"carved in stone"
If a suggestion, plan, rule, etc. is carved in stone, it cannot be changed
To make something permanent and incapable of being changed, typically a plan or idea.
These proposals are for discussion, they're not carved in stone.
We might get brunch next weekend, but we haven't carved anything in stone yet.
The deal isn't yet carved in stone, but we're confident it will go ahead as hoped.
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