Previous studies revealed critical involvement of the striatum in adapting to the environment by actions that anticipate rewards from experiences as a policy. However, it remains unclear how current policy is evaluated to explore more advantageous alternatives. Here, we show that during policy-based sequential actions in a rat reversal task, the dorsomedial striatum plays an essential role in pathway-specific manner. Recording and optical manipulation of the indirect pathway showed that late-onset activity following unrewarded suboptimal action represents a lowered valuation of the current action policy and a heightened bias to try the suboptimal action. The early-onset activity complementarily mediated policy-based suppression of unrewarded action. These results demonstrate the indirect pathway's role in monitoring unreliability of current action policy and probing alternative one. This study extends conventional understanding of consequence-guided persistence with reward-oriented action policy and provides key insights regarding how the dorsomedial striatum enables proactive and flexible adaptation to environmental changes.