arXiv:2610.25344v2 Announce Type: replace Abstract: Code-based agents can operate creative software, but mixing requests often describe an intended sound without specifying the edits needed to achieve it. We present TOMORI, a code-based mixing-agent harness for native digital audio workstation (DAW) control. Custom plugins provide REAPER project inspection, optional audio measurement, and execution of model-generated Lua code. All observations return as text; delivery includes an editable project and rendered audio. TOMORI-BENCH pairs 110 human-reviewed reference edits with three instruction forms and evaluates native DAW execution end to end through operation structure, parameter precision, and rendered-audio outcomes. Across 990 cases from three primary models, explicit instructions yield high reference agreement, while abstract requests reduce project agreement and audio improvement. Paired ablations show that the full workflow improves explicit execution over a single submission. Prompted audio measurement succeeds in all 28 selected abstract cases but yields no clear overall improvement on the reference-based metrics and increases tool calls. These results expose the gap between executable DAW control and precise realization of perceptual mixing goals.