mcp-bloat

Audit how many tokens your MCP servers eat from your context window — before your agent pays for them.

Every MCP server you connect makes your AI client load that server's tool schemas into the model's context — on every session, before you've asked a single question. A handful of servers can quietly consume a noticeable share of the context window. mcp-bloat measures it: it finds your MCP configs, connects to each server for real, and prints the token bill.

Install

pip install mcp-bloat
# or, without installing:
uvx mcp-bloat

For exact token counts (tiktoken's cl100k_base instead of a chars/4 estimate), install the accurate extra:

pip install "mcp-bloat[accurate]"

Usage

# Audit every MCP server in your known client configs
mcp-bloat

# See it work without configuring anything: audits three generated demo servers
mcp-bloat --demo

# Machine-readable output
mcp-bloat --json

# Just the per-server totals, without the per-tool breakdown
mcp-bloat --summary

# Fail CI if your servers eat more than 30,000 tokens of context
mcp-bloat --budget 30000

# Audit one specific config file
mcp-bloat --config ./mcp.json

# Audit one remote server directly, by URL — for example a server you
# use through ChatGPT or another hosted client
mcp-bloat --url https://example.com/mcp

# Look up ONE tool by name — which server(s) host it and what it costs
mcp-bloat --tool createPaymentTransaction

# If that server expects an Authorization header, pass it through
# (--header is repeatable, and --name sets the label shown in the report)
mcp-bloat --url https://example.com/mcp --header "Authorization: Bearer <token>"

# Compute the percentages against a specific model's context window —
# looked up live from OpenRouter/LiteLLM (free, no API key; cached 24h)
mcp-bloat --model gpt-5
mcp-bloat --model claude-sonnet-4-5

# …or set the window size yourself (this overrides --model)
mcp-bloat --window 128000

Which window am I using?

Every percentage in the report is relative to a context window. By default that's 200,000 tokens. Pass --model NAME to measure against the window of the model you actually use — mcp-bloat resolves the name against live model data from OpenRouter (with LiteLLM as a fallback and a small built-in snapshot when you're offline), and the report states which model and window it used. --window N skips the lookup and uses exactly N tokens. A server that eats 14% of a 200k window eats about 22% of a 128k one — same tokens, different pain.

Looking up a single tool

The full audit tells you which server is fat; sometimes you just want one tool. --tool NAME runs the audit as usual, then reports only that tool — no URL or config spelunking needed, since the tool names come from the servers themselves:

mcp-bloat --tool createPaymentTransaction

Matching is exact and case-insensitive first, then falls back to substring matches. If the same tool name is hosted by several servers, every match is shown. Each match reports the server and config it came from, the tool's total tokens, its share of that server's tool bill and of the context window, and a cost split — how much of the bill is the name + description text versus the input schema (the two parts plus the JSON structure roughly add up to the total). If nothing matches, you get the closest tool names that do exist and exit code 2. --tool combines with --config and --url to search just that target, and with --model/--window for the window percentage; with --json it prints {"tool": ..., "matches": [...]} instead.

From mcp-bloat --demo --tool run_tests:

Tool lookup: run_tests
┏━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ Tool      ┃ Server      ┃ Source                                     ┃ Tokens ┃ % of server ┃ % of 200k ctx ┃ Cost split                      ┃
┡━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┩
│ run_tests │ dev-toolbox │ Demo — mcp-bloat --demo (generated config) │    208 │        8.7% │          0.1% │ description ≈ 39 · schema ≈ 166 │
└───────────┴─────────────┴────────────────────────────────────────────┴────────┴─────────────┴───────────────┴─────────────────────────────────┘

╭─ mcp-bloat tool lookup ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ 1 match for 'run_tests' across 1 server(s).                                                                                                                  │
│ Cost split: description counts the tool's name + description text; schema counts its inputSchema. The two parts plus the JSON structure roughly add up to    │
│ the total.                                                                                                                                                   │
│ Counting method: tiktoken cl100k_base                                                                                                                        │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯

Sample output

From mcp-bloat --demo (three generated servers: a tidy one, a medium one, and a deliberately huge one):

Demo — mcp-bloat --demo (generated config)
┏━━━━━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━━━━┳━━━━━━━━━┓
┃ Server      ┃ Transport ┃ Tools ┃ Tokens ┃ % of 200k ctx ┃ Status  ┃
┡━━━━━━━━━━━━━╇━━━━━━━━━━━╇━━━━━━━╇━━━━━━━━╇━━━━━━━━━━━━━━━╇━━━━━━━━━┩
│ notes-lite  │ stdio     │     3 │    204 │          0.1% │ ok      │
│ dev-toolbox │ stdio     │    15 │  2,394 │          1.2% │ ok      │
│ mega-api    │ stdio     │    50 │ 25,164 │         12.6% │ ⚠ BLOAT │
└─────────────┴───────────┴───────┴────────┴───────────────┴─────────┘

…

dev-toolbox — tools by cost
┏━━━━━━━━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━━┓
┃ Tool           ┃ Tokens ┃ % of server ┃
┡━━━━━━━━━━━━━━━━╇━━━━━━━━╇━━━━━━━━━━━━━┩
│ grep           │    254 │       10.6% │
│ run_tests      │    208 │        8.7% │
│ list_files     │    199 │        8.3% │
│ edit_file      │    186 │        7.8% │
│ run_command    │    174 │        7.3% │
│ read_file      │    172 │        7.2% │
│ get_git_diff   │    164 │        6.9% │
│ fetch_url      │    158 │        6.6% │
│ create_branch  │    155 │        6.5% │
│ write_file     │    152 │        6.3% │
│ search_docs    │    147 │        6.1% │
│ get_git_status │    130 │        5.4% │
│ lint           │    108 │        4.5% │
│ format_code    │    107 │        4.5% │
│ package_info   │     80 │        3.3% │
└────────────────┴────────┴─────────────┘

…

╭─ mcp-bloat summary ───────────────────────────────────────────────────╮
│ Total: 27,762 tokens  (13.9% of a 200,000-token context window)       │
│ Of your 27,762 tokens, that's:                                        │
│   2.8% of Claude Sonnet 5.5's 1,000,000 window                        │
│   6.9% of GPT-5's 400,000 window                                      │
│   2.6% of Gemini 2.5 Pro's 1,048,576 window                           │
│   16.9% of DeepSeek V3's 163,840 window                               │
│ Servers audited: 3                                                    │
│ Counting method: tiktoken cl100k_base                                 │
│ Most expensive single tool: list_workflow_run (mega-api) — 690 tokens │
│ ⚠ Over 10,000 tokens on its own: mega-api (25,164)                    │
╰───────────────────────────────────────────────────────────────────────╯

After each config's server table, the report drills into that server's tools — every tool's token cost, fattest first, with its share of the server's tool bill — and the summary names the single most expensive tool of the whole run. Pass --summary to collapse the report back to per-server totals only; --json always includes the per-tool data (each server's tools array — including each tool's description/schema token split — plus a top-level most_expensive_tool).

In default mode (no --model/--window), the summary also lines your total up against several current frontier models' context windows at once — resolved from the same live model data — so you can see the cost on the models people actually use; pass --model or --window and the summary sticks to that single window instead.

Any single server over 10,000 tokens is flagged ⚠ BLOAT.

How the counting works

For each configured server, mcp-bloat:

1. Connects to the server over MCP (stdio or streamable HTTP) and performs the standard initialize handshake.
2. Calls tools/list (and prompts/list / resources/list when the server supports them — prompts and resources are surfaced to the model too).
3. For every tool, serializes {name, description, inputSchema} to JSON — the same information a client sends to the model so it can call the tool — and counts the tokens in that string.

Token counting uses tiktoken's cl100k_base encoding when the accurate extra is installed, and falls back to a chars/4 estimate otherwise. The report always states which method produced the numbers.

This is a close proxy for what clients actually send, not a byte-exact replica: individual clients wrap tool definitions slightly differently, and models with different tokenizers will count a little differently. Treat the numbers as accurate to a few percent — more than enough to find the server that's eating your context.

Servers that fail to start or don't respond within the timeout (default 20s, --timeout to change) are reported as errors; they never abort the rest of the audit. Server stderr is discarded so broken servers can't corrupt the report.

Config locations searched

• Claude Code — ~/.claude.json, ./.mcp.json
• Cursor — ~/.cursor/mcp.json, ./.cursor/mcp.json
• Claude Desktop — ~/.config/Claude/claude_desktop_config.json, ~/Library/Application Support/Claude/claude_desktop_config.json
• Windsurf — ~/.codeium/windsurf/mcp_config.json
• VS Code — ./.vscode/mcp.json (the servers key)
• Gemini CLI — ~/.gemini/settings.json, ./.gemini/settings.json (both the url and httpUrl spellings for HTTP servers)
• Codex CLI — ~/.codex/config.toml (the [mcp_servers.*] tables)

Clients that don't keep MCP servers in a local config file — ChatGPT, for instance — can still be covered: take the server's URL and pass it to --url, with --header for any authorization header the server expects.

Malformed config files produce a warning, not a crash.

License

MIT