Adaptive Music

/adaptive-music — Wire Stems to Game State

Overview

This is the wiring skill, not the generation skill. The job is to take 3–5 music stems (or full tracks) and connect them to a game state machine so the music shifts as the player's situation changes — exploration → tension → combat → victory → exploration — with crossfades, ducking, and bus routing the player never consciously notices but always feels.

This skill assumes the stems already exist. If they don't, run /music-track first for each state, or generate matching stems (same tempo, same key, same bar count) so they layer/crossfade cleanly.

When to use

• The audio bible specifies a dynamic music model (layered stems / vertical re-orchestration / horizontal sequencing).
• You have at least two music tracks for two different states and want the engine to switch between them.
• Player-state-driven crossfades, not random shuffling.

When NOT to use

• Generating the stems themselves → /music-track.
• Single static loop with no state changes → /music-track and an AudioStreamPlayer is enough.

Steps

1. Read the audio bible's dynamic music plan

Read .summer/audio-bible.md

Quote the plan back. Example:

> Bible plan: Layered stems. States CALM / TENSION / COMBAT / VICTORY. CALM → TENSION on enemy detect (2s crossfade). TENSION → COMBAT on first damage (instant drum stem). COMBAT → VICTORY on last enemy dies (drum cuts, brass swell). VICTORY → CALM after 4s. Music ducks -6 dB on dialogue.

If the plan is absent, run /audio-direction step 6 first.

2. Pick the wiring model

| Model | When | Implementation | |---|---|---| | Horizontal sequencing | 3–5 full tracks, one per state, crossfade | One AudioStreamPlayer per track on the Music bus, tween volumes on state change | | Layered stems | One base loop + drum/brass/strings stems that turn on with intensity | All stems play simultaneously at silence; mute/unmute via volume tween | | Vertical re-orchestration | Same melody, different orchestration per state, snap on bar | All variants play simultaneously muted; swap on next bar boundary | | Procedural / generative | Rules generate music in real-time | Beyond this skill; needs a custom system |

For 90% of indie games, horizontal sequencing is correct. It's cheaper to author, easier to swap, and players don't notice the difference.

3. Confirm stems exist (or generate them)

summer_search_assets(query="music", filter={ kind: "audio" })

You need (typical case):

• audio/music/calm.mp3 — exploration loop, sparse
• audio/music/tension.mp3 — same tempo/key, denser, no drums
• audio/music/combat.mp3 — same tempo/key, full drums, urgent
• (optional) audio/music/boss.mp3 — distinct from combat
• (optional) audio/music/victory.mp3 — short linear sting (4–8s)

If any are missing, run /music-track for each. Critically: same BPM, same key, same time signature, same bar length so they overlap cleanly.

4. Verify / extend the bus layout

The Music bus must exist and route to Master. If /audio-direction ran, it does. Verify:

Read default_bus_layout.tres   # or wherever the project stores it

You want this structure:

Master
├── Music         (-8 dB; ducks -6 dB when Voice is active)
├── SFX           (0 dB; routes to Reverb buses)
├── UI            (-3 dB)
├── Voice         (0 dB)
├── Ambient       (-12 dB)
├── Reverb_Small
├── Reverb_Large
└── Reverb_Outdoor

If Music bus is missing, run /audio-direction step 8.

For ducking, add a sidechain compressor on the Music bus with the Voice bus as the sidechain source. Or implement ducking in code (simpler — see step 7).

5. Build the player rig

For horizontal sequencing, instantiate one AudioStreamPlayer per state, all on the Music bus, all autoplay, all looping, all starting at -60 dB except the active state:

summer_add_node(parentPath="/root/Game/Music", type="AudioStreamPlayer", name="Calm")
summer_set_prop(path="/root/Game/Music/Calm", property="stream", value="res://audio/music/calm.mp3")
summer_set_prop(path="/root/Game/Music/Calm", property="bus", value="Music")
summer_set_prop(path="/root/Game/Music/Calm", property="volume_db", value=0.0)
summer_set_prop(path="/root/Game/Music/Calm", property="autoplay", value=true)

summer_add_node(parentPath="/root/Game/Music", type="AudioStreamPlayer", name="Tension")
summer_set_prop(path="/root/Game/Music/Tension", property="stream", value="res://audio/music/tension.mp3")
summer_set_prop(path="/root/Game/Music/Tension", property="bus", value="Music")
summer_set_prop(path="/root/Game/Music/Tension", property="volume_db", value=-60.0)
summer_set_prop(path="/root/Game/Music/Tension", property="autoplay", value=true)

summer_add_node(parentPath="/root/Game/Music", type="AudioStreamPlayer", name="Combat")
summer_set_prop(path="/root/Game/Music/Combat", property="stream", value="res://audio/music/combat.mp3")
summer_set_prop(path="/root/Game/Music/Combat", property="bus", value="Music")
summer_set_prop(path="/root/Game/Music/Combat", property="volume_db", value=-60.0)
summer_set_prop(path="/root/Game/Music/Combat", property="autoplay", value=true)

All three play simultaneously, in sync, but only one is audible. State transitions tween volumes — the music never restarts, only fades.

This is why same BPM / same key / same bar length matter: all three are at the same playhead at all times.

6. The MusicDirector script

# scripts/audio/MusicDirector.gd
class_name MusicDirector
extends Node

enum State { CALM, TENSION, COMBAT, VICTORY, BOSS }

@export var calm: AudioStreamPlayer
@export var tension: AudioStreamPlayer
@export var combat: AudioStreamPlayer
@export var boss: AudioStreamPlayer
@export var victory_sting: AudioStreamPlayer  # one-shot, not looping

@export var crossfade_seconds: float = 2.0
@export var instant_states: Array[State] = [State.COMBAT]  # snap, no fade
@export var active_db: float = 0.0
@export var inactive_db: float = -60.0

var _state: State = State.CALM
var _tween: Tween

func _ready() -> void:
	_apply_state(_state, true)

func set_state(new_state: State) -> void:
	if new_state == _state:
		return
	var instant := new_state in instant_states
	_state = new_state
	_apply_state(new_state, instant)
	if new_state == State.VICTORY:
		victory_sting.play()
		await get_tree().create_timer(4.0).timeout
		set_state(State.CALM)

func _apply_state(state: State, instant: bool) -> void:
	if _tween and _tween.is_valid():
		_tween.kill()
	_tween = create_tween().set_parallel(true)
	var dur := 0.0 if instant else crossfade_seconds
	for pair in [
		[calm,    state == State.CALM],
		[tension, state == State.TENSION],
		[combat,  state == State.COMBAT],
		[boss,    state == State.BOSS],
	]:
		var player: AudioStreamPlayer = pair[0]
		var should_be_active: bool = pair[1]
		if player == null:
			continue
		var target_db := active_db if should_be_active else inactive_db
		_tween.tween_property(player, "volume_db", target_db, dur)

Attach it under /root/Game/Music, hook the exported player slots to the four players. The director is the single point of truth for music state.

7. Drive state from gameplay signals

Where does set_state() get called from? The game state machine. Hook signals from combat / detection / encounter logic:

# scripts/audio/MusicHooks.gd
extends Node
@export var director: MusicDirector

func _ready() -> void:
	EventBus.enemy_detected_player.connect(_on_tension)
	EventBus.player_took_damage.connect(_on_combat)
	EventBus.encounter_cleared.connect(_on_victory)
	EventBus.boss_started.connect(_on_boss)
	EventBus.boss_ended.connect(_on_calm)
	EventBus.dialogue_started.connect(_on_duck)
	EventBus.dialogue_ended.connect(_on_unduck)

func _on_tension() -> void: director.set_state(MusicDirector.State.TENSION)
func _on_combat()  -> void: director.set_state(MusicDirector.State.COMBAT)
func _on_victory() -> void: director.set_state(MusicDirector.State.VICTORY)
func _on_boss()    -> void: director.set_state(MusicDirector.State.BOSS)
func _on_calm()    -> void: director.set_state(MusicDirector.State.CALM)

func _on_duck() -> void:
	var bus_idx := AudioServer.get_bus_index(&"Music")
	create_tween().tween_method(
		func(db: float) -> void: AudioServer.set_bus_volume_db(bus_idx, db),
		AudioServer.get_bus_volume_db(bus_idx), -14.0, 0.3
	)

func _on_unduck() -> void:
	var bus_idx := AudioServer.get_bus_index(&"Music")
	create_tween().tween_method(
		func(db: float) -> void: AudioServer.set_bus_volume_db(bus_idx, db),
		AudioServer.get_bus_volume_db(bus_idx), -8.0, 0.5
	)

If your project doesn't have an EventBus autoload yet, add one — adaptive music depends on event-driven signals, not polling.

Wire the connections via summer_connect_signal so they survive a scene reload.

8. Layered stems variant (alternative to step 6)

If the bible says layered stems instead of horizontal sequencing, the rig is the same (multiple AudioStreamPlayers, all looping in sync, all on Music bus) but the volume map is different:

| State | Base | Drums | Strings | Brass | |---|---|---|---|---| | CALM | 0 | -60 | -60 | -60 | | TENSION | 0 | -60 | -10 | -60 | | COMBAT | 0 | 0 | -3 | -10 | | BOSS | -60 | 0 | 0 | 0 |

The director method is the same _apply_state pattern — just more players and per-player target_db tables.

9. Bar-locked snap (vertical re-orchestration)

For the third model, snapping must happen on a bar boundary (not mid-phrase). All players run in sync; on state change, schedule the snap for the next bar:

const BPM := 128.0
const BEATS_PER_BAR := 4
const SECONDS_PER_BAR := (60.0 / BPM) * BEATS_PER_BAR

func snap_at_next_bar(callable: Callable) -> void:
	var pos := calm.get_playback_position()
	var into_bar := fmod(pos, SECONDS_PER_BAR)
	var wait := SECONDS_PER_BAR - into_bar
	get_tree().create_timer(wait).timeout.connect(callable)

Use this for boss reveals, story beats — anywhere a snap-on-bar is more dramatic than a crossfade.

10. Test the transitions

Run the game (summer_play) and trigger each transition. Listen for:

• Crossfade artifacts at crossfade_seconds = 2.0. Too fast: fades are choppy. Too slow: state changes feel disconnected from gameplay.
• Out-of-sync stems. If combat enters and the drums hit on the wrong beat, the stems weren't generated at the same BPM/bar length. Regenerate.
• Ducking that's too aggressive (music disappears) or too subtle (voice still buried). The -6 dB is a starting value; tune in step 7.
• Victory sting that loops (it shouldn't). Set victory_sting to a non-loop AudioStream.

11. Edge case: scene transitions

When the player changes scenes, the MusicDirector either:

a) Persists as an autoload. New scenes call MusicDirector.set_state() to take over. b) Per-scene, fades out the previous before the scene changes (set_state(CALM) then crossfade to -60 dB on all then change scene).

Persistent is preferred for open-world; per-scene for level-based games.

Reference card — bus map

Master
├── Music          -8 dB         (ducks to -14 dB on Voice)
│   └── (one AudioStreamPlayer per state, all looping in sync)
├── SFX            0 dB
│   └── routes to Reverb_Small / Reverb_Large / Reverb_Outdoor sends
├── UI             -3 dB
├── Voice          0 dB
├── Ambient        -12 dB
├── Reverb_Small   (return)
├── Reverb_Large   (return)
└── Reverb_Outdoor (return)

Anti-patterns

• Stems generated at different BPM/key. They will not crossfade cleanly. Same BPM, same key, same bar count.
• Restarting tracks on state change. Players hear the restart and the magic dies. Always crossfade volumes; never play() mid-state.
• No autoplay on the silent stems. They can't fade in to phase if they never started.
• Polling state in _process. Use signals. Music director listens to EventBus, not to game state every frame.
• Hardcoding volumes in set_state. Use the table-driven _apply_state so adding a new state doesn't fork the function.
• Ducking music to silence on dialogue. The bible says -6 dB (so music drops from -8 to -14), not muted. Voice should sit on a present bed, not a dead one.
• Boss music as a louder combat track. Distinct, named instrument; ideally different tempo or key. The bible's Boss music is its own track rule.

Edge cases

• Player triggers TENSION → COMBAT in 0.2s. The instant_states = [State.COMBAT] makes COMBAT snap so it lands on the swing connect. The intermediate fade is wrong — combat must feel immediate.
• Player flees combat back to TENSION → CALM. Use the regular crossfade_seconds = 2.0; the de-escalation can breathe.
• Multiple enemies aware → one dies → none aware. The state machine should hysteresis: only fall back to CALM after no_enemies_aware AND no_recent_damage_for=8s.
• Audio service not ready on _ready. If AudioServer.get_bus_index("Music") returns -1, the bus layout failed to load. Verify default_bus_layout.tres.
• Stems generated at 60s but you want a 5-minute combat. They loop. Crossfade is volume-only, not time-based, so loops are invisible to the director.

Fallback (no MCP)

Print the bus layout, the node tree, and the GDScript. User constructs in the Godot editor manually.

Handoff

> MusicDirector wired. Calm / Tension / Combat play in sync, only one audible. State driven by EventBus. Music ducks -6 dB on dialogue. Next: > - Run summer_play and trigger each transition; listen for sync issues. > - If COMBAT lands on the wrong beat, the stems are off-tempo — regenerate via /music-track with identical BPM and bar count. > - For boss-specific music, generate boss.mp3 with /music-track and the director picks it up automatically.

See also

• audio/audio-direction — defines the dynamic music plan and bus layout
• audio/music-track — generates the stems
• audio/voice-line — the voice bus that triggers ducking
• references/godot-version.md — Godot 4.5 AudioServer and Tween API

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: SummerEngine
• Source: SummerEngine/summer-engine-agent
• License: MIT
• Homepage: https://summerengine.com/

Install and usage instructions live in the source repository linked above.