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holon

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holon provides type-safety for MCP servers, e.g. invoking an MCP server as a function call your type checker understands.

This is currently supported via the Python SDK, TypeScript SDK and Go SDK.

Install

curl -fsSL https://holon-labs.dev/install.sh | sh

Or with Go 1.27+:

go install github.com/holon-sh/cli/cmd/holon@latest

Example

Declare dependencies under sh.holon.requires in your project's server.json (publisher-provided metadata). This example uses the public Context7 server.

{
  "name": "com.example/foo",
  "version": "0.1.0",
  "description": "Example Foo",
  "_meta": {
    "io.modelcontextprotocol.registry/publisher-provided": {
      "sh.holon": {
        "requires": {
          "context7": { "name": "io.github.upstash/context7", "version": "4.4.4" }
        }
      }
    }
  }
}

Generate the typed clients and type-check the calls to them.

holon types check

Or name the servers yourself. Any -s replaces the whole requires list, so server.json is not needed.

holon types check -s https://mcp.context7.com/mcp

Call the server. A wrong tool name or argument is a type error.

Python

from holon.apps import apps

await apps.context7.call_tool("resolve-library-id", libraryName="Next.js", query="app router")

await apps.context7.call_tool("resolve-library-id", libraryName=1, query="app router")
# error: No overload variant of "call_tool" of "Context7App" matches argument types "str", "int", "str"  [call-overload]

await apps.context7.call_tool("resolve-library", libraryName="Next.js", query="app router")
# error: No overload variant of "call_tool" of "Context7App" matches argument types "str", "str", "str"  [call-overload]

TypeScript

import { apps } from "holon-sh";

const libraries = await apps.context7.tools["resolve-library-id"]({ libraryName: "Next.js", query: "app router" });
libraries.content; // Context7 declares no output schema, so the complete result is returned

await apps.context7.tools["resolve-library-id"]({ libraryName: 1, query: "app router" });
// error TS2322: Type 'number' is not assignable to type 'string'.

await apps.context7.tools["resolve-library-id"]({ libraryName: "Next.js" });
// error TS2741: Property '"query"' is missing in type '{ libraryName: string; }' but required in type '{ libraryName: string; query: string; }'.

await apps.context7.tools["resolve-library"]({ libraryName: "Next.js", query: "app router" });
// error TS2551: Property 'resolve-library' does not exist on type '{ readonly "query-docs": (input: { language?: string | undefined; libraryId: string; query: string; }, options?: CallOptions | undefined) => Promise<CompleteResult<unknown>>; readonly "resolve-library-id": (input: { ...; }, options?: CallOptions | undefined) => Promise<...>; }'. Did you mean 'resolve-library-id'?

Development Servers

Let's say you have a server that you're still developing, and it's not in any MCP Registry yet.

# bar/server.py
from mcp.server import MCPServer

server = MCPServer("com.example/bar", version="0.1.0", description="Example Bar")


@server.tool()
def greet(name: str) -> str:
    return f"hello {name}"

Pass its entry file with -s.

holon types check foo -s bar/server.py

Python

from holon.apps import apps

greeting = await apps.bar.tools.greet(name="world")
greeting["result"]

await apps.bar.tools.greet(name=1)
# error: Argument "name" to "greet" of "BarTools" has incompatible type "int"; expected "str"  [arg-type]

await apps.bar.tools.hello(name="world")
# error: "BarTools" has no attribute "hello"  [attr-defined]

greeting["text"]
# error: TypedDict "BarGreetOutput" has no key "text"  [typeddict-item]

TypeScript

import { apps } from "holon-sh";

const greeting = await apps.bar.tools.greet({ name: "world" });
greeting.result;

await apps.bar.tools.greet({ name: 1 });
// error TS2322: Type 'number' is not assignable to type 'string'.

await apps.bar.tools.hello({ name: "world" });
// error TS2339: Property 'hello' does not exist on type '{ readonly greet: (input: { name: string; }, options?: CallOptions | undefined) => Promise<{ [key: string]: unknown; result: string; }>; }'.

const text: string = greeting.text;
// error TS2322: Type 'unknown' is not assignable to type 'string'.

Dependencies

A dependency is an MCP server your project calls. For each one holon obtains a contract—the JSON Schemas of the server's tools and resources—and writes a typed client apps.<alias> into the SDK package your project declares. holon types check then runs the language's type checker over your code against those clients.

A dependency is written in one of three ways.

Source code

An implementation of an MCP server, or its parent directory.

holon types check -s bar/server.py

Registry

A sh.holon.requires entry in server.json, or -s with a registry version URL. The record is fetched for its contract.

holon types check -s https://registry.modelcontextprotocol.io/v0.1/servers/io.github.upstash%2Fcontext7/versions/4.4.4

Server

An endpoint URL. The running server is asked for tools/list and resources/list.

holon types check -s https://mcp.context7.com/mcp

If the server requires a token, name it and set <ALIAS>_MCP_TOKEN. The token is sent only to that URL's origin, as a bearer token, and is not written anywhere.

export DOCS_MCP_TOKEN=...
holon types check -s docs=https://mcp.example.com/mcp

Reference

holon types generate
Generate typed clients for MCP servers.

Leverages sh.holon.requires in the publisher-provided metadata field
(https://modelcontextprotocol.io/registry/faq#can-i-add-custom-metadata-when-publishing) to
resolve dependencies, or the -s list for explicit dependencies.

Usage:
  holon types generate [DIR] [flags]

Examples:
  # Generate for the current project
  holon types generate

  # Another project
  holon types generate ../checkout

  # Name the project's own server to call it from outside the browser too, as apps.orders
  holon types generate -s orders=./server

  # Explicit dependencies: a server directory, a server entry, an MCP registry server, a remote
  #   MCP server named docs
  holon types generate -s ../orders
  holon types generate -s ../orders/server.py
  holon types generate -s https://registry.modelcontextprotocol.io/v0.1/servers/io.github.upstash%2Fcontext7/versions/4.4.4
  holon types generate -s docs=https://mcp.context7.com/mcp

  # A remote MCP server that requires a token: DOCS_MCP_TOKEN is sent to it as a bearer token
  DOCS_MCP_TOKEN=... holon types generate -s docs=https://mcp.example.com/mcp

  # Print one server's contract, or commit it beside its server.json
  holon types generate --contract ../orders
  holon types generate --contract ../orders -o ../orders/holon-contract.json

Flags:
      --contract string      Print one server's MCP contract instead of generating
  -h, --help                 help for generate
  -o, --output string        Write the --contract document to FILE, such as holon-contract.json
  -s, --server stringArray   A dependency, [ALIAS=]PATH or URL, paths relative to the working
                             directory
  -v, --verbose              Show generation details
holon types check
Generate typed clients for MCP servers, then type-check the project's calls to them.

Leverages sh.holon.requires in the publisher-provided metadata field
(https://modelcontextprotocol.io/registry/faq#can-i-add-custom-metadata-when-publishing) to
resolve dependencies, or the -s list for explicit dependencies.

Usage:
  holon types check [DIR] [flags]

Examples:
  # Generate, then type-check
  holon types check

  # Type-check the existing clients only (no network, no writes)
  holon types check --no-generate

  # Explicit dependencies, as for generate
  holon types check -s orders=../orders

Flags:
  -h, --help                 help for check
      --no-generate          Type-check the existing clients only: no network, no install, no
                             writes
  -s, --server stringArray   A dependency, [ALIAS=]PATH or URL, paths relative to the working
                             directory

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