Open Source Modding FrameworkAlpha

Mod Halo Campaign Evolved down to the tags

MJOLNIR reads the game's own data and hands it to you: a launcher that installs signed mods in one click, a Guerilla-style tag editor, and a command line that proves every byte.

Download LauncherBrowse Mods
7
Runtime Mods
101
Tag Groups
MIT
Licensed
mjolnir — powershell

$ mjolnir validate --all

12,290 tags · every invariant holds · 99.9% of values decode

$ mjolnir poke --group biped --tag spartans --field "jump velocity" --value 25

9.0 → 25.0 · written into the running game · nothing on disk

$ mjolnir roundtrip --all

12,281 / 12,281 re-serialise byte-exact · 5.77 GB

Measured, not promised — every number is a command you can run

12,290
shipped tags validated
every structural invariant, one command
12,281 / 12,281
byte-exact roundtrips
re-serialised and compared over 5.77 GB
4,787
textures decode
of 4,844 — the rest ship no pixel data
13 / 13
mission scripts recompile
HSC source, byte-for-byte back into the tag

One framework, three ways in

Play with mods, make your own, or script the whole pipeline — each tool reads your installed copy of the game directly. Nothing shipped is ever modified.

Download

MJOLNIR Launcher

One-click installs for mods and tools. Auto-detects your game, verifies every download against its published hash, and installs nothing that fails signature verification.

Get the launcher
Download

Tag Editor

Guerilla, reborn. Browse every tag with decoded values, export textures as PNG, play the ~6 GB of named Wwise audio, edit mission scripts that recompile — and write edits into the running game with Live mode.

See the editor
Build from source

mjolnir CLI

The same tag engine from a shell. List and search groups, set a field by its dotted path, pack an override container, poke the live process — and validate the whole install when you're done.

See the CLI

Make a change. See it in the game.

These are unedited screenshots of edits made with the tools on this site — the same walkthroughs are in the docs.

Halo Campaign Evolved HUD showing a 99-round assault rifle magazine

One field edit

rounds loaded maximum set to 99 in the tag editor, baked into an override container the game loads beside the shipped ones — and read straight off the HUD.

Make your first mod
Assault rifle re-textured red in Halo Campaign Evolved

One texture swap

A PNG re-encoded to the exact size the game shipped, so the override replaces one chunk of one file and moves no metadata. The rifle comes out red.

How texture swapping works

And a framework inside the running game

Seven UE4SS Lua mods ship with MJOLNIR — MJOLNIRCore is the runtime, and these stand on it. All installed and updated by the launcher.

MJOLNIRFlyCam

Free-flying detached camera on the numpad, with mouse look, speed boost and HUD toggle.

MJOLNIRConsoleEnabler

The full UE5 developer console, enabled in the shipping build.

MJOLNIRTagProbe

Read the Blam tag assets the game actually loaded — not what you inferred it would.

MJOLNIRBridge

Drive the game from outside: run Lua and console commands, read state, take screenshots.

MJOLNIRMultiplayer

Experimental map travel, listen-server loading and player administration probes.

MJOLNIRDiscovery

UFunction dumper and travel logging for charting the engine as it runs.

All of it is MIT-licensed source

A hub that takes mods seriously

mjolnircore.com is part of the framework: distribution, verification and documentation, built on the same primitives as the tools.

Signed all the way through

Authors publish from the tag editor with a per-device Ed25519 key, over an account link made by device pairing. The launcher and CLI verify every archive; unsigned builds never install.

Browse mods

Conflicts, computed

A content mod's chunk IDs are identical to the tags it overrides, so “do these mods conflict?” is an exact set intersection — checked by the hub, not declared by the author.

POST /api/v1/conflicts/check

An open API

Spec-first OpenAPI 3.1, generated from the same schemas that validate requests — CI fails if they drift. Reads need no key and CORS is open, so build against it.

Read the API reference

The format, documented

A searchable reference of all 101 tag groups — 1,779 structs, 13,250 fields — plus the research notes behind every format MJOLNIR reads: tags, textures, audio, scripts, containers.

Search the tag reference

Up and modding in minutes

1

Download the launcher

It auto-detects your Halo Campaign Evolved install and sets up the runtime.

Download
2

Pick your mods

One click to install — signed, hash-verified and conflict-checked on the way in.

Browse mods
3

Make your own

Install the tag editor from the launcher's Tools tab and follow the first-mod guide.

Read the guide

Ready to Forge?

Download the launcher, install your first mod, and join the MJOLNIR community on Discord.

Download LauncherJoin Discord