The deal, which could see Green MSPs take ministerial positions in government, is reportedly on the brink of being finalised, although there are still disagreements about how the junior party would be able to oppose certain SNP policy areas such as the extraction of oil and gas from the North Sea.
The parties have been in talks since Parliament broke for recess after Nicola Sturgeon’s government reached out to the Scottish Greens for a more formalised deal.
It would allow the First Minister to project strong climate credentials as Scotland prepares for the COP26 talks in November while the Greens also believe that the agreement would allow them to influence government policy more directly, rather than negotiating each year during budget talks.
The SNP holds 64 of Holyrood’s 129 seats, and with the seven Green seats added, it would give the parliament a comfortable pro-independence majority.
One source said: “The final five per cent is predictably the most difficult stuff which was parked until this point,” but that both parties were “optimistic” about a successful deal.
Former deals at Holyrood included the Labour-LibDem coalition governments in the first few Holyrood terms, but an agreement would be the first formal deal the SNP has struck with any other party.
The co-operation agreement is based on one similar in New Zealand politics, and would allow the Scottish Greens to be both in government, but also be able to take oppositional stances on certain issues.
A source said: “The latter is more technically tricky to work out rather than being an area we disagree on and we are trying to find a compromise. It's a totally unprecedented model of cooperation for our part of the world.”
The deal could allow the Greens to criticise the SNP’s policy on North Sea oil exploration, with pressure mounting on the Scottish government from climate campaigners to oppose the proposed Cambo oil field development near Shetland.
However under Scottish Green party rules members must approve any deal at a special conference before it can come into force.
The agreement was criticised by Scottish Conservative net zero spokesman Liam Kerr, who said the prospect was “terrifying for the 100,000 workers and their families in our oil and gas industry.”
He added: “The SNP government is working hand-in-hand with a party that’s happy to see thousands of hardworking families lose their jobs to suit their priorities.
“They care more about their separatist, ideological grievances than Scotland’s jobs and our wider economy and recovery.”
Scottish Labour leader Anas Sarwar added: "This coalition isn't a surprise it is just formalising a long standing reality where Nicola Sturgeon and the SNP hammer our public services with cuts, and the Greens nod along.
"From voting against pay rises for care workers, failing to reform the council tax, and tripling cuts to Scottish councils, this confirms the long held suspicion that the Scottish Greens are a just a branch office of the SNP.”
A spokesman for the Scottish Greens said they would not “provide a running commentary” on the talks.