MCP Tool Definition Token Overhead Calculator
Every MCP tool you expose is serialized into the model's context on each call. With a few dozen ERP tools the schemas alone can cost thousands of tokens before any user text. This calculator turns tool count and schema size into tokens, window share and money.
Your numbers
Total tools across all connected MCP servers that the client sends to the model.
Name, description and JSON input schema serialized. ERP tools with many filter fields run 600 to 1,500.
JSON and code tokenize worse than prose. 3 to 4 is typical for English JSON.
Every agent step re-sends the tool list unless it is cached.
Your results
Token counts are estimates from a characters per token ratio. Use your model's tokenizer for exact counts and your provider's price sheet for cost.
Fixed prefix share of the window as tools are added
Sweeps Number of tools exposed from 1 to 150, holding your other inputs fixed.
Tool surface review
Send us your tool list and we will suggest a leaner design for your ERP agent. Optional.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the total number of tools the client will send, not just one server's count. Paste a representative tool definition into a character counter to get the average size, then set characters per token for your model. The primary outputs are the schema tokens and the share of the context window they occupy. Drag the tools slider to see where the prefix crosses a share you are not willing to pay on every call. The chart sweeps tool count from 1 to 150 so you can find the knee in the curve before you design a server with sixty fine-grained tools.
The math behind it
Schema tokens equal tools times average characters divided by characters per token. The fixed prefix adds the system prompt. Window share is prefix over window. Cost is prefix tokens times calls per day divided by one million times the input price, times thirty for a month. The estimate ignores output tokens and any prompt caching. If your provider supports caching of a stable prefix, the cost line drops sharply for cache hits, which makes a stable, sorted tool list worth keeping.
- Long descriptions and enum lists dominate the character count.
- Tool order changes can break prefix caching.
- Selection quality tends to degrade as the list grows, not only cost.
Reducing the overhead
Merge near-duplicate tools into one with an operation argument, shorten descriptions to what the model needs to choose correctly, and remove optional parameters nobody uses. For ERP work, expose a small set of query and action tools rather than one tool per table. If the client supports it, load tool groups on demand per task. Re-run this calculator after each change and compare the monthly line, not just the percentage.
Frequently Asked Questions
How many MCP tools is too many?
There is no fixed limit. Practically, watch two numbers: the share of the context window taken by schemas and the model's tool selection accuracy in your own tests. Many teams find quality slips noticeably once the list reaches several dozen overlapping tools.
Does a tool that is never called still cost tokens?
Yes. Unless the client loads tools lazily, every connected tool's definition is sent on each model call, used or not. That is why trimming unused tools lowers cost directly.
Is characters per token 4 accurate?
It is a rough prose figure. JSON with braces, quotes and identifiers usually tokenizes at roughly 3 to 4 characters per token, so 3.5 is a safer default. Run a few real schemas through your tokenizer to calibrate.
Does prompt caching fix this?
It reduces the price of re-sending a stable prefix, but not the space it takes in the window. A large tool list still limits room for history and results, so size matters even with caching.
If your ERP MCP server has grown past what one context can carry, Netray can help restructure the tool surface and run it against private models on your own hardware.
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