The Scottish Greens have issued a stark warning to the next leader of the Scottish National party that “a sincere commitment to progressive values cannot be an optional extra”. With the SNP announcing on Monday who will replace Nicola Sturgeon as party leader and first minister after her shock resignation last month, the Scottish Green leadership gave its most definitive indication yet that the partnership deal between the two parties would be over if Kate Forbes is elected. The Scottish Green co-leaders, Patrick Harvie and Lorna Slater, told their party’s spring conference they had “so much more” to deliver in government with the SNP, but would not continue the partnership “at any cost” with a first minister who did not share their progressive commitments. The Bute House agreement that brought the Greens into government after the 2021 Holyrood elections is critical to lending the SNP the parliamentary majority it needs at Holyrood to pass its budgets. The SNP will announce its new leader on Monday afternoon after a turbulent contest that has seen deep policy divisions emerge between candidates, unprecedented personal attacks and the resignation of the party’s chief executive and Sturgeon’s husband, Peter Murrell, following damaging revelations that the media had been fed false information about membership figures. Recent polling suggests Humza Yousaf, widely regarded as the preferred candidate of the SNP leadership, is edging ahead. He is the only candidate to commit to maintaining the partnership with the Greens, and has pledged to continue the centre-left, socially inclusive agenda that defined the Sturgeon era. But his rival Kate Forbes, an evangelical Christian whose opposition to equal marriage and abortion nearly derailed her campaign in its first week, has proven popular with members after strong performances in hustings and television debates. She has been highly critical of Sturgeon’s record, challenging her model of progressive taxation and the need for a swift transition away from oil and gas exploration. Both Forbes and the third candidate, Ash Regan, a junior minister who resigned over the Scottish government’s gender recognition reform bill, have said they would drop Sturgeon’s commitment to fight Westminster’s veto of the bill in the courts, and have also stated their opposition to self-identification for trans people, a key element of securing Green cooperation. With the new SNP leader facing a formal vote to become first minister at Holyrood on Tuesday, Slater told delegates gathering in Clydebank on Saturday: “We will only vote for the SNP’s new leader to become first minister if they are committed to the politics of cooperation; if they respect and share our values of equality and environmentalism; if they will prioritise climate justice; and if they agree that trans rights are human rights and that our trans siblings cannot be used as political fodder by Westminster. “These are fundamental issues for us. They are non-negotiable.” To cheers from members, co-leader Harvie said a commitment to progressive values, including the Green-led campaign for safe access zones to abortion clinics, a full ban on conversion practices, and challenging “the UK government’s abuse of the section 35 power to block the gender recognition reform bill”, was a “necessity” in the choice of first minister. Speaking at one of his final campaign visits in Dundee on Saturday afternoon, Yousaf urged his fellow candidates not to jeopardise the Bute House agreement, saying: “I would say to anybody who ends up being the next leader of the SNP, you have to find a way of cooperating with the Greens. “If you don’t, you end up not just in a minority government, but I have to say, one of the most toxic parliaments I’ve ever been in, in terms of, I’m afraid, the opposition who will not look to cooperate with the SNP very often.” Slater said the Scottish Greens would hold a party council meeting on Monday afternoon to discuss the leadership outcome and “choose whether we want to continue in government”. During a question-and-answer session on the Bute House agreement, delegates probed the Scottish Green leadership on what more the partnership deal could deliver. Harvie responded that the agreement does contain scope for review, and that this would make sense “once the dust has settled”. “What’s in the agreement right now is a really ambitious policy delivery programme, especially when times are tight financially. Our first priority is to continue with what we’ve committed to.” Harvie also said the Scottish government’s failure to reach strict targets to cut emissions by 75% by 2030 was “the reason we need to be in the room”. He said the urgency of global heating demanded a government with “guts and political courage” that is willing to do things that might be “politically controversial”, He suggested that the current widespread criticism of the proposed deposit return scheme, which has been spearheaded by Slater, was evidence of the “immense furore” that accompanies green reform.