Undertale’s official 10th-anniversary site includes a new playable Sans battle. It is a separate browser-based Web Player—not a patch, DLC release, or expansion for the original game—and turns the encounter into a platforming challenge built around bone physics, blue-SOUL movement, and off-screen hazards.
A playable Sans encounter is waiting on Undertale’s official 10th-anniversary site
Undertale’s 10th-anniversary celebration includes a new, browser-based Sans battle. It is not a patch, DLC release, or expansion for the 2015 game. Instead, it is anniversary material presented through the official Undertale 10th Anniversary site and its “BATTLE” Web Player.
The encounter reworks Sans from a conventional boss into a physics-based platforming challenge. The player remains in blue SOUL mode, avoids or rides around bone patterns, and tries not to be pushed beyond the edges of the battle arena. Attacking Sans guarantees failure, while the final sequence ends with Sans using a bulldozer to sweep the SOUL off-screen—a deliberately comic conclusion that counts as success.
How to play the anniversary Sans fight
- Open the official Undertale 10th Anniversary page.
- Select the site’s BATTLE link.
- The battle opens at ut10-battle.undertale.com.
- Use a browser with JavaScript and WebAssembly enabled.
The public version is a Web Player rather than a downloadable update to the installed Undertale game. The supplied battle documentation does not establish universal mobile or browser compatibility, so players should treat normal JavaScript and WebAssembly support as a prerequisite rather than assuming that every phone, browser, or embedded web view will run it correctly.
What makes the new Sans fight different?
The anniversary encounter is the second of two less-serious Sans fights created for the 10th Anniversary Stream. It has its own track, “Song That Plays When Somebody Verses Sans,” and gives Sans only 1 HP and 1 defense. His listed attack value is 1, but his attacks do not damage the player’s HP.
#1 Best Overall
- Brilliant Color Illumination- With 11 unique backlights, choose the perfect ambiance for any mood. Adjust light speed and brightness among 5 levels for a comfortable environment, day or night. The double injection ABS keycaps ensure clear backlight and precise typing. From late-night tasks to immersive gaming, our mechanical keyboard enhances every experience
- Support Macro Editing: The K671 Mechanical Gaming Keyboard can be macro editing, you can remap the keys function, set shortcuts, or combine multiple key functions in one key to get more efficient work and gaming. The LED Backlit Effects also can be adjusted by the software（note: the color can not be changed）
- Hot-swappable Linear Red Switch- Our K671 gaming keyboard features red switch, which requires less force to press down and the keys feel smoother and easier to use. It's best for rpgs and mmo, imo games. You will get 4 spare switches and two red keycaps to exchange the key switch when it does not work.
- Full keys Anti-ghosting- All keys can work simultaneously, easily complete any combining functions without conflicting keys. 12 multimedia key shortcuts allow you to quickly access to calculator/media/volume control/email
- Professional After-Sales Service- We provide every Redragon customer with 24-Month Warranty , Please feel free to contact us when you meet any problem. We will spare no effort to provide the best service to every customer
That does not make the encounter harmless. The danger comes from movement and positioning:
- The blue SOUL mode never changes. The fight is built around movement affected by gravity and physical collisions rather than ordinary red-SOUL dodging.
- Bones behave like objects. They can push the SOUL around instead of simply subtracting HP on contact.
- The arena extends past the visible screen. The horizontal battle board continues beyond both sides of the display. If the SOUL is pushed off-screen, the player loses.
- Bone formations create platforming sequences. The encounter includes five principal patterns involving jumps, moving platforms, blue bones, and rapid bone arrangements.
- No items are available. There is no conventional healing or inventory strategy to fall back on.
- Attacking is a trap. If the player attacks Sans, he dodges and the fight is automatically lost. This echoes the dodge mechanic associated with Sans’s original genocide-route battle, but uses it here for a comedic anniversary encounter.
The result is closer to a short platforming obstacle course embedded inside an Undertale battle interface than to a normal HP-draining boss fight. Success depends on controlling the SOUL’s position, reading the bone patterns, and avoiding the arena’s side boundaries.
The bulldozer finale is supposed to end the fight
After the principal attacks, Sans boards a bulldozer and sweeps the SOUL off-screen. In most Undertale battles, being forced off the battle area would indicate failure. Here, it is the intended successful ending. The post-battle text changes accordingly, and the player is not prompted to retry.
Quick wins for a faster PC:
Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →That reversal is important when interpreting the encounter: being pushed away by the bulldozer is not the same outcome as falling off the battle board during an earlier sequence.
Rank #2
- Take your gaming skills to the next level: The Logitech G413 SE is a full-size keyboard with gaming-first features and the durability and performance necessary to compete
- PBT keycaps: Heat- and wear-resistant, this computer gaming keyboard features the most durable material used in keycap design
- Tactile mechanical switches: Uncompromising performance is always within reach with this wired gaming keyboard
- Premium color, material and finish: Elevate your gaming setup with this backlit keyboard featuring a sleek, black-brushed aluminum top case and white LED lighting
- 6-Key rollover anti-ghosting performance: Experience reliable key input with this anti-ghosting keyboard versus non-gaming mechanical keyboards
Is this the same Sans fight as in Undertale?
No. The anniversary battles were absent from the original Undertale release. The public version of this second anniversary fight is hosted separately on the official Undertale website, so it should be described as a new anniversary Sans fight or anniversary-exclusive battle, not as a newly added route in the base game.
It also should not be confused with the original game’s well-known Sans battle. The anniversary version changes the rules, uses a different encounter structure, and emphasizes physical pushing and platforming. It is an extra celebration piece rather than a replacement or revision of the original encounter.
Readers who want the original RPG can find the Undertale game through the official site, which lists the game’s supported console and PC availability. Platform, region, and edition availability can vary, so check the relevant storefront before purchasing.
Recommended Free Tools
What was the Undertale 10th Anniversary Stream?
The battle came from a larger two-day charity and auction livestream celebrating Undertale’s tenth anniversary. According to the official anniversary page, the event raised $361,407.76 for Doctors Without Borders, also known as Médecins Sans Frontières or MSF.
Rank #3
- Tri-mode Connection Keyboard: AULA F75 Pro wireless mechanical keyboards work with Bluetooth 5.0, 2.4GHz wireless and USB wired connection, can connect up to five devices at the same time, and easily switch by shortcut keys or side button. F75 Pro computer keyboard is suitable for PC, laptops, tablets, mobile phones, PS, XBOX etc, to meet all the needs of users. In addition, the rechargeable keyboard is equipped with a 4000mAh large-capacity battery, which has long-lasting battery life
- Hot-swap Custom Keyboard: This custom mechanical keyboard with hot-swappable base supports 3-pin or 5-pin switches replacement. Even keyboard beginners can easily DIY there own keyboards without soldering issue. F75 Pro gaming keyboards equipped with pre-lubricated stabilizers and LEOBOG reaper switches, bring smooth typing feeling and pleasant creamy mechanical sound, provide fast response for exciting game
- Advanced Structure and PCB Single Key Slotting: This thocky heavy mechanical keyboard features a advanced structure, extended integrated silicone pad, and PCB single key slotting, better optimizes resilience and stability, making the hand feel softer and more elastic. Five layers of filling silencer fills the gap between the PCB, the positioning plate and the shaft,effectively counteracting the cavity noise sound of the shaft hitting the positioning plate, and providing a solid feel
- 16.8 Million RGB Backlit: F75 Pro light up led keyboard features 16.8 million RGB lighting color. With 16 pre-set lighting effects to add a great atmosphere to the game. And supports 10 cool music rhythm lighting effects with driver. Lighting brightness and speed can be adjusted by the knob or the FN + key combination. You can select the single color effect as wish. And you can turn off the backlight if you do not need it
- Professional Gaming Keyboard: No matter the outlook, the construction, or the function, F75 Pro mechanical keyboard is definitely a professional gaming keyboard. This 81-key 75% layout compact keyboard can save more desktop space while retaining the necessary arrow keys for gaming. Additionally, with the multi-function knob, you can easily control the backlight and Media. Keys macro programmable, you can customize the function of single key or key combination function through F75 driver to increase the probability of winning the game and improve the work efficiency. N key rollover, and supports WIN key lock to prevent accidental touches in intense games
The stream was more ambitious than a standard broadcast. Its specially modified Undertale build included new locations, new non-player characters, altered sequences, new music, and references or cameos connected to Deltarune. Programming for the anniversary battles was credited to the Three Musketeers, with additional programming by Fred Wood for the pacifist Sans battle incentive, according to the event documentation.
The public Web Player and the complete modified livestream build are not the same thing. The official anniversary page exposes the battle along with event replays, downloadable music, downloadable graphics, donation information, and auction-related material. It does not present the entire modified livestream build as a downloadable version of Undertale.
Related anniversary material
The official anniversary page functions as a hub rather than only a link to the battle. In addition to the Web Player, it connects visitors with replays and downloadable creative assets from the event. Fans looking for physical items can also browse official Undertale merchandise and collector’s editions through Fangamer, which is linked from the official Undertale ecosystem.
The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Fangamer’s catalog has included Undertale physical games, collector’s editions, books, music, vinyl, plushes, figures, clothing, posters, pins, and other merchandise. It has also listed tenth-anniversary-era editions with names including Amalgamate, So Hyped, Bonetrousle, and Onslaught. Inventory and availability can change, so those products should be treated as current catalog listings rather than a guarantee that every anniversary item remains in stock.
Rank #4
- Hybrid blue mechanical gaming switches – The tactile click of a blue mechanical switch plus a smooth membrane – guaranteed for 20 million keypresses
- OLED smart display – Customize with gifs, game info, discord messages, and more.
- Aircraft-grade aluminum alloy frame – Manufactured for unbreakable durability and sturdiness
- Dynamic per-key RGB illumination – Gorgeous color schemes and reactive effects for every key
- Premium magnetic wrist rest – Provides full palm support and comfort
What the anniversary fight means for Undertale fans
The new Sans encounter is best understood as a compact, official celebration of Undertale’s design language. It takes a character associated with one of the game’s most demanding battles and removes the usual lethal damage loop, replacing it with slapstick physical hazards, screen-edge danger, and a deliberately absurd ending.
It is therefore worth playing for fans who want additional official Undertale content, but it does not signal a new Undertale expansion or a change to the original game. The clearest distinction is simple: the original Undertale remains its own game, while the anniversary site offers a separate Web Player built for the tenth-anniversary event.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently Asked Questions
How do you play the new Undertale anniversary Sans fight?
Open the official Undertale 10th Anniversary page and select the “BATTLE” Web Player. The battle is hosted at ut10-battle.undertale.com and requires JavaScript and WebAssembly.
Crashes, No Sound, or Screen Glitches?
Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteWindows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallIs the 10th-anniversary Sans fight part of the original Undertale game?
No. The encounter is anniversary material hosted separately as an official Web Player. It was absent from the original Undertale game and is not presented as a patch, DLC, or expansion.
Best Value
- 【Dreamy Rainbow Gaming Keyboard】K521 Gaming Keyboard Adopts a Different LED Backlight Design, Upgraded on the Traditional LED Backlight Effect, Making the Light More Penetrating, Giving You a More Dazzling Visual Effect, Making Your Gaming Process More Enjoyable
- 【One Touch Opens & Visual Feast】The K521 Red Dragon Keyboard has a One-Touch on/off Lighting Button for Added Convenience. It also has a Three-Position Adjustable Breathing Mode and a Four-Position Adjustable Brightness Lighting Mode
- 【Mechanical Feeling & Fast Tapping】The PC Keyboard Keys are Designed for Mechanical Feeling, Giving You a Better Feel During Use and the Ability to Trigger Keys Quickly, Allowing You to Win All Your Games
- 【19 Keys Anti-Ghosting Keyboard】Anti-Ghosting Ensures Every Button Can Be Triggered. This Allows You to Trigger Key Combinations In The Game Accurately, And Each Skill Can Be Accurately Released to Increase Your Winning Rate. Redragon K521 Will Be Your Perfect Partner
- 【12 Multimedia Combination Keys】The K521 Wired Gaming Keyboard is Equipped with 12 Multimedia Keys That Can Greatly Enhance Your Gaming/Office Efficiency and Make It More Convenient to Use
How much money did the Undertale 10th Anniversary Stream raise?
The event raised $361,407.76 for Doctors Without Borders, also known as Médecins Sans Frontières or MSF, according to the official anniversary page.
How does the anniversary Sans battle work?
The player remains in blue SOUL mode, while bones act as physical objects that can push the SOUL. Falling or being pushed off the battle board causes an earlier loss, but the final bulldozer sequence is the intended successful ending.
The Bottom Line
Bottom line: Undertale’s official 10th-anniversary site adds a separate, playable Sans encounter—not a patch or DLC update. It runs in a JavaScript- and WebAssembly-enabled browser and replaces normal boss damage with blue-SOUL movement, bone physics, off-screen hazards, and a bulldozer finale. The battle originated in a charity anniversary stream that raised $361,407.76 for Doctors Without Borders.
Free tools Windows power users keep installed
One-click scans. No signup required.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
