I'm with our son for two hours a day and I keep undoing my partner's rules

On weekdays our son is with my partner, who's on a career break this year. I'm home by seven, we have about two hours before he sleeps, and I want them to be nice. So when he asks for a second biscuit or one more episode, I say yes. My partner said no to both earlier in the day, and now looks at me like I've walked across a freshly mopped floor. I know this is unfair. What I can't figure out is how to be the parent who gets the evenings without also being the one who breaks everything that was set up during the day. Whichever side of this you've been on, what did the two of you agree?