rigc — a rig compiler that lets an AI agent author Spine skeleton data

firejune

I bought a Spine Professional licence this week, and would rather hear from the people
whose format this is built on before saying more.

What it is. rigc is a rig compiler: rig and motion specs in, Spine 4.3 skeleton data
out, round-tripped through @esotericsoftware/spine-core and 40 named assertions before
a byte reaches disk. The narrow problem: an agent cannot tell whether its rig succeeded.
A size: disagreeing with the PNG collapses every UV, and the file loads clean and
plays.

assembly
Fourteen part PNGs, one rig spec, one motion spec — compiled, nothing posed by hand.

Two key poses read out of pictures, two in-betweenings, a ballot picking one.

scene
A 2.5D head turn, each feature at its own depth.

What was measured. The yardstick is your own examples: 1-weight-and-mass …
8-follow-through as a difficulty ladder, spineboy as the graduation exam. The authoring
agent gets images/, a brief and rendered reference frames, never the reference JSON —
our own renders of your exports, committed under each example's own license.txt. Eleven
candidates over 56 frame sets hold under the current gate; worst attributable slot
drift peaks at 5.5550 px against a 6.0 px bar.

What the graduation demonstrates is that the tool, the guide and the protocol reach the
bar across a bounded series of honest attempts, each residual diagnosed and fixed —
spineboy took five, the last inheriting the previous specs. It is not that an agent
authors a spineboy-scale rig from the brief alone in one run: two from-zero attempts on
2026-09-03 both failed, at 9.3289 and 18.9753 px against that bar.

Licensing. rigc links spine-core and validation cannot be switched off, so the
Spine Runtimes License applies to it like any runtime integration. Using rigc, or
shipping its output, requires a Spine editor licence and rigc changes that in neither
direction: an AI drafts, a human refines in Spine. Two facts anyone can check. The
project's contribution rules forbid a --no-validate bypass on licensing grounds: rigc
"must remain structurally incapable of being used as a substitute for" the editor
(CONTRIBUTING.md, written out in CLAUDE.md). And every page rigc preview generates
embeds the Spine Runtimes copyright, licence URL and editor-licence requirement
(src/preview.ts).

I am asking for your opinion and nothing else: whether this is reasonable beside the
editor, whether the licensing reads correctly, and whether the ladder measures anything
meaningful.

firejune/rigc · https://www.npmjs.com/package/spine-rigc ·
firejune/rigcblob/main/NOTICE.md

Notes for the poster (not part of the post)

Why Showcase. Section description is "Show off what you create in Spine" (139
topics), and the pinned sticky by Mario, https://esotericsoftware.com/forum/d/6620, frames
it as the place to promote your own work: "While our showcase forum is most excellent to
show off what's possible with Spine to the Spine community…". There is no forum-wide
rules page and no self-promotion policy — /forum/rules, /guidelines, /faq all 404
— so the sticky is the closest thing to a charter, and it blesses this.

Nearest-neighbour precedents, all in Showcase and all tools that emit or audit Spine JSON:

fxpine, particle effects → Spine JSON — https://esotericsoftware.com/forum/d/30462
Blender-Spine-IO, a Spine 4.3 JSON exporter — https://esotericsoftware.com/forum/d/29425 (Mario replied)
SpineOptima, a web export auditor — https://esotericsoftware.com/forum/d/29480 (Nate ×2, Davide)
Aseprite-Spine Script — https://esotericsoftware.com/forum/d/24339 (Harald, Nate, Luke)
"Seeking feedback and opinions on new tool" — https://esotericsoftware.com/forum/d/30533

Runtimes is the wrong section — its description scopes it to "using the Spine
Runtimes", and the tool posts that landed there are runtime-integration helpers, not
standalone tools. Announcements is first-party only in practice (33 topics).

⚠️ State the licensing precisely, because staff do scrutinise it. On the
Blender-Spine-IO thread Mario raised a licence concern unprompted, and Nate has corrected
redistribution readings directly elsewhere on the forum. The licensing paragraph above is
written to be checkable rather than reassuring: every clause in it resolves to a public
file — NOTICE.md for the chain, CONTRIBUTING.md + CLAUDE.md for the bypass rule,
src/preview.ts for the embedded notice.

Where the two checkable facts live, if a reply asks for line references:

The bypass rule is written out in CLAUDE.md (the "Validation through spine-core is
not optional" bullet, reason 2, Licensing) and stated in short form in
CONTRIBUTING.md ("No bypass of the round trip", which links CLAUDE.md). Both are
committed to the public repo; cite CONTRIBUTING.md first, since that is the file a
contributor is pointed at.
The generated-page notice is the HTML comment in src/preview.ts — Spine Runtimes
copyright, the licence URL, and "requires each user of a product integrating them to
hold a Spine Editor license". Every rigc preview output carries it.

Two lines cut for the 400-word budget, restorable if the section allows more.

The earlier-thread parenthetical: "(I posted a DOM-first runtime experiment here a
little while back — https://esotericsoftware.com/forum/d/30563 — this is the authoring
side of the same curiosity.)" The post does not depend on it and it presumes no
familiarity, but it does establish prior good-faith presence, so it is the first thing
to put back.
The JSON-format-page quote that used to close the licensing paragraph: "Your
JSON-format page is the ground I stand on: 'Also, Spine can import data in this
format, allowing interoperability with other tools.'" Verbatim from
https://esotericsoftware.com/spine-json-format, opening paragraph — the invitation the
whole project rests on. Cut only because the two checkable facts that replaced it are
stronger than a quotation, being things a reader can verify about this repo.

Provenance, if it is raised in replies. bench/reference/ holds 1,293 PNG frames the
project renders from the examples' own exports; a frame carries those images' pixels, so
committing one is redistribution, and each example's license.txt grants exactly that
"as long as they are accompanied by this license file" — all eight copies are present,
written there by the render script rather than left to memory, and the non-commercial
condition rides along. 7-anticipation publishes no license.txt upstream, so its
frames are never committed and git check-ignore enforces it. The example projects
themselves are fetched, never committed. Sources: README.md §Licensing (the 📐
paragraph), NOTICE.md §Rendered reference frames, bench/reference/README.md.

Misaki

firejune For now, I think the only point we can answer definitively is whether your interpretation of the Spine license is correct, so I will address that part.

If the compiler were completed and then distributed or sold publicly, the people using that compiler would also be indirectly using the spine-core runtime. Therefore, yes, those users would need to hold a valid Spine license.

I do not think there is much else we can say officially about the other points. However, if you search through some of the previous discussions on this forum, you should be able to get a sense of our views on using AI to generate animation. You are not the first person to experiment with this, and we have also received many requests to add this kind of functionality ourselves.

If we believed this approach had strong potential to become a genuinely useful and practical feature, we would likely already be working on something along those lines ourselves.

Nate

Do not dump huge LLM generated posts here. You didn't even bother to remove the LLM's notes for YOU that were not intended for you to post. That means I have now read your post more thoroughly than you have. We are likely to delete future posts like this.

If you expect humans to read and respond your post, write it using your human voice or don't post at all.

firejune

Nate Fair, and sorry. I pasted the whole draft file without checking — those notes were never meant to be posted, and the edit window is already closed so I can't even clean it up. My mistake.

The tool is real, I built it and I use it myself. I just wanted to share what I made. Judge the work, not my lazy post.

Nate

If you want good feedback, you'll need to present things in a sane way. No human should spend their finite time on this planet reading verbose generated junk. If you can't do better than the generated junk, work on that, don't just accept reliance on AI.

Automating rigging (and animating) will need feedback so the LLM can judge what they are doing. Without that I doubt the output can be very useful. You could try feeding it screenshots of its rigging, but you likely need to exercise joints and give multiple screenshots, eating a lot of tokens.

Animators are very meticulous and likely not a good audience for automation. AI is unlikely to come close to what the animator would do for many reasons. Research animation theory, the 12 principles and our Animating with Spine videos, and you can see that how you animate depends on many things: physical properties, what the character is thinking, why they are taking actions, what the animator wants to convey to the user, how much time they have to do it, etc. You can't make great animation without these considerations.

A tool focused on rigging rather than animating has an easier job and may be able to save time somewhere, but the rigging is defined by what movements you need in the animations. It's typically iterative, where you rig, animate, see the rig needs adjustment, repeat. If your tool can't animate, it probably can't rig well.

Currently the most clearly useful place for AI is probably back painting. Everything else is much less clear. In the future, once AI can automate compute usage exactly like a human, it can just use Spine directly and do everything that way, or probably more likely, bypass Spine entirely. There won't be a market for AI or non-AI tools. I suggest getting a head start on being a plumber. It's not so bad, you just can't eat sandwiches anymore.