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Great writeup — you've clearly diagnosed this correctly. The root cause is exactly what you described: SameSite=Lax blocks the session cookie on cross-site POST, so CSRF verification fails before the request even gets a chance to process the JWT.

This isn't an edge case either — it's a fundamental incompatibility between how Rails handles CSRF by default and what LTI 1.3's OIDC flow actually requires. The spec mandates response_mode=form_post across origins, so there's no workaround on the LMS side.

The two fixes you listed are both valid, but they have different tradeoffs:

Option 1 — skip_before_action :verify_authenticity_token on just the LTI redirect endpoint is the more surgical fix. You're not disabling CSRF globally, just on the one action where it structurally can't work. The JWT signature itself serves as the integrity check here anyway, so you're not really losing security — you're just removing a check that was never going to pass.

Option 2 — SameSite=None; Secure on the session cookie works too but is broader — it affects the whole session, not just the LTI flow, which might have other implications depending on how Classroom manages sessions elsewhere.

Most LTI tool implementations go with Option 1 scoped to the redirect action specifically.

Also worth noting — issue #72219 that you referenced has been open since October 2023 with no official response or fix, and reports are still coming in as recently as early 2026 across multiple Moodle versions. So you're not missing anything in your config — this is a confirmed, long-standing bug on GitHub Classroom's end.

Your breakdown of the SameSite=Lax + CSRF interaction is honestly the most technically detailed explanation that thread has seen. Most reports there are just "I get a 500" with no root cause. It's worth dropping your full analysis as a comment there — having a clear, reproducible diagnosis from an Open edX integration on top of all the Moodle reports makes it a lot harder for the Classroom team to keep deprioritizing this.