I'm unsure about this interpretation. Glyph of warding states that you cast the stored spell as part of the creation of the glyph, and Contingency states that you cast the contingent as part of casting the Contingency spell. So if you cast Contingency for storing in a glyph, you should also cast the contingent spell as part of Contingency's casting. Not casting the contingent spell would make Contingency's casting fail, as you do not fit the casting conditions.

Or would that mean that you have to cast a spell alongside Contingency when storing it in a glyph of warding, but that additional spell is effectively lost? Actually, more importantly : the glyph is casting the spell, not the character triggering the glyph, so that target couldn't cast a spell alongside Contingency, since they are not casting it. With that in mind, the glyph would cast the spell, and it would fail instantly, because the glyph cannot cast another spell alongside. This interpretation makes it impossible to use Contingency effectively in a glyph of warding.

@Matthieu I clarified why casting the contingent spell etc. is an effect.

@Matthieu regarding your second point, glyph of warding doesn't state the the glyph casts the spell, just that 'the stored spell is cast'.

It indeed does simply state "the stored spell is cast", but then who or what is casting the spell? Since this unknown caster is the only one which can cast another spell alongside the stored spell, they're the only one which can make the spell not instantly fail. The contingent spell is not a spell effect, it is effectively part of the casting process.

To elaborate on that last point I mention, the rule you have quoted is the general rule for how spells are written. Contingency explicitely states a specific exception to how spells are cast by giving an additional action to be done "as part of the casting". Because of this explicit exception, the casting of the contingent spell is not an effect, but a condition of the casting.

@Matthieu effects happening as part of the casting is the normal behavior of spells. The rules don't go into this much detail, so I got nothing to quote, but spells go out of their way to explain when part of their effects happen at a different time or as part of a different action, those are exceptions. So 'You cast that spell--called the contingent spell--as part of casting contingency' is an exception for the contingent spell: its casting happens doesn't involve additional actions. But it's not an exption for contingency: its effect happens as part of its casting, as normal.

Glyph of warding introduces the exception that the effect happens later, separate from the first casting.

The part about casting the contingent spell "as part of the casting" sounds like an absolutely explicit part of the casting process. Even if it is in the "effects" part of the spell's description, it is a condition necessary for the spell to cast properly. Other spells have casting conditions in their description, which are often part of the spell's verbal components, for instance.

the first one that comes to mind, for instance, is "disintegrate"

"A thin green ray springs from your pointing finger to a target that you can see within range." If you do not point your finger to a target that you can see within range, the spell cannot cast properly.

But "pointing the finger" isn't a spell effect.

This is the same situation. "Casting the contingent spell" is effectively a component of the spell.

Even if the contingent spell, or the stored spell, does not take effect, you need to effectively cast it properly.

I think that you are drawing a like where there isn't one. Where is this casting process defined? Where does it start and end? What I know is that the component are found in one part of the spell's block and the effects are in another and that's what I'm basing my answer on.

Effect itself is also not well defined. I think that pointing the finger is part of the effect of the spell.

But it's not part of the failed saving throw's effect

The dex save as disintegrate is an effect of the spell. The damage is an effect of the save. Theyare all an effect of the spell alongside pointing the finger.

But taking a step back these problems only matter because of stuff like glyph of warding. Can you point your finger at the target? No because there is no target yet as you put disintegrate into the glyph. Does anything point its finger at the glyph's triggerer? Obviously not, so that part of the spell seemingly vanishes. Even for the simplests cases glyph of warding requires some fudging one way or the other

@Ruse I don't think I want to get into the argument of why a somatic component of a spell isn't an "effect" of the spell, so let's leave it at that disagreement, like you propose. I'd be curious to see how that reading takes into account the components for the conserved spell though.

If I follow your interpretation, when casting Glyph or Contingency and a spell, I should be able to store any spell in them even if I cannot perform the somatic component for those stored spells : they're effects of the main spell, and not a necessary component.

With this interpretation, just like how "pointing your finger" would be an effect of the spell, meaning the spell makes you point your finger at the target, here the casting of the secondary spell is an effect of the spell, meaning the spell makes you cast the stored spell, even if you are not currently capable of doing so.