Spine runtime errors, explained
The messages the official Spine 4.2 runtimes throw while loading a skeleton or playing it — what each one means, where in the runtime it comes from, and how to fix the file or the code. Web (spine-ts: Phaser, PixiJS, spine-player) and Unity (spine-csharp) word some of them differently, and some fail in one runtime and load silently in the other; each page shows both. To check a whole skeleton.json at once, use the Spine JSON validator.
Spine error: Region not found in atlas
Why Spine throws Region not found in atlas while loading a skeleton: an image name or path that the .atlas file does not list, and how to find which one.
Spine error: Couldn't find bone for slot
Spine fails to load a skeleton with Couldn't find bone X for slot Y (Unity: Slot bone not found). What the slot points to, and how to fix the JSON.
Spine error: Parent bone not found
Unity fails a Spine skeleton with Parent bone not found, while the web runtime loads it and quietly makes the bone a second root. Why, and how to fix the order.
Spine error: Couldn't find slot for skin
Spine stops loading with Couldn't find slot X for skin Y (Unity: Slot not found) when a skin lists images for a slot the skeleton does not have.
Spine: Cannot read properties of undefined (reading 'bones')
A Spine skeleton fails with Cannot read properties of undefined (reading 'bones') or Timeline attachment not found: a deform key points to a missing mesh.
Spine error: Color hexadecimal length must be 8
Unity rejects a Spine skeleton with Color hexadecimal length must be 8, received: … while the web runtime plays it. The slot color format and the fix.
Spine in Unity: The given key 'hash' was not present in the dictionary
Unity fails a Spine skeleton.json with KeyNotFoundException: the given key 'hash' was not present. Unity needs skeleton.hash; the web runtime does not.
Spine error: Animation not found
Animation not found comes from game code, not the file: setAnimation, addAnimation or setMix got a name the skeleton lacks. How to list the real names.
Spine error: Skin not found
Skin not found in Spine: a skin name passed to setSkin, or a file whose animation or linked mesh names a skin that is not in the skeleton. Where to look.
Spine error: Attachment not found, for slot
Attachment not found: X, for slot: Y comes from setAttachment in your code: the active skin and the default skin have no image by that name in that slot.