Use 'in the bag' to express that success is 100% sure, with no doubt.

Absolutely Certain

'In the bag' is an expression and doesn't involve a real bag.

Not Literal

Best for casual conversations, 'in the bag' isn't usually formal.

'In the bag' suggests prior actions have almost secured future results.

Until it's official, using 'in the bag' may be premature—things can change.

The phrase carries a feeling of relief that a goal will be met.