Where the tokens actually go
An MCP tool call has three cost components: the tool schemas the model reads to choose a tool, the arguments it sends, and the payload that comes back. The third one dominates, by a wide margin, and it has a compounding property people miss: the result stays in the conversation. Ask five follow-up questions and you pay for that payload five more times as input context.
Now consider what "get my sales data" means against raw marketplace APIs. Orders are one endpoint, financial events another, advertising a third. Each returns nested JSON with dozens of fields per record, most of which nobody asked for. Amazon also throttles per operation, documented in the SP-API usage plans and rate limits reference, so the assistant is not only paying tokens, it is waiting between pages.
Cheap and accurate point the same way
Every token you avoid sending is also a chance for the model to mis-add a column that you avoid. When a server returns a computed contribution margin, the model cannot get the fee logic wrong, because it never touches it. Cost control and correctness are the same design decision.
What a well-designed server does for you
The protocol itself is neutral on all of this. The MCP specification describes how tools are discovered and called; it says nothing about how much you should return. That is the vendor's choice, and it is the choice worth interrogating before you connect anything.
Concretely, look for:
- Metric filters as first-class arguments, so thresholds run in the database.
- Sorting and limits on every list-returning tool.
- Time grains, so a twelve-month series is one call.
- A metric catalog the assistant can read once instead of guessing field names.
- Comparison periods built in, so variance is computed rather than derived in the prompt.
Nova MCP is built to those rules, which is why a working session usually costs less than people expect. The Nova MCP tools page lists the tools and their filters, and the prompt library shows the phrasing that keeps payloads small.