4 of the most confusing terms in AI, defined:
Model: a blob of parameters, written during training. Does next-token prediction and nothing else. Stateless.
Harness: everything around the model that turns it into an agent: tools, system prompt, context window management, etc.
Environment: the world the agent acts on. Anything outside the harness that the agent perceives and acts on via tools.
Agent: a model, harnessed, in an environment.
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Opus is a model.
Claude Code and Claude Web are different agents, because their harnesses differ - even though the models are the same.
The file system is an environment. MCP servers add tools to the environment.