Agentic systems are no longer just model-driven — they are increasingly runtime-driven through tools, sandboxes, and orchestration layers. When agents are writing code, executing tests, calling external tools, and validating outcomes, the reliability of the entire system becomes a function of the runtime and environment, not just the model.
Controlling the runtime gives Anthropic vertical control across:
- Packaging and distribution - Execution semantics - Performance characteristics - Security surface area - Operational stability
Of course, owning a JavaScript runtime does not magically make runtime deterministic. The dominant sources of variability still sit lower in the stack: OS behaviour, native dependencies, filesystem semantics, scheduling, hardware differences, and network effects. Runtime ownership is necessary, but not sufficient, for true runtime determinism.
What this does signal is a strategic shift in where value is being captured.
Model capability is no longer the only critical layer.
The execution plane is becoming just as strategic too, can see this in ChatGPT 5’s dynamic model routing capability as well as ecosystem wide Tool Calling and MCP adoption, which are all aimed at increasing LLM determinism through traditional software stacks.
Vertical integration here looks far less like a tooling move and far more like a control-plane strategy: consolidate the stack, reduce dependency entropy, and shrink the surface area where behaviour can diverge.
Intelligence may live in the model, but reliability, scalability, and trust are increasingly properties of the system that executes it.