Gameplay tweaks: patcher features in the engine

At a glance

Carrier

legaia_engine_session::session_switches::GameplayTweaks, carried whole by both play hosts

Native window

At launch: the launcher’s Gameplay tweaks and Battle tuning rows, or the play-window flags

Browser play page

At any time: its Gameplay tweaks and Battle tuning panels

Lands on

WorldToggles, through SessionSwitches::apply; retail-faithful mode holds every switch at retail’s value (WorldToggles::pin_retail)

Stacks with a patched disc

Yes: the engine’s EXP rate multiplies a disc already patched with --exp-scale

Two ways a patcher feature reaches the engine

The engine reads its game data from whatever disc it is given, and it never executes the game’s MIPS code. That splits the patcher in two.

• Data edits reach the engine on their own. A randomized drop table, a re-priced shop, a rescaled monster record or a re-routed door is bytes the engine parses at boot or scene entry.
• Code hooks do not. The engine runs its own Rust ports of the routines a hook patches, so a hook’s behaviour exists in the port only when the port implements it.

Classification

Every patcher feature falls in exactly one class. Class (b) is either carried (a switch below) or open (ranked at the end).

The full per-flag table is in docs/subsystems/gameplay-tweaks.md.

The carried switches

A launcher or page slider cycles a fixed step list that starts at retail; the flags take any value in range. The two fixes default on because a game the defect never touched plays identically with them: the Fortress fix changes nothing on a save without the stale flag, and the readout guard is an identity while retail’s readout invariant holds. The balance changes default to retail.

Shiny Seru

At each formation battle entry the engine rolls 2%: one capturable monster gets +35% HP / ATK / DEF / SPD, and capturing it marks the learned spell shiny (+35% damage). Off skips the roll and its rand() draw, which retail never takes, so every later battle-entry roll stays on retail’s position in the stream.

Run-away EXP

Any successful escape (Run, an item or a spell) banks 5% of the fled formation’s EXP into each member’s total, as the hook banks it: no level-up runs until the next won battle.

EXP rate

Each monster record’s base EXP is scaled before the victory sum with the patcher’s arithmetic (round half up, zero stays zero, otherwise 1..=65535); retail’s 3/4 scale and the per-member split run on the scaled sum.

Fortress battles fix

The encounter condition walk reads system flag 0x15 as clear in deroa and chitei2, falling past the Floating Castle’s copied "no battles" group (field oddities). The patch rewrites the table; the engine leaves it alone.

Tetsu spar order fix

In town01 a field script reads flag 0x20C (the spar counts) as set once the spar and the measuring both are, so a spar with Tetsu fought before Mei measures Vahn still lets the hunters return. Retail never counts it and the story stops (field sequence breaks). The flag bank is left as retail wrote it.

Boss casts respect jewels

Seven capture-class boss casts roll through the resist-respecting wrapper FUN_801DD4B0 instead of the bypass FUN_801DD6B4. That swaps the whole roll, not only the resist flag - the two wrappers read different attacker and defender terms (battle formulas).

Battle freeze guard

Before each frame of the HP-readout ramp, a party slot whose readout differs from live HP with nothing pending gets the difference re-seeded, so state 0x51’s settle check opens (exit gates).

Bonus equipment drops

Once per won battle, a 5% roll puts one equipment id straight into the bag, from the same pool the patcher embeds.

Save anywhere

The pause menu’s Save row works in every scene, not only where the scene header raises the save-allow bit. A port enhancement with no patcher counterpart.

Battle tuning

Each slider is one scale at the reader the patcher’s edit changes, with the patcher’s arithmetic. The patch scales the stored record; the engine scales at the reader, so a fight sees the same numbers but nothing on disc or in a save moves. Tetsu, the unwinnable sparring partner, is pinned against both enemy sliders, as the patcher pins him.

Verification

Each switch has a disc-free test that it changes behaviour when on and that its retail value is the unmodified path, across engine-core, engine-battle-vm, engine-session, engine-shell and web-play; the bonus-drop pool is pinned against the patcher’s by a disc-gated test. Tier 3 of the host-drift gate requires both hosts’ switch sites to pass the tweaks field, and the host frame trace digests the tweaks, so a switch thrown on one host only shows as a divergence.

Open

Ranked by how much of the engine each needs:

1. Split stat scale--enemy-stat-scale’s per-stat and regular: / boss: forms, over the patcher’s monster classes.
2. Seru trading without a patched discAn engine-built offer configuration in place of the one parsed from the executable.
3. Enemy HP barsA battle-HUD draw over each living monster, on both hosts.
4. Super Arts on the move list, oscillating APBattle-menu row sources.
5. Enemy ally (charm)A monster acting on the party’s side through the round scheduler, with the victory fixup already in place.

See also

• Randomizer and patcher - every patcher flag.
• Engine reimplementation - the fidelity-versus-enhancement model and every toggle’s default.
• Playing: launcher options - where the rows sit.