Composition
Composition is how layers of prompts are assembled into a rendered message list for the model. resolveExecutionBundle / resolvePromptComposition build renderedComposition used by runPrompt (see runPrompt.ts and composition.ts).
Each layer is a promptLayerSchema (type, content, mode append/replace).
Trace
Preview and run detail expose composition trace data (blocks, rendered prompt) in skillVersionSchema.preview and in contract trace sections on runs.