?

A real-world benchmark comparing three approaches to implementing a chunked translation feature in a WordPress plugin: Claude Opus 4.7 alone, GLM 5.1 alone via Ollama Cloud, and GLM 5.1 following a plan written by Opus. While the mixed approach costs ~41% of Opus alone in tokens, it required 3.5× more active monitoring time and ~18 corrective interventions vs. 0 for Opus. The plan helped with 'what' to build but left the 'how' gap open, resulting in worse developer experience and sequential rather than parallel dispatch. The conclusion: model mixing pays off only for simple, well-scoped tasks — for anything involving async coordination, parallelism, or external dependencies, using the capable model alone is more cost-effective when developer time is factored in.