Question 1

Why does connecting three MCP servers cost more than the sum of what each server would cost on its own?

Accepted Answer

It doesn't cost more per server, but it does cost more per turn than most people expect, because the client doesn't send only the tool schemas for the server you're about to use — it sends the schemas for every MCP server currently connected to that session, bundled into a single union, on every single turn. If you connect a GitHub MCP server with 26 tools, a Slack MCP server with 11 tools, a Postgres MCP server with 6 tools and a filesystem MCP server with 8 tools, the client resends all 51 tools' worth of schema on turn one, turn two, and every turn after that, regardless of which one tool actually gets called that turn. Three or four servers each individually cheap to run can add up to a schema payload of tens of thousands of tokens sitting in every request before the conversation even starts, the same union-of-connected-tools mechanic documented for heavy single servers like GitHub, whose tool definitions alone can run into the tens of thousands of tokens, just multiplied across however many servers happen to be wired into the same client session.

Question 2

If a server's tools were never called this session, why does it still get attributed a cost?

Accepted Answer

Because the cost being priced here isn't the cost of calling a tool — it's the cost of the schema sitting in every request whether or not it gets used. The client doesn't selectively include only the schemas for tools it expects to call; it includes the full schema for every tool on every connected server, every turn, and that block of tokens gets billed as ordinary input tokens regardless of what the model actually does with it. This calculator attributes each server's share of the monthly schema bill in proportion to that server's share of the total connected schema — tools times average tokens per tool, divided by the sum across all servers — not in proportion to how often its tools get invoked. A server with a large tool count and verbose schemas can end up responsible for most of your monthly bill even at zero actual calls, simply by staying connected, while a server you use constantly but that exposes two small tools costs almost nothing by comparison. That mismatch between 'expensive to have connected' and 'expensive to use' is the whole point of pricing attribution this way instead of by call volume.

Question 3

Does turning on prompt caching solve the multi-server schema cost problem?

Accepted Answer

It solves the dollar magnitude, not the underlying shape of the cost. Caching lets the first turn of a session pay full input price for the schema block, then read that identical block back from cache at a steep discount — typically around 90% off — for every turn after, as long as the exact same set of tools stays connected with nothing reordered or changed mid-session. That discount is real: on a typical multi-server setup it can turn a monthly bill in the thousands of dollars into a few hundred. But it does not change which server is responsible for the largest share of that bill, because the attribution is still proportional to each server's share of the connected schema, cached or not — a heavy server connected but rarely used is still the biggest line item, just at a smaller number. And caching only works while the tool list stays identical turn to turn; add, remove, or reorder a connected server mid-session and the next request misses cache and pays full price again.

Question 4

How is this different from the MCP server cost calculator and the MCP tool schema token overhead calculator already on this site?

Accepted Answer

All three price something related to MCP but none of them overlap. The MCP server cost calculator prices what it costs to build, host and maintain one MCP server as infrastructure — engineering hours, a monthly hosting bill, maintenance retainer — a number that doesn't change based on how any LLM conversation uses it. The MCP tool schema token overhead calculator prices the recurring token tax of one server's tool schemas being resent on every turn of a session, the single-server version of the schema tax. This calculator is the multi-server version: it models what happens once you connect more than one MCP server to the same client session at the same time, where the client resends the union of every connected server's schemas together on every turn, then splits that combined monthly bill back out by each server's share of the total connected schema — the attribution problem that only exists once more than one server is in the room.

MCP マルチサーバーのコスト帰属計算ツール

接続された MCP サーバー

セッション量と料金

サーバーごとの、帰属される月額コスト

これが他のツールにどのように接続されるか

この計算機の仕組み

よくある質問

3 台の MCP サーバーを接続すると、各サーバー単体の費用の合計よりも費用がかかるのはなぜですか?

サーバーのツールがこのセッションで呼び出されていないのに、なぜ依然としてコストが発生するのでしょうか?

プロンプトキャッシュを有効にすると、マルチサーバースキーマのコストの問題は解決しますか?

これは、このサイトにすでにある MCP サーバーコスト計算ツールや MCP ツールスキーマトークンオーバーヘッド計算ツールとどう違うのですか?

サーバー名	ツール	平均スキーマトークン/ツール