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What it does

You give it a modpack source — a Modrinth slug, an FTB pack ID, a direct .mrpack URL, or a local file — and it resolves the manifest, downloads all mod files concurrently, verifies their integrity, and drops them into a folder. There's a CLI for one-off use and a full async Python API for integration.

The resolver is provider-agnostic by design. Built-in support covers Modrinth and FTB. Additional providers can be plugged in without touching the library.

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Installation

pip install packlayer

Requirements: Python 3.14+

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Usage

CLI

packlayer install mr:fabulously-optimized

packlayer install https://modrinth.com/modpack/fabulously-optimized

packlayer install ftb:79

packlayer install ./mypack.mrpack

packlayer install mr:fabulously-optimized --minecraft 1.20.1 --dest ./dest

All options

Flag	Description
--dest <path>	Output directory (default: ./dest)
--version <version>	Pin a specific modpack version (e.g. 6.0.1)
--minecraft <version>	Filter by Minecraft version (e.g. 1.20.1)
--side <client|server|both>	Which side to install for (default: client)
--no-optional	Skip optional mods
-v, --verbose	Enable debug logging

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Python API

One-shot

import asyncio
from packlayer import install_modpack

asyncio.run(install_modpack("mr:fabulously-optimized", "./mods"))

Client

import asyncio
from packlayer import PacklayerClient

async def main():
    async with PacklayerClient(minecraft_version="1.20.1") as client:
        versions = await client.list_versions("mr:fabulously-optimized")
        modpack  = await client.resolve("mr:fabulously-optimized", modpack_version=versions[0].version_number)
        results  = await client.install(modpack, "./mods")
        print(f"{results.total} files installed")

asyncio.run(main())

Progress tracking

from packlayer import PacklayerClient

async def main():
    async with PacklayerClient() as client:
        modpack = await client.resolve("mr:fabulously-optimized")

        def on_start(total: int) -> None:
            print(f"downloading {total} files")

        def on_progress() -> None:
            print(".", end="", flush=True)

        await client.install(
            modpack, "./mods",
            on_start=on_start,
            on_progress=on_progress,
        )

on_progress is called once per installed file (mods and overrides). Both sync and async callables are accepted.

Install options

from packlayer import PacklayerClient, InstallOptions

async def main():
    async with PacklayerClient() as client:
        modpack = await client.resolve("ftb:79")
        await client.install(
            modpack, "./mods",
            options=InstallOptions(
                side="server",
                include_optional=False,
            ),
        )

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Reference

install_modpack

async def install_modpack(
    source: str,
    dest: str | PathLike[str],
    *,
    minecraft_version: str | None = None,
    modpack_version: str | None = None,
    concurrency: int = 8,
    on_start: Callable[[int], None] | None = None,
    on_progress: ProgressCallback | None = None,
    options: InstallOptions | None = None,
    extra_resolvers: list[ModpackResolver] | None = None,
    default_resolver: ModpackResolver | None = None,
) -> InstallResult

Parameter	Description
source	Local path, direct URL, mr:<slug>, Modrinth project URL, or ftb:<id>
dest	Destination directory. Created if it does not exist
minecraft_version	Filter versions by Minecraft version (e.g. "1.20.1")
modpack_version	Pin a specific modpack version (e.g. "6.0.1"). Uses latest if omitted
concurrency	Max simultaneous downloads. Default: 8
on_start	Callback invoked with the total file count before downloading starts
on_progress	Callback invoked after each installed file (sync or async, no arguments)
options	Controls which files are installed. See InstallOptions
extra_resolvers	Additional resolvers registered before built-ins
default_resolver	Fallback resolver when no registered resolver claims the source

PacklayerClient

class PacklayerClient:
    def __init__(
        self,
        *,
        minecraft_version: str | None = None,
        concurrency: int = 8,
        extra_resolvers: list[ModpackResolver] | None = None,
        default_resolver: ModpackResolver | None = None,
        config: PacklayerConfig | None = None,
    ) -> None

Must be used as an async context manager.

Method	Description
resolve(source, *, modpack_version)	Resolve a modpack without downloading files
install(modpack, dest, *, on_start, on_progress, options)	Install a resolved modpack to disk
list_versions(source)	Return available versions, newest-first
resolver_for(source)	Return the resolver that would handle source
resolvers()	Return all registered resolvers in priority order

Models

Modpack

Field	Type	Description
name	str	Display name
version	str	Version string
minecraft_version	str	Target Minecraft version
files	tuple[ModFile, ...]	Mod files to be downloaded
overrides	tuple[Override, ...]	Non-mod files to be written into the instance directory

ModFile

Field	Type	Description
url	str	Download URL
filename	str	Bare filename
size	int	Expected file size in bytes
hash	str | None	Hex digest, verified post-download if provided
hash_type	"sha512" | "sha1" | None	Algorithm used for hash
optional	bool	Whether the file is optional
side	"client" | "server" | "both"	Which side this file targets

Override

Field	Type	Description
path	str	Destination path relative to the instance root (e.g. "config/sodium.json")
side	"client" | "server" | "both"	Which side this override applies to
data	bytes | None	Raw file contents bundled inline (e.g. from .mrpack zips). Mutually exclusive with url
url	str | None	Remote URL to fetch the file from (e.g. FTB overrides). Mutually exclusive with data

ModpackVersion

Field	Type	Description
id	str	Provider-assigned version ID
version_number	str	Human-readable version string
name	str	Release display name
loaders	tuple[str, ...]	Supported mod loaders
game_versions	tuple[str, ...]	Compatible Minecraft versions
date_published	str	ISO 8601 publish timestamp

InstallOptions

Field	Type	Default	Description
include_optional	bool	True	If False, optional files are skipped
side	"client" | "server" | "both"	"client"	Files incompatible with this side are skipped

Exceptions

All exceptions inherit from PacklayerError.

Exception	Description
LocalFileNotFound	Local path does not exist
InvalidMrpack	File is not a valid .mrpack archive
SlugNotFound	No project matches the given slug or ID
NoVersionFound	Project exists but has no compatible version
NoResolverFound	No registered resolver claimed the source
HashMismatch	Hash digest mismatch after download
NetworkError	Network failure

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Supported providers

Provider	Source format	Auth required
Modrinth	mr:<slug>, Modrinth project URL, direct .mrpack URL, local .mrpack	No
FTB	ftb:<id>, feed-the-beast.com URL	No

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Plugin system

packlayer dispatches resolution to a registry of ModpackResolver instances. extra_resolvers are registered before built-ins, giving them higher priority.

Implementing a resolver

from packlayer.interfaces.resolver import ModpackResolver
from packlayer.domain.models import Modpack, ModpackVersion

class MyResolver(ModpackResolver):
    def can_handle(self, source: str) -> bool:
        return "myprovider.com" in source

    async def resolve(self, source: str, *, modpack_version: str | None = None) -> Modpack:
        ...

    async def fetch_versions(self, source: str) -> list[ModpackVersion]:
        ...

can_handle must be exclusive — return True only for sources this resolver definitively owns.

Registering

async with PacklayerClient(extra_resolvers=[MyResolver()]) as client:
    modpack = await client.resolve("https://myprovider.com/modpacks/mypack")
    await client.install(modpack, "./mods")

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