Optional. For a generated model, Claude looks at it from four sides beside the photos: turns it to face the way the photos do when it faces away (normalised again, no credits, confirmed by a second look), and lists in the output row what a shopper would notice as wrong, a quality rating and a builder that would make the product exactly. A yawDegrees you set always wins. For a flat build, Claude finds the face in the images. For a slatted-armchair build, it reads the counts and heights and, with build.cutCrops, cuts the wood and fabric. For a drum-shade build, Claude reads how the shade is made (inner layer, diffuser and its opening, spokes, ring, holder, rolled rims) from the photos, the maker's construction photos and notes, and checks it once against the built model; params you give win. For a boxy-sofa build, Claude looks at the photos and the dimensions and fills in every param you did not give (arm width, which end the chaise is on, back cushions, tufting, scatter cushions) before the sofa is built, and corrects them once if the model then differs grossly from the photo. Params you give always win. At most two Claude calls per sofa, billed to this key; the output row lists which params came from Claude, your input or the builder's defaults. Without a key (here or as ANTHROPIC_API_KEY in the environment) the sofa is built from your params alone.