Sans knows that timelines are changing, but Undertale does not establish that he remembers each reset. His dialogue shows awareness of an anomaly and suspicion that the player caused it; that is different from retaining memories of earlier runs.
What does Sans know about resets?
During the Genocide-route fight, Sans says, “our reports showed a massive anomaly in the timespace continuum. timelines jumping left and right, stopping and starting…” He then asks the player, “that’s your fault, isn’t it?” (Undertale Wiki fight transcript.)
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That exchange establishes that Sans knows about timeline changes and suspects the player is responsible. But he refers to “our reports,” not memories of events he personally lived through. The game does not explain who created those reports, what they record, or how Sans receives the information.
Does Sans remember previous runs?
Not clearly, at least not in the way Flowey does. The Undertale Wiki’s save-file article describes Flowey as retaining full knowledge of the protagonist’s actions across loads, while other characters have only vague memories (Undertale Wiki save-file article). That distinction supports saying Sans is not shown with Flowey’s complete memory of earlier runs. It does not rule out fragments or intuitions.
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The same article cites Legends of Localization Book 3: UNDERTALE, page 89, for the rule that the Underground character with the most Determination can use the save/load system. This helps explain Flowey’s place in the game’s save/load history; it does not directly settle how much Sans remembers.
Can Sans infer that the player has repeated events?
Yes. In another interaction, Sans notices the player’s bored expression after they have heard his speech before. He offers a secret codeword as a way to demonstrate that the player is a time traveler (Undertale Wiki Sans page). This can be read as Sans observing the player and drawing a conclusion, rather than remembering a previous timeline himself.
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Awareness, inference, and memory are different
- Awareness: Sans knows timelines are jumping, stopping, and starting, as his report indicates.
- Inference: He can suspect the player’s role and recognize signs that the player has experienced an interaction before.
- Memory: The game does not confirm that Sans personally retains detailed memories of prior runs. Flowey is the clearer example of that kind of continuity.
There is no cited statement from Toby Fox that directly confirms whether Sans remembers resets. The safest reading is that he is aware of timeline anomalies and can infer things about the player, while any personal memory he may have remains unconfirmed.
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