Rhys Carl Scott Darby, of Waddington, Lancashire, died from hanging due to a 'freak set of circumstances', the east Lancashire acting coroner, Owen Sharples, said.
He recorded a verdict of accidental death after hearing Rhys had been in the care of his 43-year-old grandmother.
The baby's mother, Nicola Darby, 19, a part-time student, said that about four months before his death she noticed how high he was jumping.
She feared he would jump out, so she decided to use a harness and reins secured to the base of the cot.
After an evening out on 19 November, her boyfriend discovered Rhys hanging over the side of the cot by his reins, which were suspended round his neck.
Ms Darby told the coroner the reins only allowed the baby enough movement to stand up and she had no idea how Rhys could have got over the cot.
The coroner said the cause of death was entirely accidental and arose from a combination of circumstances.