Fallout 4

Fallout 4 (game_id fallout4, Steam App ID 377160) is a fully shipped Bethesda title in modde: scanning, conflict detection, plugin load order, and save tracking all work end to end. Its overall status is Done in the supported-games table and the parity audit. It is one of five Creation Engine games modde drives through a single data-driven BethesdaGame plugin, sharing its deploy strategy and conflict logic with Skyrim SE/AE, Fallout 76, and Starfield.

Engine & overall status

Because Fallout 4 is part of the Bethesda plugin family, it inherits the full Creation Engine feature set documented for Skyrim SE/AE and in the Conflicts & load order guide: plugins.txt read/write, a pure-Rust LOOT masterlist parser, binary plugin header validation (Form 43 / missing-master detection), and archive-aware conflict analysis.

Install detection

modde locates a Fallout 4 install through its registered launcher IDs, in this priority order:

1. Steam — App ID 377160, install directory steamapps/common/Fallout 4. This is the primary, best-tested detection path.
2. Heroic (GOG) — GOG app ID 1998527297, for the GOG build (on Linux/macOS run through Heroic + Proton/Wine; on Windows it runs natively).

There is no Epic launcher mapping for Fallout 4. The install root is the directory that contains Fallout4.exe and the Data/ folder; you can also point modde at it explicitly with gameDir (home-manager) or --game-dir (CLI) when auto-detection cannot reach it.

Proton prefix

On Linux, Fallout 4 runs under Proton, so the Windows-side paths modde needs (plugins.txt, the save folder, the per-user My Games tree) live inside the Steam compatibility prefix rather than in your real home directory. modde resolves them under the App-ID-keyed compatdata prefix:

~/.local/share/Steam/steamapps/compatdata/377160/pfx/drive_c/users/steamuser/

plugins.txt is read from and written to:

.../compatdata/377160/pfx/drive_c/users/steamuser/AppData/Local/Fallout4/plugins.txt

and saves from the My Games tree (see Save tracking). If the prefix has not been created yet — for example you have installed the game but never launched it — these paths will not exist, and modde degrades gracefully: the scanner falls back to a plain directory walk and the save tracker reports no saves rather than erroring.

Mod directory & deploy strategy

The mod directory is Data/ under the install root (mod_directory returns install.join("Data")). Every plugin, archive, and loose asset Fallout 4 loads lives here.

modde deploys with its standard symlink farm: mods are staged in ~/.local/share/modde/staging/<profile>/, conflicts are resolved into a single winning layout, and that layout is symlinked into Data/. Your real game install is never overwritten, and a rollback restores the previous deployment atomically. See Deployment & VFS for the full build → materialize → deploy pipeline and modde rollback.

.ba2 archives are deployed as-is, not extracted — the Creation Engine loads them natively, so modde places the archive straight into Data/ and lets the engine mount it. This matches the bare-layout recognizer, which treats a top-level Data/, loose .esp/.esm/.esl/.ba2 files, or the usual asset folders (meshes/, textures/, scripts/, interface/, sound/, materials/, strings/, …) as a valid Bethesda mod root even when the archive was packed without a containing folder.

What scanning finds

The Fallout 4 scanner (FALLOUT4_SCANNER, a BethesdaScanner) discovers installed mods by walking Data/ and cross-referencing plugins.txt:

1. Authoritative pass. It reads plugins.txt from the Proton prefix and, for every listed plugin that exists on disk, emits a plugin/<filename> mod at confidence 0.95. For each plugin it also pairs companion archives that share the plugin’s stem — both <stem>.ba2/<stem>.bsa and the texture-archive convention <stem> - Textures.ba2 — so a plugin and its assets are reported as one mod.
2. Unmanaged pass. It then scans Data/ for any .esp/.esm/.esl files not present in plugins.txt (disabled or hand-dropped plugins) and reports them at confidence 0.8, again pulling in same-stem archives.

Scan results feed duplicate detection: each discovered mod exposes a Data-relative footprint (plugin/Foo.esp → foo.esp) so modde can spot files that are already deployed loose in the game folder versus managed by a profile. See Scanning installed mods for the shared workflow.

Conflict classification

Conflicts are classified by the shared BethesdaCollisionClassifier, which is archive-aware: it reads the file listing inside each .ba2/.bsa and folds those entries into the same collision map as loose files, so two archives that both ship textures/foo.dds are flagged even though neither was extracted.

Severity is assigned per file extension:

This is the same severity ladder modde uses to decide which collisions to surface loudly. See Conflict detection for how severities drive the conflict report and how load-order priority resolves a winner.

Mod-safety classification

Independently of collisions, modde tags whole mods as save-breaking when they contain .esp/.esm/.esl/.pex/.dll/.psc, and cosmetic when they only contain art/sound/archive/config assets. This drives the warnings you see before enabling a mod mid-playthrough.

Plugin & load-order handling

Fallout 4 has a real plugin system (has_plugin_system() is true), and modde manages it as a first-class, dual-pane concern:

• plugins.txt read/write. modde parses the standard format — *Plugin.esp = enabled, Plugin.esp = disabled, #… = comment — from the Proton prefix and writes it back in the same *-prefixed form.
• Dual-pane order. Mod install priority (“which mod’s files win”) is kept separate from plugin load order (“which .esp loads first”), stored in a dedicated plugin_order table per profile — MO2’s core insight.
• LOOT masterlist. modde ships a pure-Rust parser for LOOT’s public YAML masterlist; for Fallout 4 it pulls the loot/fallout4 repository and turns the after/requires/incompatible rules for your active plugins into modde load-order rules.
• Header validation. A binary header reader inspects the first ~1 KB of each plugin to extract the form version, ESM/ESL flags, and master list, flagging Form 43 plugins and missing masters before they cause a crash on load.

# Auto-sort the Fallout 4 load order against the LOOT masterlist
modde loot sort --game fallout4

# Validate plugin headers (Form version, missing masters)
modde loot validate --game fallout4

Not yet shipped for any Bethesda title: plugin pinning (“lock load order”), a dedicated Archives tab, archive content preview/packing, and per-mod INI tweak editing. LOOT sorting prints rules to the terminal rather than a rich report. See the MO2 parity audit for the full feature comparison.

Save tracking

Save tracking for Fallout 4 is Done and opted into the per-profile save layer (with_save_profiles(true)).

modde locates saves inside the Proton prefix at:

.../compatdata/377160/pfx/drive_c/Users/steamuser/Documents/My Games/Fallout4/Saves

The Fallout 4 save tracker (FALLOUT4_SAVE_TRACKER) scans that directory for .ess save files (.bak backups are skipped) and, for each one, parses the binary header to fingerprint it:

• It verifies the FO4_SAVEGAME magic so a stray Skyrim/Starfield save in the same folder is not mislabelled.
• From the header it reads the save number (the slot ID that increments each save) and the player name (a length-prefixed UTF-8 string), producing a label like Sole Survivor — Save 7.
• Slot type is inferred from the filename: Autosave… → auto, Quicksave… → quick, everything else → manual.
• A save whose header cannot be parsed is still captured, just without a label.

Captures are summarized newest-first and grouped by character, so a capture message reads like capture: 3 saves — Sole Survivor (slots 5, 6); …. See Save tracking & profiles for committing, browsing, and restoring save snapshots.

Installer specifics

• FOMOD is fully supported. When a downloaded mod contains a fomod/ModuleConfig.xml, modde detects it and runs the installer — interactively via the GUI wizard, or non-interactively from a saved choices file. Generate a template with modde fomod generate, inspect options with modde fomod inspect, and replay choices with --fomod-config. See the FOMOD installer guide.
• BA2 archives are deployed verbatim (never repacked or extracted), and their contents are still indexed for conflict analysis as described above.
• Loose-file mods (a bare Data/ tree or top-level asset folders) are recognized by the Bethesda bare-layout policy without any installer.
• Fallout 4 does not use REDmod (Cyberpunk), Unreal .pak, or SMAPI installers — those belong to other engine families. Its installer story is FOMOD + loose files + BA2, the standard Creation Engine toolkit.

Gaming tools & overlays

Fallout 4 launches through modde’s Tools & Overlays layer like any other game. The settings that matter most for this title:

• proton — pin a Proton/GE-Proton runner, set extra_env, override the Wine prefix, or force DLL overrides. Fallout 4 Script Extender (F4SE) and many script-extender-dependent mods need the right runner and, occasionally, forced overrides; dll_override_mode=forced with forced_dll_overrides=… is how you apply them through modde’s launcher integration.
• mangohud, vkbasalt, gamemode, reshade — performance overlay, post-processing, the Feral GameMode daemon, and the ReShade shader framework, enabled and configured per game with modde tool enable/configure.
• Overwrite capture. Tools that rewrite files in Data/ (xEdit/FO4Edit, BodySlide, Material Editor) can be run through modde tool run … --game fallout4 so their output is captured into an __overwrite__ mod and survives redeployment.

modde ships no OptiScaler profile for Fallout 4 (optiscaler_profiles is empty for this title). OptiScaler can still be applied as a generic tool, but there is no curated Fallout-4-specific upscaling profile the way there is for Stellar Blade.

Linux / Proton notes & gotchas

These notes apply to the game’s runtime on Linux and macOS, where Fallout 4 runs through Proton/Wine. On Windows the game runs natively — there is no Wine prefix, and plugins.txt, the My Games/Fallout4 tree, and the INI files live at their native Windows locations. modde itself runs natively on all three platforms.

• Launch the game once first. Until Proton has created the compatdata/377160 prefix, plugins.txt, the My Games/Fallout4 save tree, and the per-prefix INI files do not exist. Run the game once so the prefix is populated, then let modde manage it.
• INI files live in the prefix. Fallout4.ini, Fallout4Prefs.ini, and Fallout4Custom.ini are under the prefix’s My Games/Fallout4, not your Linux $HOME. modde patches them comment- and formatting-preserving rather than rewriting them wholesale. Creating Fallout4Custom.ini and enabling loose-file loading is still your responsibility, exactly as on Windows.
• Case sensitivity. Bethesda assets assume Windows’ case-insensitive paths. The bare-layout recognizer matches the standard asset directories case-insensitively, but a poorly packed mod with an unexpected directory name can still mis-deploy on a case-sensitive Linux filesystem — check the conflict report if assets do not load.
• Script extender. F4SE is installed into the game root like on Windows and launched through the script-extender executable; configure the runner with the proton tool and, if a plugin needs it, forced DLL overrides.
• GOG via Heroic. The GOG build (Heroic app ID 1998527297) works, but Steam
  • Proton is the most exercised path; the GOG prefix layout differs, so verify the detected gameDir and save path if you use it.

Worked example

Home-Manager profile

{ inputs, ... }:
{
  imports = [ inputs.modde.homeManagerModules.modde ];

  programs.modde = {
    enable = true;

    profiles.fo4-vanilla-plus = {
      game = "fallout4";
      # Leave gameDir unset (or installMode = "await-game") until Steam/Heroic
      # has installed Fallout 4 and you have launched it once to build the prefix.
      installMode = "await-game";
    };
  };
}

Once Steam has installed Fallout 4 and you have run it once, set gameDir to the install root (the folder containing Fallout4.exe and Data/) and rebuild; modde deploys the profile through its activation script.

CLI

# Install a Nexus mod into a Fallout 4 profile, applying saved FOMOD choices
modde install mod https://nexusmods.com/fallout4/mods/12345 \
  --profile fo4-vanilla-plus \
  --fomod-config my-choices.toml

# Discover what is already in Data/
modde scan --game fallout4

# Sort and validate the load order against the LOOT masterlist
modde loot sort --game fallout4
modde loot validate --game fallout4

# Deploy, then launch through Proton
modde deploy --profile fo4-vanilla-plus --game fallout4
modde play --game fallout4

# Capture xEdit/FO4Edit edits into an __overwrite__ mod
modde tool run /path/to/FO4Edit --game fallout4 -- -quickautoclean

See also

• Supported games — the status baseline and full game list
• Skyrim SE/AE, Starfield — sibling Creation Engine titles
• Deployment & VFS — the symlink-farm deploy pipeline
• Conflict detection — severities and winner resolution
• Scanning installed mods — what the scanner reports
• Save tracking & profiles — committing and restoring saves
• FOMOD installer — interactive and declarative installs
• Tools & Overlays — Proton, OptiScaler, ReShade, overwrite capture
• Parity audit — what is Done vs Partial