MCP Nexus

The discovery and routing layer that keeps MCP servers out of your context window until you actually need them.

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The problem

The Model Context Protocol lets an LLM talk to any number of servers — GitHub, Slack, Postgres, your internal tools. The catch: most clients load every tool definition from every configured server at startup. Six servers can mean 70+ tool schemas and thousands of tokens spent before the conversation even starts, most of which the model never touches in a given session.

What MCP Nexus does

MCP Nexus sits in front of your MCP servers as a thin discovery layer. Instead of loading everything up front, the LLM asks for what it needs — by server or by tool — and MCP Nexus resolves the request and connects on demand. You keep your existing MCP servers unmodified; MCP Nexus only changes how (and when) their tools reach the model's context.

Two modes cover the two ways teams actually want this to work:

	Discovery Mode	Dynamic Mode
Granularity	Whole server	Individual tool
Tools always in context	4 (mcpd_find, mcpd_list, mcpd_connect, mcpd_get_schema)	1 (find_tools)
Best for	"Connect me to GitHub" style workflows	Cherry-picking one tool from many servers
After resolution	LLM talks to the server directly — MCP Nexus exits the data path	MCP Nexus lazy-connects and stays in the loop per tool call

v1.0.0 adds Lazy Schema Loading: Discovery Mode can hand back a stub tool list (names only, no schemas) and fetch a single tool's full schema only when it's about to be called — about 92% fewer tokens than loading everything.

How it works

Discovery Mode — server-level selection

Step 1  LLM -> mcpd_find("github issues")
               MCP Nexus searches the registry
               returns: { id: "github", tools: ["create_issue", "search_repos", ...] }

Step 2  LLM -> mcpd_connect("github")
               MCP Nexus starts the GitHub MCP server
               returns: 20 tools now available as github__create_issue, etc.

Step 3  LLM -> github__create_issue({ title: "...", body: "..." })
               MCP Nexus proxies to the GitHub MCP server, returns the result

Scenario	Tools in context	~Tokens
All 6 servers loaded directly	78	~4,778
Discovery Mode (before connect)	4	~305
Discovery Mode (after connect — 1 server)	4 + 20	~1,578

Lazy Schema Loading

No proxy, no changes to the target MCP server required:

1. mcpd_find("github") → choose a server
2. mcpd_connect("github", lazy_mode=true) → get a stub list (20 tools, no schemas, ~132 tokens)
3. mcpd_get_schema("github", "create_issue") → fetch one full schema (~80 tokens)
4. github__create_issue(...) → direct call, as always

Savings: ~92% vs. a full load (292 vs. 2,064 tokens for a typical 2-tool session). Pass --sync-on-start so the registry has schemas cached ahead of time via nexus-sync.

Dynamic Mode — tool-level selection

Step 1  LLM -> find_tools("create issue, post slack message")
               MCP Nexus searches the tool index across all servers
               returns: create_issue (github), post_message (slack)
               both tools added to tools/list

Step 2  LLM -> create_issue({ title: "Bug #42" })
               MCP Nexus lazy-connects to the GitHub MCP server
               executes create_issue, returns the result

Step 3  LLM -> post_message({ channel: "#eng", text: "Done" })
               MCP Nexus lazy-connects to the Slack MCP server
               executes post_message, returns the result

Scenario	Tools in context	~Tokens
All 6 servers loaded directly	78	~4,778
Dynamic Mode (before find)	1	~100
Dynamic Mode (2 found tools)	3	~300

Installation

# Core — keyword search, stdio transport
pip install mcpnexus

# With HTTP and SSE transport (remote MCP servers)
pip install mcpnexus[http]

# With semantic search (sentence-transformers)
pip install mcpnexus[embeddings]

# Full installation
pip install mcpnexus[all]

# Development
pip install mcpnexus[dev]

Quick start

1. Build your registry

The registry is a lightweight JSON catalog of your MCP servers and their tool summaries. Build it from your MCP client's config (e.g. Cursor: ~/.cursor/mcp.json, Claude Desktop: ~/Library/Application Support/Claude/claude_desktop_config.json):

nexus-sync --config /path/to/your/mcp-config.json --output registry/mcpd-registry.json

2. Point your MCP client at MCP Nexus

Discovery Mode (4 tools, connect to one server at a time):

{
  "mcpServers": {
    "nexus-server": {
      "command": "nexus-server",
      "args": ["--registry", "/path/to/mcpd-registry.json", "--sync-on-start"]
    }
  }
}

Dynamic Mode (1 tool, cherry-pick tools across all servers):

{
  "mcpServers": {
    "nexus-gateway": {
      "command": "nexus-gateway",
      "args": ["--registry", "/path/to/mcpd-registry.json"]
    }
  }
}

Or invoke the Python module directly (avoids PATH issues):

{
  "mcpServers": {
    "nexus-gateway": {
      "command": "python",
      "args": ["-m", "mcpnexus.dynamic.server", "--registry", "/path/to/mcpd-registry.json"]
    }
  }
}

3. Measure the token savings

nexus-benchmark --registry registry/mcpd-registry.json

(Generate your registry first with nexus-sync.)

Architecture

┌─────────────────────────────────────────────────────────────┐
│                        LLM / AI Client                       │
└──────────────────────┬──────────────────────────────────────┘
                       │ MCP (stdio / JSON-RPC 2.0)
          ┌────────────┴─────────────┐
          │                          │
   ┌──────▼──────┐           ┌───────▼──────┐
   │  Discovery  │           │   Dynamic    │
   │    Mode     │           │    Mode      │
   │             │           │              │
   │ mcpd_find   │           │ find_tools   │
   │ mcpd_list   │           │              │
   │ mcpd_connect│           │ LazyPool     │
   │ mcpd_get_schema│        │              │
   └──────┬──────┘           └───────┬──────┘
          │                          │
          └────────────┬─────────────┘
                       │
          ┌────────────▼─────────────┐
          │        Shared Core        │
          │                           │
          │  Registry (mcpd-registry) │
          │  KeywordSearchEngine      │
          │  ToolSearchEngine         │
          │  HybridSearch (TF-IDF +   │
          │    sentence-transformers) │
          │  NexusConnector           │
          │   ├─ stdio transport      │
          │   ├─ streamable-http      │
          │   └─ SSE transport        │
          └───────────────────────────┘

Design principles

A discovery layer, not a permanent proxy. In Discovery Mode, once mcpd_connect resolves, the LLM gets direct tool access to the connected server. In Dynamic Mode, server connections stay lazy — a server process starts only when one of its tools is actually called.

Offline-first registry. Tool summaries (name, description, tags) are captured at sync time. Searches run against the cached registry with zero network traffic; full tool schemas load only on connection.

Search degrades gracefully. Keyword search (TF-IDF with synonyms) is the default — always available, no extra dependencies. Semantic search is optional (pip install mcpnexus[embeddings]): when installed, sentence-transformers embeddings blend with keyword results, which helps for natural-language queries like "narzędzie do czytania stron internetowych" → Playwright. Keyword-first keeps installs frictionless when you don't need it.

Registry format

The registry file (mcpd-registry.json) is a JSON catalog of MCP servers:

{
  "mcpd_version": "1.0",
  "metadata": {
    "name": "My MCP Registry",
    "description": "Personal registry of MCP servers"
  },
  "servers": [
    {
      "id": "github",
      "name": "GitHub MCP Server",
      "description": "Official GitHub MCP server (remote). Repositories, issues, pull requests, and code search",
      "version": "remote-2025-11",
      "transport": {
        "type": "streamable-http",
        "url": "https://api.githubcopilot.com/mcp/",
        "headers": { "Authorization": "Bearer ${GITHUB_MCP_PAT}" }
      },
      "tags": ["github", "git", "code", "issues"],
      "tools_summary": [
        {
          "name": "issue_write",
          "description": "Create or update an issue or pull request",
          "tags": ["issues", "create"]
        }
      ],
      "estimated_tools_count": 90,
      "enabled": true,
      "last_synced": "2026-06-11T00:00:00Z"
    }
  ]
}

Full schema: registry/schemas/mcpd-schema.json

Project structure

mcpnexus/
├── mcpnexus/                    # Python package
│   ├── __init__.py              # Public API and version
│   ├── models.py                # Shared dataclasses
│   ├── registry.py               # Registry loader (mcpd-registry.json)
│   ├── connector.py              # MCP connector — stdio, HTTP, SSE transports
│   ├── sync.py                   # Registry builder (sync from mcp.json)
│   ├── benchmark.py              # Token savings measurement
│   ├── search/
│   │   ├── keyword_search.py    # Server-level TF-IDF search
│   │   ├── tool_search.py       # Tool-level TF-IDF search
│   │   ├── embeddings.py        # Sentence-transformer embedding engine
│   │   └── hybrid.py            # Hybrid keyword + semantic search
│   ├── discovery/
│   │   └── server.py            # Discovery Mode MCP server
│   └── dynamic/
│       ├── server.py            # Dynamic Mode MCP server
│       ├── tool_index.py        # O(1) tool lookup index
│       └── lazy_pool.py         # On-demand connection pool
├── registry/
│   ├── mcpd-registry.example.json  # Example registry
│   └── schemas/
│       └── mcpd-schema.json     # JSON Schema for registry validation
├── docs/
│   ├── specification.md         # Protocol specification
│   ├── architecture.md          # Architecture deep-dive
│   ├── registry-format.md       # Registry format reference
│   └── dynamic-mcp.md           # Dynamic Mode guide
├── examples/
│   ├── cursor-config-discovery.json
│   ├── cursor-config-dynamic.json
│   └── README.md
├── tests/                       # 491 tests, 100% coverage
└── pyproject.toml

Development

git clone https://github.com/KrzysztofAugiewicz/MCPNexus.git
cd MCPNexus
pip install -e ".[dev]"

# Run tests
pytest

# Run tests with coverage
pytest --cov=mcpnexus --cov-report=term-missing

# Run end-to-end integration test
python test_e2e.py

# Benchmark token savings (generate registry first with nexus-sync)
nexus-benchmark --registry registry/mcpd-registry.json

CLI reference

Command	Description
nexus-server	Start the Discovery Mode MCP server
nexus-gateway	Start the Dynamic Mode MCP server
nexus-sync	Build or update the registry from an mcp.json config
nexus-benchmark	Measure token savings for a given registry

All commands accept --help for the full option reference.

Transport support

Transport	Install extra	Use case
stdio	(core)	Local process-based MCP servers
Streamable HTTP	mcpnexus[http]	Remote HTTP MCP servers
SSE	mcpnexus[http]	Legacy remote servers (Server-Sent Events)

Transport type is resolved automatically from the registry entry's transport.type field.

Documentation

• Protocol Specification — Tool schemas, message formats, handshake sequence
• Architecture — Data flow, shared core, design decisions
• Registry Format — Full registry schema reference
• Dynamic Mode Guide — Dynamic Mode deep-dive

Publishing to PyPI

Releases are published automatically when a GitHub Release is created. Prerequisites:

1. Add PYPI_API_TOKEN to repository secrets (create at pypi.org/manage/account/token)
2. Create a release with a tag (e.g. v1.0.0)

The publish workflow builds and uploads to PyPI.

Contributing

Contributions are welcome. Please read CONTRIBUTING.md before opening a pull request. For bug reports and feature requests, use GitHub Issues.

Authors

• Krzysztof Augiewicz — główny maintainer — LinkedIn · GitHub
• Kacper Pisarczyk — współtwórca — LinkedIn
• Sebastian Pawłowski — wsparcie technologiczne — LinkedIn
• Mateusz Wiszniowski — współtwórca

Full details in AUTHORS.md.

License

MIT