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Theo Browne describes three eras of agentic coding through Sonnet 3.5, Opus 4.5 and Mythos 5. In his view, the progression runs from reliable tool calls to longer tasks and then to orchestrating and verifying delegated work.
- 1Tool callsSonnet 3.5 was not the first model with tool calls, but Theo found them reliable enough for daily coding work.
- 2Long-running workTheo says Opus 4.5 could work for longer without losing track of its task.
- 3OrchestrationHe describes Mythos 5 as a jump toward delegating parts of a task to other models and verifying their work.
Conclusiones
- Theo associates Sonnet 3.5 with reliable tool calls for everyday coding.
- He sees Opus 4.5 as a step toward longer-running tasks.
- He associates Mythos 5 with breaking work up, delegating it and verifying the results.
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Three eras of coding agents
Theo connects three model releases with tool calling, long-running work and orchestration. This is his framework for understanding their progress.
I think of these as eras. Sonnet 3.5 represents the tool-call era: it was not the first model able to call tools, but, in my experience, it was the first to do so consistently and reliably enough for everyday coding.
Then came Opus 4.5, which could work on much longer-running tasks without losing track of what it was doing.
Mythos 5 feels like another jump, this time to orchestration. In my experience it understands not only the codebase but how to use other models: it can split up work, spawn additional agents, and have the results verified afterward when asked.
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