Verifying Without a Compiler: Cowork's Harness vs Claude Code's, and Why the Slice Verifier Stays

Two-question synthesis on verification where no mechanical checker exists. (1) Cowork and Claude Code share primitives (skills, MCP, sub-agents, computer use) but sit on opposite ends of the verifier ladder, so the harness weight redistributes: Claude Code leans on a deterministic post-hoc verifier stack (tests, compiler, diffs, spec-drift checks) that both catches errors and bounds damage before merge; Cowork's outputs have no such rung, so its harness substitutes judgment-encodings for mechanical checks — the loaded design system as the nearest thing to a style linter, evals and LLM-judges for quality, human review concentrated at decision checkpoints — while the pre-action classifier gate becomes load-bearing because errors ship directly into live SaaS state with no red test in between. Failure modes split accordingly: loud (build breaks) vs silent (a polished deck that reads fine — the failures-that-look-like-success class), which is why accountability redesign matters more for Cowork, not less. (2) The planner needs the horizontal-slice verifier by design, not just empirically through 4.7: 'every slice produces end-to-end feedback' is a mechanically checkable invariant (does it touch schema+service+UI?), and checkable invariants belong in the deterministic checker regardless of model trust — the verifier is a constraint (doesn't compound, costs nothing to keep, catches the training-prior regression toward horizontal layering), while 'please slice vertically' is a behavior request, the transient form of the same discipline. Trust-the-model applies to prompt lines; verifiers are the durable class