mcp-tax is a Python command-line utility that estimates how much context your configured MCP servers’ tool definitions may use, then lets you disable selected servers for a Claude Code session. Its author, Hao Li, describes the tool in a DEV Community article published September 30, 2026. The displayed token figures are rough estimates—not exact model-token counts or a measure of the size of tool results.
Why MCP tools can add context overhead
MCP servers can expose tools for an AI client to discover and use. The official Model Context Protocol TypeScript SDK documentation describes tool definitions with names, descriptions, and input schemas that are handed to a model. A large or detailed catalog can therefore add tool-definition overhead.
That does not establish that every MCP host loads every definition into every request. The cost depends on how the host and model handle tool definitions. The practical question mcp-tax addresses is narrower: what is the approximate size of the tool schemas exposed by the stdio servers in your configuration?
How mcp-tax measures a server
Li says mcp-tax reads ~/.claude.json and, if present, the project-level ./.mcp.json, then lists configured servers. For stdio servers, it communicates with each server, sends initialize, requests tools/list, and measures the compactly serialized tool-list schema.
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- Discover: Read the global and optional project configuration and identify configured servers.
- Connect: Start each supported stdio server and perform the MCP initialization handshake.
- List tools: Request the server’s tool definitions.
- Estimate: Count characters in the compact JSON representation and divide by four, rounding the result.
The division-by-four figure is an order-of-magnitude gauge. Character counts do not equal tokenizer counts, and Claude Code may wrap schemas differently. Use the estimate to compare servers in the same audit, not as an exact context-window reading or billing meter.
What the author’s example shows
In Li’s 2026 article, the example reports the following schema sizes. These are illustrative figures from the tool author, not an independently reproduced result or a representative MCP benchmark.
| Server in the example | Tools | Schema characters | Estimated tokens at characters ÷ 4 |
|---|---|---|---|
| GitHub | 51 | 118,203 | 29,551 |
| PostgreSQL | 9 | 12,440 | 3,110 |
| Total | 60 | 130,643 | 32,661 |
Li’s total is 32,661 estimated tokens for the 60-tool example. Compared with an assumed 200,000-token context window, that is about 16.3%. The comparison is illustrative: it does not establish that those tokens are always loaded into a model’s context, nor that other MCP setups have similar overhead.
Disable servers for a Claude Code session
Li describes two controls: mcp-tax off <server> to mark a server disabled and mcp-tax on <server> to re-enable it. The disabled names are stored in ~/.config/mcp-tax/disabled.json.
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To launch with disabled servers filtered out, the documented form is:
mcp-tax run -- <Claude Code arguments>
The utility is described as generating a filtered mcpServers configuration and launching Claude Code with those servers removed, without modifying the real ~/.claude.json. Li says he did not have Claude Code installed to verify claude --mcp-config; if that option changes, mcp-tax prints the filtered configuration path so it can be passed manually. Check the behavior against the Claude Code version installed on your system.
Coverage and limits to check before relying on it
- Transport: The audit covers stdio servers, not SSE or streamable HTTP servers.
- Operating system: Li says the
select(2)stdout-pipe handling works on Linux and macOS, not Windows. - What it measures: It estimates tool-schema size, not runtime tool results. A small catalog can still return large outputs.
- Token accuracy: The characters-divided-by-four method is approximate and may differ from actual tokenization and host-specific schema wrapping.
- Test status: Li reports that seven smoke tests passed, including tests with a fake stdio MCP server using newline-delimited JSON-RPC 2.0 framing. That is the author’s report, not an independent test of the utility.
For the implementation details and the author’s complete caveats, see Hao Li’s September 30, 2026 DEV Community article about mcp-tax.
When the estimate is useful
mcp-tax is most useful as a triage tool: identify which supported stdio servers expose the largest schemas, then decide whether you need those servers in a particular session. A large estimate can prompt a closer look at whether a server’s full catalog is necessary; it does not, by itself, prove a context problem. Likewise, a low schema estimate says nothing about the size of data returned when tools run.
The available figures do not establish typical MCP context overhead across users or deployments. Treat Li’s example as an illustration of the calculation, and interpret your own result as a comparison among the servers mcp-tax can audit.
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