toolbill

Measure the token cost of MCP tool definitions before they enter a model's context window.

Why

Every MCP tool definition (its name, description, and JSON schema) is sent to the model on every turn, whether or not the tool is used. Most clients never show you how much that costs.

Four small reference servers add up to 7,994 tokens per turn. One tool alone, sequentialthinking, accounts for 1,005 of them, and more than half of that is its description.

toolbill shows you the bill: per server, per tool, and split into description, schema, and annotations, so you know what to trim or disable. It only measures. It never proxies, rewrites, or changes your setup.

toolbill is a read-only Python CLI and library for auditing MCP server tool definitions. It discovers supported local client configurations, starts local stdio servers, calls tools/list, and reports total and per-tool token counts.

Install and run

python -m pip install toolbill
toolbill audit

By default, audit discovers MCP configs for Claude Desktop, Claude Code, Cursor, VS Code, and GitHub Copilot CLI, and starts their configured stdio servers. The official mcp Python SDK handles the stdio connection and MCP protocol.

Example audit summary:

SERVER               STATUS  TOOLS  TOKENS  % OF TOTAL  HEAVIEST TOOL
filesystem           OK         14   2,852       35.7%  read_media_file (294 tokens)
memory               OK          9   2,408       30.1%  search_nodes (327 tokens)
everything           OK         13   1,729       21.6%  gzip-file-as-resource (249 tokens)
sequential-thinking  OK          1   1,005       12.6%  sequentialthinking (1,005 tokens)
TOTAL                           37   7,994      100.0%  sequentialthinking (1,005 tokens)

Measured on 2026-10-10 against the latest published versions of the four official reference servers. Run the command yourself to reproduce:

toolbill audit --config examples/reference-servers.json --timeout 60

Use --dry-run to see which servers would be launched without running them. HTTP servers are listed as SKIP; HTTP transport is not supported in this release. Command arguments and URL credentials are redacted in dry-run output. Environment variable values are never displayed.

Each server has a 15-second timeout by default. Change it with --timeout SECONDS. A server that fails to launch, initialize, times out, or fails to list tools gets a FAILED row; the audit continues with other servers. Server stderr is suppressed unless --verbose is passed. Failed servers make the CLI exit with status 1.

The summary includes each server's share of total tokens and its heaviest tool. The tool table shows the 10 heaviest tools overall by default. Use --all-tools to show every tool, or --json to print the full report (all servers and tools) as JSON:

toolbill audit --all-tools
toolbill audit --json

To audit a specific config file instead of the discovered configs:

toolbill audit --config ~/.cursor/mcp.json
toolbill audit --config .mcp.json --timeout 30

Python API

import asyncio
from toolbill import audit_config, audit_server

async def main():
    report = await audit_config("~/.cursor/mcp.json")
    print(f"{report.total_tokens:,} tokens across {report.tool_count} tools")

    for server in report.servers:  # sorted, heaviest first
        print(f"{server.name:<16} {server.tokens:>7,} ({server.share:.1%})")
        for tool in server.heaviest(5):
            print(tool.name, tool.tokens, tool.breakdown)

    filesystem = await audit_server(
        command="python",
        args=["my_mcp_server.py"],
        timeout=15,
    )
    print(filesystem.tokens, filesystem.status)

asyncio.run(main())

audit_config(path, *, model="openai", timeout=15, verbose=False, tokenizer=None) returns an AuditReport. audit_server(command, args=None, *, env=None, cwd=None, name=None, model="openai", timeout=15, verbose=False, tokenizer=None) returns a ServerReport. The first release supports the openai model option and uses tiktoken's o200k_base encoding. Pass an object implementing count(text: str) -> int as tokenizer to use a compatible custom counter.

Reports expose total_tokens, tool_count, servers, and lookup by server name (report["github"]). Each server has name, tokens, tool_count, share, status, tools, and heaviest(n). A tool has name, tokens, and a breakdown for description, schema, and annotations. Failed/skipped servers have a safe summary in error and contribute no tokens.

Tokenization details

For each MCP Tool returned by tools/list, toolbill first converts the SDK object to its protocol-field JSON object, omitting absent and null fields. It tokenizes the complete tool definition as compact UTF-8 JSON: recursively sorted object keys, no insignificant whitespace, and non-ASCII characters preserved. No client-specific wrapper or provider framing is added.

Breakdown counts are measured independently: description is counted as its plain text; schema is the compact JSON object containing inputSchema and/or outputSchema; annotations are counted as their compact JSON value. These field counts do not necessarily sum to the complete definition's token count.

tools/list pagination is followed until the server returns no cursor, so each listed tool is counted. Counting measures the tool definitions only; provider framing overhead is not included.

Configuration discovery and safety

The config fixtures in tests/fixtures follow the documented formats for each supported client:

The portable workspace .mcp.json format (mcpServers) is also discovered once and labeled MCP portable, since more than one client can read it.

MCP servers are executable programs. Review a server configuration before auditing it. --dry-run never starts a server; it omits environment values and redacts credential-looking argument and URL values. The same redaction applies to error output. Server stderr remains hidden by default; --verbose displays it, which may reveal whatever the server itself writes there.

Development

python -m pip install -e ".[test]"
pytest

The GitHub Actions workflow runs the test suite on supported Python versions. See CONTRIBUTING.md for guidelines and CHANGELOG.md for release notes.

License

MIT. See LICENSE.