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Anvil

Bellows

All plugins
Status
Released
Latest version
0.7.018 Oct 2026
Requires
Anvil>=0.2.0
Game builds
eu4-1.37.5-steam-win64
Changes gameplay
No
Source
happy-dwarf-team/bellows
Download Bellows 0.7.0

Windows 64-bit · 774 KB · released · All releases

SHA-256 checksum82f92118d13f75b988f2ca20b44dde00cb803184b84f17c2f7c8d8a41a263e3c

Compare it with Get-FileHash .\bellows-windows-x64.zip in PowerShell, or with SHA256SUMS.txt of the release.

Bellows (formerly EU4 Startup Boost) shortens the time from launching the game to the main menu. Since 0.7.0 it also speeds up the running game: a simulated year takes about a quarter less time. Game state, logs and loaded content stay identical to vanilla; only the time changes. Like a smith’s bellows, it gets the forge hot sooner.

What it does

The plugin loads early, before the game’s own code runs, and speeds up the work the game repeats on every start, and one check the game repeats every day. Each part is a module:

Module What it does Default
mimalloc Replaces the game’s memory allocator with a faster one, for the whole session on
verifypath_listing Answers the game’s path checks from directory listings on
exists_index Answers “does this file exist?” for the modifier icon check on
metascript_subst Substitutes $parameter$ placeholders in scripted effects and triggers in a single pass on
trigger_intern Interns the game’s triggers at the end of loading with a faster index on
token_dict_cache Caches the script tokenizer’s keyword lookups instead of walking a large tree character by character on
open_warmup Opens the files the game will need on parallel threads, so the slow first access after a reboot does not block the start on
ram_prefetch Reads and unpacks the DLC archive members the game will need into memory on background threads; uses up to 1 GB of extra RAM during start-up on
attr_cache Remembers “file not found” answers for game and mod files during start-up off
range_witness Simulation, not start-up. Answers the country range check (can a country still reach a province?) from the province that was in range last time, instead of walking all of the country’s provinces again; same answers as the game’s own code on

open_warmup and ram_prefetch learn which files to read per mod set. range_witness keeps what it learns only for the current session. A module that fails turns itself off while the game continues.

The diagnostic module range_profile (off by default) only observes the range check and writes counters to anvil/data/bellows/; it is meant for development, not for playing.

Install

  1. Install Anvil 0.2.0 or newer.
  2. Download bellows-windows-x64.zip from the latest release and extract it into <EU4>/anvil/plugins/; it contains the bellows folder. SHA256SUMS.txt next to the archive holds its checksum.
  3. Start the game as usual.

Delete the bellows folder to uninstall.

Configuration

On its first start the plugin writes anvil/data/bellows/config.toml and never overwrites it. Each module is on, off or verify; verify compares the module’s answers with the game’s own code, for checking a module rather than for playing.

  • [ram_prefetch] budget_mb sets the memory budget of ram_prefetch (64–8192, default 1024).
  • token_dict_cache and range_witness are not listed in the file and are on for existing files too. Add token_dict_cache = "off" or range_witness = "off" under [modules] to switch one off, or range_witness = "verify" to compare every answer with the game’s.
  • When the game is ready, the plugin writes its counters to anvil/anvil.log.

Upgrading

  • From EU4 Startup Boost (0.5.0 or earlier): delete the anvil/plugins/eu4-startup-boost/ folder and extract Bellows. On its first start Bellows moves your settings and learned profiles from anvil/data/eu4-startup-boost/ to anvil/data/bellows/. While the old folder is still installed, Bellows changes nothing and says so in anvil/anvil.log.
  • A config.toml written by 0.3.0 or earlier keeps trigger_intern = "off". Set it to "on", or delete the file to get the current defaults.

Tested and measured

Start-up

All start-up measurements: EU4 1.37.5 (Steam, Windows 64-bit) with 10 UI, graphics and music mods, on the development machine, warm starts (not the first after a reboot), from the game process starting to the main menu. Details and run names are in the design notes.

Case Without the plugin With the plugin Starts
Started directly from the game folder 33.6 s 11.9 s (first start: 14.9 s) 1 each
Started through the Paradox launcher 30.9 s and 30.1 s 14.8 s 2 + 1, interleaved

These starts ran with the plugin as of 0.3.0. Through the launcher the Steam overlay also loads; it probably accounts for most of the difference to a direct start (not verified).

Single modules were measured on a copy of the game folder:

  • ram_prefetch: median 12.24 s with it and 15.16 s without it, 3 + 3 interleaved starts.
  • token_dict_cache (0.5.0): 0.38 s shorter on average over 16 pairs of starts with the cache on and off.

In every series the game’s setup.log, error.log and system.log were identical with and without the plugin. In a verification start, the modules compared their answers with the game’s own code: 0 differences over 3.6 million calls. Five in-game years were played with the plugin (before 0.7.0, so without range_witness) without crashes or new errors, and an autosave from that session loaded correctly.

Not measured yet: the first start after a reboot. The research build that the plugin’s techniques come from took it from ~61 s to ~15 s.

Simulation

range_witness was measured in the lab on EU4 1.37.5 (Steam) with a save from 17 March 1719 (Russia), 8 worker threads in each of the game’s two pools and only this module on. Tongs ran the game through the year 1720 at speed 5 and timed it, one game launch per trial, interleaved:

Simulated year 1720 Without range_witness With range_witness
Trials 27.29 / 27.58 / 27.91 s 20.68 / 20.48 / 21.02 / 20.05 s
Average 27.6 s 20.6 s (−25.5%)

One trial without the module was rejected because an event paused the game during the measured year. The game’s main thread used 19.1 s of CPU time per year instead of 25.5 s; 92% of the range checks were answered from the remembered province.

In verify sessions the module compared its answers with the game’s own code: 0 differences over more than 1.4 million compared answers, including one session driven year by year from 17 March 1719 to 1 January 1727. Details are in the design notes.

Not verified yet: other saves, mod sets and much later dates.

Supported game builds

  • EU4 1.37.5 (Steam, Windows 64-bit), eu4-1.37.5-steam-win64

On any other build Anvil skips the plugin and the game starts normally.

Happy Dwarf Team

Anvil and its plugins are MIT-licensed. Unofficial community project, not affiliated with Paradox Interactive.