High renewable penetration and stringent decarbonisation targets are reshaping power system planning. Although carbon pricing and emission constraints are widely adopted

most existing planning models represent carbon incentives in a mechanism-specific or reduced-form manner

making policy comparisons inconsistent and obscuring how cooperation and controllable transfers respond under different carbon designs. This paper develops a mechanism-switchable carbon policy representation under unified emission accounting

enabling consistent comparisons across major carbon policies. An integrated planning–operation co-optimisation model is formulated to capture generation and network expansion

controllable interarea transfers

storage and demand-side flexibility and chronological operational constraints. Interarea cooperation is modelled explicitly through reserve procurement

mutual-aid costs and coordination frictions

allowing the marginal value of cooperation to be quantified in a policy-consistent way. A multi-cut L-shaped Benders decomposition ensures computational tractability. Case studies on an IEEE 118-bus three-area system show that a moderate carbon tax reduces emissions by 45.7% mainly via photovoltaic expansion

whereas deep decarbonisation requires a diversified portfolio and stronger interarea deliverability

revealing a policy-dependent role of controllable flows.