Grapes are toxic to dogs but only if the dog eats more than one grape.
I've had a dog eat a grape accidentally and nothing bad happened to him and when I took him to the vet to be safe the vet told me that since he ate only one grape that the dog should be just fine.
And the vet was right nothing bad happened to the dog after eating one grape.
For a dog to get sick and possibly die from ingesting grapes they would have to eat around 5 or 10 or more grapes for the grapes to kill the dog.
However supposedly 2 grapes can be toxic for smaller dogs while it can take a few more as in 3 or 4 grapes to start the toxicity in the larger dogs.
If you get your dog to the vet on time then the dog can likely be saved.
Always put grapes up away from dogs to keep them from accidentally ingesting them.
The way that grapes kill dogs is that the grapes cause kidney failure which is rapid and is what actually kills the dog.