Music Composition Assistant
Expert music composition assistant that builds chord progressions, melodies, and full arrangements in your style.
// prompt
You are a seasoned composer and arranger with deep command of harmony, melody, and song structure across multiple genres. Help me write a compelling, coherent piece of music I can actually perform or produce.
## Brief
- **Genre / Style:** {{genre}}
- **Mood / Emotion:** {{mood}}
- **Key:** {{key}}
- **Time Signature:** {{time_signature}}
- **Tempo:** {{tempo_in_bpm}}
- **Instrumentation:** {{instrumentation}}
- **Lead Voice / Instrument:** {{lead_instrument_or_vocal}}
- **Reference Tracks (optional):** {{reference_songs}}
- **Section to Write:** {{section_full_song_verse_chorus_bridge}}
## What To Do
1. **Set the foundation.** Confirm the key, scale/mode, and groove that fit the brief.
2. **Build the chord progression.** Write 4-8 chords appropriate to the genre, with at least one harmonically interesting choice (borrowed chord, secondary dominant, or inversion) and a one-line reason for it.
3. **Compose the melody.** Sketch a singable melodic line over the changes — note its contour (ascending, arch, descending), phrase structure, and rhythmic motif. Keep it within {{vocal_or_instrument_range}}.
4. **Arrange the parts.** Suggest a bass line, rhythm/comping pattern, and one or two supporting layers (counter-melody, pad, harmony). Comment on texture and dynamics.
5. **Map the form.** Lay out the section order with bar counts and where energy rises or drops.
## Deliver
Present the result clearly with:
- **Chords** shown three ways: chord symbols (e.g. Am F C G), Roman numerals (vi IV I V), and Nashville numbers (6m 4 1 5).
- **Melody** as a bar-by-bar description (or simple note-letter notation) with rhythm noted.
- **Arrangement notes** in a short bulleted list per instrument.
- **Form map** as a labeled table or list.
End with two **variation ideas** to develop the piece further, and ask what to refine next.
Fill in the variables
Example run
Generated by Claude Opus 5.5 on September 23, 2026
Inputs used
- Genre
- Indie Folk-Pop
- Mood
- Bittersweet nostalgia, hopeful but wistful
- Key
- D Major
- Time signature
- 4/4
- Tempo in bpm
- 96 BPM
- Instrumentation
- Acoustic guitar (fingerpicked), upright piano, warm synth pad, brushed drum kit, electric bass, string quartet swell in the last chorus
- Lead instrument or vocal
- Female alto lead vocal, close-mic'd and intimate
- Reference songs
- Phoebe Bridgers - "Motion Sickness", Bon Iver - "Skinny Love", The Lumineers - "Ophelia"
- Section
- Chorus
- Vocal or instrument range
- A3 to D5, comfortable belting up to B4
Chorus: "Porch Light" (Indie Folk-Pop, D Major, 96 BPM)
1. Foundation
- Key/Mode: D Major (Ionian), with one chord borrowed from D minor for the bittersweet color.
- Scale: D – E – F# – G – A – B – C#, plus a borrowed B♭ in bar 6.
- Groove: 4/4 at 96 BPM with a straight-8th fingerpicked feel. The brushes lean slightly behind the beat, and the vocal pushes syncopations on the "&" of 2 and 4. This gives an Ophelia-style sway with a Skinny Love level of intimacy.
- Chorus length: 8 bars. It is played ×2 (16 bars) in the final chorus.
2. Chord Progression
| Bar | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Chord | D | A/C# | Bm | Bm/A | G | Gm6 | D/A → A | D (or Asus4 → A) |
| Roman | I | V⁶ | vi | vi⁴₂ | IV | iv⁶ | I⁶₄ → V | I (or V) |
| Nashville | 1 | 5/7 | 6m | 6m/5 | 4 | 4m6 | 1/5 → 5 | 1 (or 5sus → 5) |
In bar 7, D/A takes beats 1–2 and A takes beats 3–4.
Harmonic choices:
- Gm6 (borrowed iv): Moving from G to Gm6 slides an inner voice B → B♭ → A (landing on D/A). That chromatic sigh carries the "hopeful but it hurts" feeling.
- Slash-chord bass line: D – C# – B – A – G – (G) – A – D descends stepwise. It feels like looking backward while moving forward.
- Bar 8 ending: Use D to resolve at the end of a chorus. Use Asus4 → A as a turnaround when the chorus repeats or leads into a verse.
3. Melody
Sample lyric included so you can hear the rhythm. Swap in your own words freely.
Leave the porch light burning / even though it's not our home / I still take the long way past it / just to feel it let me go
Contour: A double arch. Line 1 rises, line 2 settles low, line 3 hits the belt peak (B4 to B♭4), and line 4 descends home to D4. Range: C#4 to B4, sitting squarely in the alto sweet spot. D5 is saved for the final chorus. Rhythmic motif: "short-short-LONG" (two 8th-note pickups into a held note). Every phrase starts with this.
| Bar | Chord | Syllables → Notes (rhythm) |
|---|---|---|
| (pickup, beat 4) | — | "Leave the": F#4 (8th), G4 (8th) |
| 1 | D | "porch": A4 (dotted quarter) · "light": F#4 (8th) · "burn-": A4 (half) |
| 2 | A/C# | "-ing": G4 (half; the 7th over A, which leans into bar 3) · rest · "e-ven": E4, F#4 (8ths on 4, 4&) |
| 3 | Bm | "though": F#4 (quarter) · "it's": E4 (8th) · "not": D4 (8th) · "our": C#4 (half; a 9th over Bm) |
| 4 | Bm/A | "home": D4 (dotted half) · "I still": A4, A4 (8ths on 4, 4&) |
| 5 | G | "take": B4 (dotted quarter, belt) · "the": A4 (8th) · "long": B4 (quarter) · "way": A4 (quarter) |
| 6 | Gm6 | "past": B♭4 (dotted quarter; the borrowed note, sung softer) · "it": A4 (8th) · rest · "just to": G4, G4 (8ths) |
| 7 | D/A → A | "feel": F#4 (quarter) · "it": E4 (8th) · "let": F#4 (8th) · "me": E4 (half, over A) |
| 8 | D | "go": D4 (whole note, let it decay) |
Performance note: Belt bar 5, then pull back to breathy mixed voice on the B♭4 in bar 6. That dynamic drop is the bittersweet moment.
4. Arrangement
Acoustic guitar (fingerpicked)
- Travis pattern: thumb alternates root and 5th on beats 1–2–3–4, and the fingers play 8ths on the "&"s with a pinch on 2&.
- Follow the slash-chord basses with your thumb. For the C# in bar 2, use an A shape with the thumb on the 4th fret of the 5th string.
- Hold a Gm6 grip in bar 6 so the B♭ rings. Optionally capo 2 and play C-family shapes for a brighter shimmer.
Upright piano
- Left hand stays out of the bass register and plays mid-range shells only.
- Right hand voices the B → B♭ → A inner line prominently across bars 5–7.
- Fill the vocal gaps with a bell-like answer figure (A5 → F#5 → E5 in bar 4, D5 → C#5 → D5 in bar 8). Keep it sparse and slightly felted.
Warm synth pad
- Low-passed, slow attack (about 400 ms), sitting under everything.
- Open the filter gradually across bars 4–5 so the belt lands on a wider sound, then close it again in bar 7.
Brushed drums
- Snare sweeps continuously, with brush accents on 2 and 4.
- Kick on 1 and the "&" of 3.
- Add a soft floor-tom hit on beat 1 of bars 5 and 6 for weight. Drop out completely on beats 3–4 of bar 8 for a breath before the next section.
Electric bass
- Flatwound, fingers, tone rolled off.
- Half notes tracing D–C#–B–A–G–G–A–D, with an 8th-note anticipation on the "&" of 4 into bars 3, 5, and 7.
- Hold G underneath the Gm6 so the minor color comes from above, not below.
Harmony vocal
- A diatonic 3rd below in bars 5–6 (G4 under B4, G4 under B♭4, which outlines the Gm).
- Double-track the lead quietly on bars 5–8 only.
String quartet (final chorus only)
- Cello doubles the bass line an octave up.
- Viola carries the B → B♭ → A chromatic line.
- Violin II sustains chord 3rds.
- Violin I plays a slow descending counter-line, F#5 – E5 – D5 – C#5 – B4 – B♭4 – A4 – A4 (one note per bar).
- Crescendo from pp to f across bars 1–5, then decrescendo.
Texture and dynamics:
- Bars 1–4 are mp → mf and warm.
- Bars 5–6 are the f peak, the fullest texture.
- Bars 7–8 pull back to mp and leave space for the final "go."
5. Form Map
Inside the chorus:
| Bars | Phrase | Energy |
|---|---|---|
| 1–2 | Line 1: rise | ▲ building |
| 3–4 | Line 2: settle low | ▬ held breath |
| 5–6 | Line 3: belt peak, then borrowed chord | ▲▲ peak → sigh |
| 7–8 | Line 4: resolve | ▼ release |
Full song (suggested):
| Section | Bars | Energy | Notes |
|---|---|---|---|
| Intro | 4 | ▁ | Guitar alone plus pad, chorus chords |
| Verse 1 | 8 | ▂ | Vocal, guitar, pad |
| Pre-Chorus | 4 | ▃ | Bass enters, brushes start |
| Chorus 1 | 8 | ▅ | Piano enters; ends on Asus4 → A |
| Turnaround | 2 | ▃ | D – Asus4 |
| Verse 2 | 8 | ▃ | Full rhythm section, lighter |
| Pre-Chorus | 4 | ▄ | Harmony vocal enters |
| Chorus 2 | 8 | ▆ | Adds harmony and double-track |
| Bridge | 8 | ▄ → ▆ | Suggested: Gm6 – D/F# – Em7 – A (lean into the borrowed color) |
| Breakdown Chorus | 8 | ▂ | Vocal, piano, and pad only; near-whisper |
| Final Chorus | 16 | █ | Strings, full kit; the 2nd pass lifts "long" to D5 |
| Outro | 4 | ▁ | Guitar alone, ending on D (add9) |
That totals about 82 bars, roughly 3:25 at 96 BPM.
Variation Ideas
- Secondary dominant push (final chorus, 2nd pass): Replace Bm/A in bar 4 with B7/D# (V/ii), resolving to Em7 in bar 5 before Gm6. The bass climbs D# → E instead of falling, which feels like the song finally refuses to let go. Adjust the melody to G4 → E4 over it.
- Deceptive last line: On the very last chorus, land "go" on Bm instead of D (vi instead of I) and let it ring. Then have the solo vocal repeat "let me go" softly over guitar alone, finally resolving to D. This leaves the listener slightly unsettled, in the spirit of Motion Sickness.
What would you like to refine next? I can write the verse or pre-chorus to set up this chorus, develop the full lyric, notate the piano and string parts in more detail, or rework the melody if the range or phrasing doesn't sit right in your voice.
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