MCP is a way to define tools that works with many apps and has a lot of extra functionality built in, it's not the only way, but it's popular because many apps support it. You can also make tools using the opencode API or any other API, and you can give them large descriptions that take up a lot of context. No matter how you define the tools, they are injected into the context of the model using the same chat template provided by the developers of the model.