| name | vst3-review |
| description | Reviews VST3 plugin implementations for spec compliance, host compatibility, and correctness. Use when the user asks to review a VST3 plugin, check bus arrangements, audit parameter handling, or verify their process() callback is safe. Trigger on phrases like "review my VST3 plugin", "check VST3 bus arrangement", "is my VST3 process safe", "audit my IAudioProcessor", or when you see IComponent, IAudioProcessor, IEditController, ProcessData, or Vst:: namespaced types in the code.
|
VST3 Plugin Review
VST3's split controller/processor model and bus negotiation are the most common sources of host incompatibility. Issues are rarely compile-time errors — they surface as broken recall, silence, or crashes in specific DAWs.
Step 0 — Run universal checks first
Invoke audio-dsp-review and audio-numerics-review before this skill. This skill adds VST3-specific spec compliance and host compatibility checks on top; it does not replace realtime-safety or numerics reviews.
Step 1 — Identify plugin structure
Locate the three core interfaces and their ownership:
| Interface | Thread | Responsibilities |
|---|
IComponent | Main thread | Bus layout, activation, state save/load, unit info |
IAudioProcessor | Audio thread in process(); main thread otherwise | Bus arrangement negotiation, latency, processing setup |
IEditController | Main thread | Parameter definitions, UI, MIDI mapping, setComponentState |
IConnectionPoint | Main thread | Processor↔controller messaging via notify() |
Step 2 — Scan for VST3-specific violations
| Violation | Where | Risk |
|---|
process() allocating memory or acquiring a lock | IAudioProcessor::process | Realtime violation — causes xruns and glitches |
Latency not reported via getLatencySamples() | IAudioProcessor | Host delay compensation broken — early mixdown or timing drift |
Bus arrangement not matching what was negotiated in setBusArrangements | setBusArrangements / process | Channel count mismatch — buffer overread or silent output |
ParamID not stable across save/load and plugin versions | IEditController | Parameter recall broken — automation and presets silently map to wrong params |
Calling IHostApplication or IComponentHandler from audio thread | process() | Not thread-safe — host component handler is main-thread only |
IParamValueQueue not fully consumed each process() callback | process() | Parameter updates lost — automation steps skipped |
| Bypass parameter not passing audio through cleanly | IAudioProcessor / controller parameter setup | Incorrect bypassed output — artifacts or silence when host automates bypass |
kOfflineProcessing mode not handled separately from realtime | setProcessing / process | Offline rendering broken — wrong timing, denormal handling, or lookahead |
setComponentState not applied by controller on load | IEditController::setComponentState | Controller shows stale values after preset load |
setState / getState on IComponent and IEditController not symmetric | Both | Preset corruption — one side saves data the other ignores |
Step 3 — Write the review
## VST3 Plugin Review: `[file / processor class]`
### Verdict
[Spec-compliant | Has critical violations | Warnings only] — [one sentence summary]
### Critical Violations
**[Category]: [description]**
`file:line` — `offending code`
Why: [one sentence on spec / thread safety / host compat risk]
Fix: [concrete VST3-idiomatic suggestion]
### Warnings
[same format]
### What's Done Well
[correct patterns — clean bus negotiation, stable ParamIDs, latency reported, etc.]
### Recommended Fixes (priority order)
1. ...
Quick fix table
| Violation | Fix |
|---|
Allocation in process() | Move to setupProcessing() or constructor; use pre-sized buffers |
| Missing latency report | Override getLatencySamples() and return a stable value; call restartComponent(kLatencyChanged) if it changes |
| Bus mismatch | Store the accepted arrangement from setBusArrangements; validate numInputs/numOutputs against it in process() |
Unstable ParamID | Use a fixed enum or stable CRC of the param name string; never use a container index |
| Params not consumed | Iterate inputParameterChanges fully each block; apply at sample-accurate offsets |
| Unsafe bypass | Mark the bypass parameter with ParameterInfo::kIsBypass; when bypassed, copy inputs to outputs with no modification and keep consuming parameter changes |
For deep detail on bus negotiation, parameter ID stability, latency reporting, controller↔processor communication, and per-DAW compatibility quirks see references/vst3-pitfalls.md.
