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mcp-rtk

MCP proxy that cuts 60-90% of tokens from tool responses

Drop it in front of any MCP server. Same tools, way fewer tokens.

Quick start • Presets • Configuration • Contributing presets

The problem

MCP servers return raw API responses. A single list_issues call from GitLab can dump 180K+ tokens of JSON into your context: full user objects with avatar URLs, null fields everywhere, nested metadata nobody asked for. You pay for all of it.

What mcp-rtk does

It sits between Claude and your MCP server as a transparent proxy. Tool calls go through unchanged, but responses get compressed through an 8-stage filter pipeline before they hit your context window.

Claude ←(stdio)→ mcp-rtk ←(stdio)→ upstream MCP server

The LLM gets the same information in a fraction of the tokens.

Install

cargo install mcp-rtk

Quick start

Wrap your existing MCP server command with mcp-rtk --. One line change in your Claude Code config.

Before (~/.claude.json):

{
  "mcpServers": {
    "gitlab": {
      "command": "npx",
      "args": ["-y", "@nicepkg/gitlab-mcp"],
      "env": { "GITLAB_PERSONAL_ACCESS_TOKEN": "glpat-..." }
    }
  }
}

After:

{
  "mcpServers": {
    "gitlab": {
      "command": "mcp-rtk",
      "args": ["--", "npx", "-y", "@nicepkg/gitlab-mcp"],
      "env": { "GITLAB_PERSONAL_ACCESS_TOKEN": "glpat-..." }
    }
  }
}

mcp-rtk detects the upstream server from the command and loads the matching preset automatically.

Filter pipeline

Every response passes through 8 stages, in order:

Stage	Effect
`keep_fields`	Whitelist: only retain specified fields
`strip_fields`	Blacklist: recursively remove fields like `avatar_url`, `_links`
`condense_users`	`{id, name, username, avatar_url, ...}` becomes `"username"`
`strip_nulls`	Drop all `null` and empty string values
`flatten`	`{"data": [...]}` becomes `[...]`
`truncate_strings`	Cap long strings (descriptions, diffs) at N chars
`collapse_arrays`	Keep first N items, append `"... and X more"`
`custom_transforms`	Regex-based string replacements

Even without a preset, generic defaults apply: null stripping, user condensing, flattening, and field removal. This alone typically saves 30-40%.

Presets

Presets are TOML files with tool-specific filter rules. mcp-rtk ships with community-maintained presets and auto-detects which one to use based on the upstream command.

Preset	Detected from	Tools covered
`gitlab`	`gitlab-mcp`, `gitlab`	45+ tools across MRs, issues, pipelines, commits, projects, labels, releases
`grafana`	`mcp-grafana`, `grafana`	15+ tools for dashboards, datasources, Prometheus, Loki

You can force a specific preset:

mcp-rtk --preset gitlab -- node my-custom-gitlab-server.js

What a preset looks like

Each preset defines per-tool rules that get merged on top of the generic defaults. Here's an excerpt from the GitLab preset:

[tools.list_merge_requests]
keep_fields = ["iid", "title", "state", "author", "source_branch", "target_branch", "web_url"]
max_array_items = 20
condense_users = true

[tools.get_merge_request_diffs]
keep_fields = ["old_path", "new_path", "diff", "new_file", "deleted_file"]
truncate_strings_at = 2000
max_array_items = 50

[tools.list_issues]
keep_fields = ["iid", "title", "state", "author", "labels", "assignees", "web_url"]
max_array_items = 20
condense_users = true

The full GitLab preset covers merge requests, pipelines, jobs, issues, commits, files, projects, members, labels, releases, and events.

Creating your own preset

If you use an MCP server that doesn't have a built-in preset, you can write your own as a TOML config:

# ~/.config/mcp-rtk/my-server.toml

[filters.default]
strip_nulls = true
condense_users = true
truncate_strings_at = 800
max_array_items = 25
strip_fields = ["avatar_url", "_links", "metadata"]
flatten = true

[filters.tools.list_items]
keep_fields = ["id", "name", "status", "created_at"]
max_array_items = 15

[filters.tools."get_*"]        # glob pattern — matches all get_ tools
truncate_strings_at = 1500
strip_fields = ["internal_id", "legacy_data"]

Then pass it with --config:

mcp-rtk --config ~/.config/mcp-rtk/my-server.toml -- your-mcp-server

Contributing a preset

Found good filter rules for an MCP server you use? Submit a preset so others can benefit too.

Create a TOML file in config/presets/ named after the server (e.g., github.toml)
Add your tool-specific filter rules (look at gitlab.toml or grafana.toml for reference)
Register it in src/config.rs in the PRESETS array with detection keywords
Open a merge request

The more MCP servers get presets, the more tokens everyone saves.

Configuration

For power users who want to tweak filter behavior beyond what presets provide.

mcp-rtk --config ~/.config/mcp-rtk/custom.toml -- npx @nicepkg/gitlab-mcp

[filters.default]
strip_nulls = true
condense_users = true
truncate_strings_at = 500
max_array_items = 20
strip_fields = ["avatar_url", "_links", "time_stats"]
flatten = true
custom_transforms = [
  { pattern = "https://gitlab\\.com/[^ ]+", replacement = "[link]" }
]

[filters.tools.get_merge_request_diffs]
truncate_strings_at = 2000

[filters.tools.list_merge_requests]
keep_fields = ["iid", "title", "state", "author", "source_branch", "target_branch"]
max_array_items = 15

[tracking]
enabled = true
db_path = "~/.local/share/mcp-rtk/metrics.db"

User config overrides preset rules. Tool-specific strip_fields and custom_transforms are concatenated with defaults, everything else replaces them.

Tool names support glob patterns: * matches any sequence, ? matches a single character. Exact matches always take priority over patterns.

Commands

`mcp-rtk gain`

See how many tokens you've saved.

mcp-rtk gain                # summary with per-tool breakdown
mcp-rtk gain --history      # last 50 calls with individual details
mcp-rtk gain --export json  # machine-readable JSON output for scripting

`mcp-rtk discover`

Scan your Claude Code session logs to find MCP servers that aren't proxied yet and estimate how many tokens you could save.

mcp-rtk discover            # last 30 days
mcp-rtk discover --days 7   # last week

`mcp-rtk presets`

Browse the built-in presets without digging through source code.

mcp-rtk presets list          # show all available presets
mcp-rtk presets show gitlab   # print the full TOML for a preset

`mcp-rtk validate-preset`

Check a preset or config file for syntax errors and potential issues before using it.

mcp-rtk validate-preset my-preset.toml

`mcp-rtk dry-run`

Test filters on JSON from stdin without running a proxy. Stats go to stderr, filtered JSON to stdout.

echo '{"id":1,"title":"test","avatar_url":"...","_links":{}}' | mcp-rtk dry-run --preset gitlab --tool list_issues
cat response.json | mcp-rtk dry-run --config custom.toml --tool get_merge_request

Safety

Responses are filtered, never blocked. A few guardrails keep things predictable:

10 MB response cap: oversized responses get truncated before parsing
128-level recursion limit, matching serde_json's default
String truncation respects UTF-8 character boundaries
Non-JSON responses pass through with only string truncation applied
Images and resources are forwarded unchanged

License

MIT

mcp-rtk 1.2.0