AI API cost review path for prompt caching and tool calls

An AI assistant should use an MCP tool when the answer depends on external state, current data, private files, or an action that cannot be proven from chat context alone.

The Model Context Protocol gives AI applications a standard way to connect to external systems such as files, databases, search engines, calculators, and specialized prompts. That does not mean every question needs a tool. If the user asks for general explanation, the assistant can answer directly. If the user asks what is inside a repository, calendar, issue tracker, spreadsheet, or live service, a tool is usually the safer path.

A practical decision table has four rows. Use chat context for definitions, stable concepts, and reasoning over text already provided. Use a retrieval or file tool when the answer depends on a document. Use an API or database tool when the answer depends on current records. Use an action tool only after the user can see and approve the operation.

The risk of avoiding tools is hallucinated certainty. The assistant may sound confident while guessing about a file, ticket, or setting it has not read. The risk of overusing tools is unnecessary exposure and slow interaction. A weather lookup needs a weather source; a definition of "cache hit" does not need a database.

Teams should write tool-use rules around evidence. A good rule says, "If the answer names a current value, read it." A weak rule says, "Use tools whenever possible." Evidence-based routing keeps the assistant useful without turning every question into an external call.

The final check is user visibility. If the tool can change state, spend money, message someone, or expose private data, the interface should show what will happen before it happens. MCP standardizes the connection pattern, but product design still decides how much consent and explanation the user sees.

When an AI assistant should use an MCP tool instead of answering from chat context

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