The two passages differ in emphasis, but the Quran does not plainly say that Jacob knew Joseph was alive. It says Jacob rejected the brothers' account as untrustworthy and responded with patience, which is not the same as possessing certain knowledge of Joseph's fate.
Why the objection appears strong
Genesis presents Jacob as accepting the bloodstained garment and grieving deeply. He tears his clothes, refuses consolation, and says that he will mourn Joseph until his own death. The Quran, however, records the brothers' story about a wolf, then has Jacob say, "Rather, your souls have enticed you to something," followed by "beautiful patience." Read quickly, this can sound as though Jacob immediately knew Joseph had not died.
They came to their father at night, weeping. They said, "O our father, we went racing and left Joseph with our possessions, and a wolf devoured him. But you would not believe us, even if we were truthful." And they brought his shirt stained with false blood. He said, "Rather, your souls have enticed you to something, so beautiful patience. And Allah is the One whose help is sought against what you describe."
Quran 12:16-18
Genesis likewise says:
Then Jacob tore his garments, and put sackcloth on his loins and mourned for his son many days. All his sons and all his daughters rose up to comfort him, but he refused to be comforted and said, "No, I shall go down to Sheol to my son, mourning."
Genesis 37:34-35
What Jacob's words actually establish
The Quranic wording establishes that Jacob recognized something dishonest in the brothers' presentation. Their account was suspicious: they had left Joseph alone, returned without him, and produced a garment marked with what the Quran calls false blood. Jacob's response can mean, "You have contrived this matter," without meaning, "I know Joseph is alive and know what happened to him."
This distinction matters. A person may reject a specific explanation while still fearing that the missing person has died. Classical Muslim commentators commonly understood Jacob to have detected the brothers' deceit, while leaving the precise fate of Joseph unknown. His "beautiful patience" describes how he bore the trial before Allah; it is not a technical statement that he felt no grief or had certainty of rescue.
Does this prove an error?
Only if one adds an assertion the Quran itself does not make: that Jacob knew Joseph was alive. Genesis explicitly narrates prolonged mourning, whereas the Quran selects the moment of suspicion and patience. Those details can coexist: Jacob could distrust the wolf story, suspect the brothers, and yet grieve as though Joseph might be dead.
If someone insists that the Quran requires Jacob to know Joseph was alive, then there is a tension with Genesis. But that reading is stronger than the Arabic text warrants. The passage plainly supports Jacob's distrust and steadfastness; it does not plainly deny his anguish or establish certain knowledge. Thus the comparison does not demonstrate a concrete contradiction or, by itself, a mistaken retelling.