How should the ambiguous ending of Tarkovsky's Solaris be interpreted?

The film gives you two readings and refuses to pick one for you. That refusal is the point, not a flaw to be solved.

Two readings, both licensed by the same image

In the closing scene, Kelvin appears to have returned to his father's house on Earth. Then the camera pulls back and shows that the house sits on a small island within Solaris's ocean, and rain is falling inside the house. Nothing in the image itself tells you whether to feel relief or dread.

Read one way, this is a tragedy. Kelvin has given up on resolving anything with the people who are actually alive and actually flawed, and instead has settled into a copy of a place, made for him by an alien intelligence that has been feeding him copies of a woman he failed. On this reading, staying on Solaris is a retreat into a manufactured comfort, a kind of solipsism dressed up as homecoming. He gets to keep the feeling of return without doing the harder work of returning.

Read the other way, this is the first real success in the film. Every human effort at contact with Solaris up to that point had failed. Scientists had bombarded the ocean with radiation, tried to communicate through instruments, treated it as an object to be studied or neutralized. The ocean's response, sending each crew member a visitor built from their most painful memory, looked at first like an attack. But if the final image is the ocean's best attempt to give Kelvin something like peace, using the only materials it has, which are his own memories, then this is contact succeeding, not failing. An alien mind has reached toward a human one and produced something that functions, however strangely, as reconciliation.

Why the film will not resolve this for you

A conversation on Very Bad Wizards works through exactly this fork. One host leans toward the tragic reading, treating Kelvin's choice to remain as a self-deceptive escape from unresolved human relationships. The other resists closing the question, arguing the scene can just as easily be read as a genuine, unprecedented act of contact between humanity and something truly alien. Neither position is presented as obviously wrong, and that is worth taking seriously as a feature of the film rather than an oversight.

The reason the ambiguity holds is that both readings depend on the same unresolved question the whole film has been asking: can you ever really know an alien mind, or are you only ever encountering your own projections reflected back? Solaris never lets you confirm which is happening. The ocean might be a genuine other, reaching out clumsily. Or it might be a mirror, incapable of anything but returning your own guilt and longing in a new shape. The film gives you no instrument, literal or narrative, that settles it.

What the ambiguity is doing

If you take the ending as needing a solution, you will end up arguing about plot mechanics: is the island real, is Kelvin dead, is this a dream. Those questions have no textual answer. The more useful question is what each reading costs you. The tragic reading asks you to notice how easily comfort can substitute for repair, and how a copy of what you lost can feel like enough. The contact reading asks you to take seriously that another mind, however alien, might reach you in a form you did not choose and could not have predicted, and that this might still count as real.

The film's honesty is in holding both open. It does not tell you that Kelvin was wrong to stay, and it does not tell you he was right. It leaves you with the same problem it leaves him: deciding what counts as real contact, and what only counts as consolation.