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Andre Faria
Andre Faria

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MCPs Are Eating Your Context Window (And What To Do About It)

#ai #agents #automation #openclaw

I was looking at my OpenClaw token usage data when I noticed something odd. The numbers were dominated by cache reads, tens of millions of tokens per week, on a setup where the actual conversations were relatively short. The output tokens, the ones where the model is actually thinking, were a small fraction of the total.

The culprit turned out to be something I had not thought to question: MCP servers.

This article is about what MCP tool schemas actually cost, why most people miss it, and how skills solve the problem by loading lazily instead of front-loading everything into every turn. The numbers are real, measured from a real setup, priced against real provider rates.

1. What MCP servers actually inject

Model Context Protocol is a standard for connecting AI agents to external services. The idea is straightforward: define a set of tools, and the model can call them. OPNsense integration? Here are 133 tools. TrueNAS SCALE? Here are 278. GitHub? Here are 101.

The problem is how those tools reach the model. Every tool ships a JSON schema describing its name, description, parameters, types, enums, and constraints. When an MCP server is active, every single one of those schemas gets serialised and injected with every API call, whether you are going to use any of them or not. This is not a quirk of any particular client. It is how the MCP spec works. The tools array goes with every request.

Here is what that looks like in practice, measured from my homelab setup:

Component Tools Estimated tokens
TrueNAS MCP 278 ~27,800
OPNsense MCP 133 ~13,300
Playwright MCP 35 ~3,500
Native agent tools 25 ~2,500
Workspace files (AGENTS.md, SOUL.md, etc.) n/a ~3,400

Total first-turn context: approximately 41,000 tokens. Workspace files account for 8% of that. The other 92% is tool schemas.

Run 215 turns per day (a moderate multi-agent setup) and you are pushing roughly 9 million context tokens daily just to describe tools you rarely use.

2. This is not a homelab problem

A few well-known MCP servers to put scale in perspective:

MCP Server Total tools Default/active Tokens (full) Source
GitHub MCP 101 52 ~64,600 / ~30,300 Official discussion #1182
TrueNAS MCP 278 All ~27,800 Measured
OPNsense MCP 133 All ~13,300 Measured
Datadog MCP 40+ (16 core) 16 core ~4,000+ Datadog docs
AWS MCP suite 50+ across 20 servers Per server (5-15) ~1,500-3,000 each AWS Labs repo
Atlassian Rovo MCP ~12-20 All ~3,000-5,000 Estimated
Stripe MCP ~20 official All ~5,000 Stripe docs
Slack MCP 13 All ~3,250 Speakeasy catalog
Notion MCP ~14 All ~3,500 Docker MCP catalog
Sentry MCP ~10-15 All ~2,500-3,750 Estimated
PostgreSQL MCP ~5-12 All ~1,250-3,000 MCP reference server
Kubernetes MCP ~15-25 All ~3,750-6,250 Community

A developer running GitHub MCP, Slack MCP, and a Postgres MCP alongside their native tools is starting every single message with roughly 40,000 tokens of context overhead before they have typed a word. GitHub MCP alone at full capacity burns 64,600 tokens, consuming 32% of Claude Sonnet's 200K context window before the conversation starts.

3. This affects every tool that uses MCP

This is not an OpenClaw issue. It is a consequence of how MCP works architecturally, and it affects every AI tool that integrates with MCP servers:

Tool MCP support Injection pattern Notes
Claude Code Full native Eager, every API call Issue #44536: ToolSearch experiment, 85% reduction when enabled
Codex CLI Full Eager, per turn Used in Datadog + Codex integration examples
OpenCode Full Eager by default; lazy via opencode-mcp-tool-search plugin Same underlying problem; community plugin fixes it
OpenClaw Full Eager, per turn What this article is about

The MCP spec itself requires the tools array to be sent with each API call. The only documented escape valve is "ToolSearch", a meta-tool that lets the model search for tools by name rather than receiving all schemas upfront. Claude Code introduced this experimentally, with a reported 85% token reduction. GitHub MCP reduced its default toolset from 101 to 52 tools specifically in response to user complaints about context overhead.

4. What it costs per provider

On a flat-rate plan like GitHub Copilot, this overhead is invisible. You pay a fixed monthly fee regardless of token volume. But most serious usage of Claude, GPT, or Gemini goes through the API, where every token has a price.

Provider Model Input $/M Cached input $/M Output $/M Context window
Anthropic Claude Sonnet 4.6 $3.00 $0.30 $15.00 1M tokens
Anthropic Claude Haiku 4.5 $1.00 $0.10 $5.00 200K tokens
OpenAI GPT-5 $1.25 ~$0.31 $10.00 272K tokens
OpenAI GPT-4.1 $2.00 $0.50 $8.00 1M tokens
Google Gemini 2.5 Pro $1.25 ~$0.25 $10.00 1M tokens
Google Gemini 2.5 Flash $0.30 — $2.50 1M tokens
AWS Bedrock Claude Sonnet 4.6 $3.00 ~$0.30 $15.00 1M tokens
AWS Bedrock Amazon Nova Pro $0.96 $0.20 $3.84 300K tokens
Azure OpenAI GPT-4.1 ~$2.00 ~$0.50 ~$8.00 1M tokens
OpenRouter (aggregator) pass-through model-dependent pass-through varies

What 44,500 tokens of MCP overhead costs per message at different providers, assuming prompt caching is active (best case):

Provider + Model Per message (cached) Per message (uncached) Monthly (215 turns/day, 22 days)
Anthropic Claude Sonnet 4.6 $0.013 $0.134 $62 (cached) / $622 (uncached)
OpenAI GPT-4.1 $0.022 $0.089 $104 (cached) / $416 (uncached)
OpenAI GPT-5 $0.014 $0.056 $65 (cached) / $260 (uncached)
Google Gemini 2.5 Flash $0.013 $0.013 $62 (no caching)
AWS Bedrock Nova Pro $0.009 $0.043 $42 (cached) / $200 (uncached)

These are costs from overhead alone, before any actual work is done. On Sonnet without prompt caching, 44,500 tokens per message at 215 turns/day adds up to over $600/month in context overhead.

Prompt caching helps significantly for repeated context (the tool schemas do not change turn-to-turn, so they cache well). But even at the cached rate, the overhead is material at scale.

5. Skills: lazy loading as the fix

The alternative is skills. In OpenClaw and in tools like oh-my-openagent for OpenCode, a skill is a markdown file that tells the model how to use a capability. Only a name and a short description enter the context upfront. The full instructions are loaded when the model actually needs them.

A skill entry in the context looks like this:

truenas: Manage TrueNAS SCALE: storage, sharing, services, VMs, alerts, replication.
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That is roughly 24 tokens. Compare that to the ~27,800 tokens for the TrueNAS MCP schema.

The model retains full capability. When it needs to interact with TrueNAS, it reads the skill and executes shell commands: midclt websocket calls, curl against the REST API, or short Python scripts. The capability is the same. The context cost is not.

The token savings from replacing three MCP servers:

Replaced Tokens before Tokens after Saved per turn
TrueNAS MCP ~27,800 ~24 ~27,776
OPNsense MCP ~13,300 ~24 ~13,276
Playwright MCP ~3,500 ~24 ~3,476
Total ~44,600 ~72 ~44,528

First-turn context drops from ~41,000 tokens to roughly ~10,000. A 75% reduction in baseline overhead per turn.

6. What skills look like in practice

A skill is a SKILL.md file with a short frontmatter description and usage instructions. The model reads it when needed. The skill documents three things: how to authenticate, what the primary command pattern is, and what the fallback is when the primary does not cover the full surface.

Credentials live in the environment, not in the skill file. In OpenClaw, env vars are declared in openclaw.json and injected into every agent turn. Other frameworks use .env files, secrets stores, or per-agent config blocks. The skill does not care how the variables arrive, only that they exist at runtime.

{ "env": { "TRUENAS_URL": "https://truenas.host:50443", "TRUENAS_API_KEY": "..." } }
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For API key auth, that is all the setup needed. For OAuth-based services, the approach shifts to pre-authenticated CLI state: gh auth login stores credentials in ~/.config/gh/hosts.yml; jira init writes an API token to ~/.config/.jira/.config.yml. After that one-time setup, skill calls carry no credentials in the command itself.

Each skill documents a primary path and a fallback. For TrueNAS that is midclt (websocket) with curl as fallback:

# Primary: dedicated CLI
midclt -u ws://truenas.host:50443/api/current --api-key "$TRUENAS_API_KEY" call pool.query

# Fallback: curl REST
curl -sk -H "Authorization: Bearer $TRUENAS_API_KEY" "$TRUENAS_URL/api/v2.0/pool" | jq .
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For more complex operations (bulk queries, job polling, conditional logic), a short Python script is cleaner than chained shell commands:

import os, sys
sys.path.insert(0, '/home/user/.local/lib/python3.14/site-packages')
from truenas_api_client import Client

with Client(os.environ['TRUENAS_WS_URL'], api_key=os.environ['TRUENAS_API_KEY']) as c:
    for ds in c.call('zfs.dataset.query', [], {'select': ['name', 'used']}):
        print(ds['name'], ds['used'])
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Each skill also ships a check.sh that verifies the CLI is installed, env vars are set, and the host is reachable before the agent tries to use it. Validation moves from MCP schema enforcement (happens automatically before every call) to check.sh (happens at load time, once). For stable infrastructure with a single operator that is a reasonable trade. For production systems with many contributors and rapidly evolving APIs, MCPs may still be the right call.

7. Real audit numbers

Before starting this work I pulled six days of session data from my setup:

  • 92 million cache read tokens in six days
  • Average daily cost at Sonnet direct API rates: $15/day
  • Projected monthly: $285-390/month

This was on a flat-rate plan where none of it showed up in the bill. But GitHub Copilot is actively transitioning to usage-based billing. When that change completes, token volume will directly translate to cost for the first time.

The right time to fix token obesity is before you are paying per token, not after.

I also found a secondary problem during the audit: AGENTS.md had grown to 99% of the 12,000-character per-file bootstrap limit, meaning it was being silently truncated on every turn. The workspace files, which everyone assumes are the main context cost, were actually only 8% of the total. The other 92% was tool schemas that nobody had looked at.

8. The replacement stack

For reference, this is what replaced the three MCP servers in my setup:

TrueNAS: truenas_api_client (official iXsystems library) and midclt CLI for websocket API access. REST API via curl as fallback. Full coverage of the 278-tool surface.

OPNsense: opn-cli (community Python CLI) for firewall, HAProxy, routes, and DNS. Raw curl against the OPNsense REST API for NAT, VLANs, DHCP, and ACME, which opn-cli does not cover.

Playwright: shot-scraper for screenshots, JS eval, and HTML extraction. Python playwright library for full browser automation: form fills, login flows, file downloads.

All three follow the same pattern: a primary CLI or library path with documented fallback commands for anything the primary does not cover. The skill documents both paths. The model chooses based on what the task requires.

9. Conclusion

MCP servers are a reasonable architecture for giving agents access to external services. The problem is the cost model: every tool schema defined by an active MCP server gets injected with every API call, whether those tools are relevant to the current task or not. As the ecosystem adds more MCP servers (GitHub, Datadog, Atlassian, Stripe, Slack, Sentry, AWS, Kubernetes), the baseline context overhead per message compounds.

On flat-rate plans, this is invisible. Under per-token billing, it is a significant and growing cost that starts before any work has been done.

Skills sidestep this by being lazy. A skill entry is a name and a description, a few dozen tokens. Full instructions load when needed. The model calls CLIs and APIs directly. The capability is the same; the upfront cost is not.

The numbers from this setup: 44,500 tokens saved per turn, a 75% reduction in baseline context overhead, and a monthly saving of roughly $62 under Sonnet cached pricing, or $622 at uncached rates. On a flat rate today, not relevant. On usage-based billing, very much so.

A note on GitHub Copilot: Copilot Pro+ at $39/month is a flat rate that absorbs all token volume. If you stay within the request limits, this overhead is financially invisible. The analysis in this article applies to direct API usage with Anthropic, OpenAI, Google, AWS Bedrock, or any other pay-per-token provider. If you are on Copilot and not planning to switch, the context window fill rate argument still applies: you hit context limits sooner. But the cost argument does not, until Copilot's usage-based transition completes.

Further reading:

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