Vst3 Review

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.

Source & license

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

• Author: kunitoki
• Source: kunitoki/sonic-skills
• License: Unlicense

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