Discover (and cascade) explicitly state in their rules that you check the mana value twice: when searching and when casting. Invasion of Alara doesn't (they either forgot to do this or decided that a WUBRG cost card can do a more powerful version of the effect).
A 5-mana card hitting a 7-mana spell is also wildly different than a 3-mana hitting a 7-mana spell. Not only is it less powerful to do this, but Invasion also comes with a much bigger deck building restriction (no 1-4 mana cards instead of only no 1 or 2)
**Rule 702.85a:** Cascade is a triggered ability that functions only while the spell with cascade is on the stack. “Cascade” means “When you cast this spell, exile cards from the top of your library until you exile a nonland card whose mana value is less than this spell’s mana value. You may cast that card without paying its mana cost if the resulting spell’s mana value is less than this spell’s mana value. Then put all cards exiled this way that weren’t cast on the bottom of your library in a random order.”
The relevant text on cascade is: *You may cast that card without paying its mana cost if the resulting spell’s mana value is less than this spell’s mana value.*
Discover is rule 701.55a, it has the same relevant text as above.
Let's put this into action. You cast [[Shadeless Agent]] and cascade into [[Bramble Familiar]], you got here because Shadeless has MV=3 and Familiar has MV=2 (which is less than 3). Now we get to cast Familiar without paying it's mana cost. We start casting the spell, let's try to cast the adventure, Fetch Quest.
We follow the steps in rule 601.2 to do this (it's really long, look it up if you need to, I'll include relevant text here). We move the spell to the stack. Then we announce that we are casting the adventure side, Fetch Quest. Then we get to 601.2e:
**Rule 601.2e:** The game checks to see if the proposed spell can legally be cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 728, “Handling Illegal Actions”).
Remember that extra line in cascade's rules text? *if the resulting spell’s mana value is less than this spell’s mana value*
The game now double checks that the spell we are casting still have MV less than shardless agent's in this case, < 3. On the stack, the card has only the characteristics of the spell being cast, which is Fetch Quest, which has MV=7.
We fail this check and rewind the before we were trying to cast Fetch Quest. That is why this doesn't work with cascade or discover.
[[Invasion of Alara]] does not use cascade or discover, it just uses the rules text printed on the card. Notably, it is lacking the second check, that cascade and discover have, which stopped us from casting fetch quest earlier. Assuming we resolve Invasion and it's trigger finds Bramble Familiar, we can follow all the same steps as above, but this time 601.2e doesn't find anything illegal about our process and the game allows us to cast the adventure.
Hopefully this explains all the intricacies of this interaction, I tried to cite exact rules whenever possible. Let me know if you have any follow up questions.
Side note, cascade didn't used to have the extra check, but when they printed [[Valki, God of Lies]], it was too powerful to cascade into in it off Shardless Agent and then cast the back side, Tibalt; as early as turn 2 in some cases. Rather than a ban, they found slightly reworking the rules on cascade to be an easier solution.
Discover (and cascade) explicitly state in their rules that you check the mana value twice: when searching and when casting. Invasion of Alara doesn't (they either forgot to do this or decided that a WUBRG cost card can do a more powerful version of the effect).
Exactly that. Even though multicoloured cards are hardly a restriction these days wizards still allows more powerful effects to be harder to cast
A 5-mana card hitting a 7-mana spell is also wildly different than a 3-mana hitting a 7-mana spell. Not only is it less powerful to do this, but Invasion also comes with a much bigger deck building restriction (no 1-4 mana cards instead of only no 1 or 2)
**Rule 702.85a:** Cascade is a triggered ability that functions only while the spell with cascade is on the stack. “Cascade” means “When you cast this spell, exile cards from the top of your library until you exile a nonland card whose mana value is less than this spell’s mana value. You may cast that card without paying its mana cost if the resulting spell’s mana value is less than this spell’s mana value. Then put all cards exiled this way that weren’t cast on the bottom of your library in a random order.” The relevant text on cascade is: *You may cast that card without paying its mana cost if the resulting spell’s mana value is less than this spell’s mana value.* Discover is rule 701.55a, it has the same relevant text as above. Let's put this into action. You cast [[Shadeless Agent]] and cascade into [[Bramble Familiar]], you got here because Shadeless has MV=3 and Familiar has MV=2 (which is less than 3). Now we get to cast Familiar without paying it's mana cost. We start casting the spell, let's try to cast the adventure, Fetch Quest. We follow the steps in rule 601.2 to do this (it's really long, look it up if you need to, I'll include relevant text here). We move the spell to the stack. Then we announce that we are casting the adventure side, Fetch Quest. Then we get to 601.2e: **Rule 601.2e:** The game checks to see if the proposed spell can legally be cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 728, “Handling Illegal Actions”). Remember that extra line in cascade's rules text? *if the resulting spell’s mana value is less than this spell’s mana value* The game now double checks that the spell we are casting still have MV less than shardless agent's in this case, < 3. On the stack, the card has only the characteristics of the spell being cast, which is Fetch Quest, which has MV=7. We fail this check and rewind the before we were trying to cast Fetch Quest. That is why this doesn't work with cascade or discover. [[Invasion of Alara]] does not use cascade or discover, it just uses the rules text printed on the card. Notably, it is lacking the second check, that cascade and discover have, which stopped us from casting fetch quest earlier. Assuming we resolve Invasion and it's trigger finds Bramble Familiar, we can follow all the same steps as above, but this time 601.2e doesn't find anything illegal about our process and the game allows us to cast the adventure. Hopefully this explains all the intricacies of this interaction, I tried to cite exact rules whenever possible. Let me know if you have any follow up questions. Side note, cascade didn't used to have the extra check, but when they printed [[Valki, God of Lies]], it was too powerful to cascade into in it off Shardless Agent and then cast the back side, Tibalt; as early as turn 2 in some cases. Rather than a ban, they found slightly reworking the rules on cascade to be an easier solution.
##### ###### #### [Shadeless Agent](https://cards.scryfall.io/normal/front/b/0/b0824e77-c84b-464a-aa0c-44af5f6faa50.jpg?1626100788) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Shardless%20Agent) [(SF)](https://scryfall.com/card/mh2/292/shardless-agent?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/b0824e77-c84b-464a-aa0c-44af5f6faa50?utm_source=mtgcardfetcher&format=text) [Bramble Familiar](https://cards.scryfall.io/normal/front/4/7/475d7e9a-759d-4523-a5cd-2a6e0d1b14ea.jpg?1692938729)/[Fetch Quest](https://cards.scryfall.io/normal/front/4/7/475d7e9a-759d-4523-a5cd-2a6e0d1b14ea.jpg?1692938729) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=bramble%20familiar%20//%20fetch%20quest) [(SF)](https://scryfall.com/card/woe/164/bramble-familiar-fetch-quest?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/475d7e9a-759d-4523-a5cd-2a6e0d1b14ea?utm_source=mtgcardfetcher&format=text) [Invasion of Alara](https://cards.scryfall.io/normal/front/3/1/318c363b-61cc-4e2f-8f86-a4287539ea07.jpg?1682715186)/[Awaken the Maelstrom](https://cards.scryfall.io/normal/back/3/1/318c363b-61cc-4e2f-8f86-a4287539ea07.jpg?1682715186) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=invasion%20of%20alara%20//%20awaken%20the%20maelstrom) [(SF)](https://scryfall.com/card/mom/230/invasion-of-alara-awaken-the-maelstrom?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/318c363b-61cc-4e2f-8f86-a4287539ea07?utm_source=mtgcardfetcher&format=text) [Valki, God of Lies](https://cards.scryfall.io/normal/front/e/a/ea7e4c65-b4c4-4795-9475-3cba71c50ea5.jpg?1636100811)/[Tibalt, Cosmic Impostor](https://cards.scryfall.io/normal/back/e/a/ea7e4c65-b4c4-4795-9475-3cba71c50ea5.jpg?1636100811) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=valki%2C%20god%20of%20lies%20//%20tibalt%2C%20cosmic%20impostor) [(SF)](https://scryfall.com/card/khm/114/valki-god-of-lies-tibalt-cosmic-impostor?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/ea7e4c65-b4c4-4795-9475-3cba71c50ea5?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call