Two answers in one plant
Chickens can eat tomatoes — the ripe fruit is a juicy, well-loved treat that hens will squabble over. But the tomato plant demands a split answer, because the same plant that produces the fruit carries a genuine poison in its green parts. Ripeness is the whole question here: red and ripe is feed, green and growing is not, and a keeper's job is simply to keep the flock on the right side of that line.
Solanine: why the green parts are out
Tomatoes belong to the nightshade family, and like their relatives they defend their unripe fruit and foliage with solanine, a compound that makes green tomatoes, leaves, stems and vines unsafe for chickens to eat. A hen who snatches a leaf or two through a fence is unlikely to come to serious harm — solanine tastes bitter, which limits how much gets eaten — but a flock with free access to tomato plants can eat enough green material to become genuinely unwell. The safe policy is not complicated: the plants are never a treat, never forage, and never something to toss into the run as garden waste.
Serving the ripe fruit
A ripe tomato needs only washing and, for cherry tomatoes, halving — whole small round fruit goes down in one gulp, and halved is the safer, fairer way to share it. Garden tomatoes at the split, over-soft stage that the kitchen rejects are flock gold, provided they are ripe rather than rotting; a fermenting or mould-furred tomato goes to the compost instead. Tomatoes are sweet and acidic, so portions stay treat-sized — a big tomato shared across the flock is a fair event — and a hen who gorges on acidic fruit may show a day of loose droppings as the receipt.
The vegetable patch problem
Most tomato trouble starts with geography: hens free-ranging a garden will find the tomato bed, because hens find everything. They will eat the ripe fruit you were saving — annoying but harmless — and they may sample leaves and green fruit on the way. Fence the bed, or fence the flock's range to exclude it; either barrier solves the problem completely. Never throw tomato prunings, suckers or pulled plants into the run as greenery, and when the season ends and the plants come out, they go to the compost heap the hens cannot reach, not onto the flock's scratching ground.
If a hen has raided the patch
A hen who has eaten some green tomato material usually shows it first as digestive upset — loose droppings, a drop in appetite, a bird quieter than herself. Remove access, offer plain water and normal feed, and watch the flock over the following day. Most mild cases settle; a hen who is weak, unsteady, off her legs or not eating by the next morning needs a vet promptly, and it helps to be able to say what she got into and roughly how much of it.
Patch rules only work if everybody who lets the hens out knows them, so set them down in KinsLe's Care Teams — the tomato bed stays fenced, ripe fruit is shared deliberately, and the flock's favourite red treat never turns into a green accident.