I tried to follow the premise example in the thread stickied in this subforum, but there's a lot of commentary in each premise. I warn you that this is a long and dense read and I tried to organize and word it the best I could.
Question: In
40K 9th Edition, can a Space Wolves army include a Vindicare Assassin in a patrol detachment and still benefit from the Savage Fury ability?
Premise 1: Codex Supplement: Space Wolves,
pg. 45. Detachment Abilities
An SPACE WOLVES Detachment is one that only includes models with the SPACE WOLVES keyword (excluding models with the AGENT OF THE IMPERIUM or UNALIGNED keyword).
The Vindicare Assassin is an
AGENT OF THE IMPERIUM, so it can be included in a
SPACE WOLVES patrol detachment, provided it follows the other rules for its inclusion.
Premise 2: Codex Supplement: Space Wolves,
pg. 45. Detachment Abilities
If every unit in your army (excluding UNALIGNED units) has the SPACE WOLVES keyword, then every unit that has the Combat Doctrines ability and is in a SPACE WOLVES Detachment gains the Savage Fury ability, below.
Savage Fury (for any who are familiar with Space Marines, but not necessarily Space Wolves) is the Space Wolves' "Super Doctrine" ability. The Vindicare Assassin is neither a
UNALIGNED or
SPACE WOLVES unit. So by being part of the army it prevents units with the Combat Doctrines ability in
SPACE WOLVES detachments from gaining the Savage Fury ability, according to the two rules listed so far.
Premise 3: War of the Spider,
pg. 50. Agent of the Imperium
If your army is Battle-forged, you can include 1 AGENT OF THE IMPERIUM unit in each IMPERIUM (excluding FALLEN) Patrol, Battalion and Brigade Detachment in your army without those units taking up slots in those Detachments.
The Vindicare Assassin is still considered part of the army, it just doesn't take up a slot in a detachment. So it is still preventing units with the Combat Doctrines ability in
SPACE WOLVES detachments from gaining the Savage Fury ability, according the three rules list so far.
Premise 4: War of the Spider,
pg. 50. Agent of the Imperium
... and it does not prevent other units from your army benefiting from abilities that require every model in your army to have that ability (e.g. Combat Doctrines).
This is the second part of next sentence, but I'm going to address it before for the first part of the sentence. This part of the sentence allowed the Space Marines in an 8th edition army to still benefit from Combat Doctrines, because in 8th edition the Combat Doctrines ability required every unit in the army to also have the Combat Doctrines ability in order to gain the bonus from it, Codex: Space Marines (2019),
pg. 109. In 9th edition, Codex: Space Marines (2020),
pg. 125, shifts the criteria to gaining the bonus from combat doctrines to every unit having the
ADEPTUS ASTARTES keyword, allowing exemptions for an
AGENT OF THE IMPERIUM. So in 9th edition this part of the sentence does just seem to be obsolete, at least for Combat Doctrines, and Savage Echoes doesn't require that every unit also has Savage Echoes to gain the bonus, so I would say it's irrelevant.
Codex: Space Marines (2019),
pg. 109. Combat Doctrines
If you have a Battle-forged army, units only benefit from this bonus if every unit from your army has the ability (excluding SERVITOR and UNALIGNED units).
Codex Space Marines (2020),
pg. 125. Combat Doctrines
If every unit from your army has the ADEPTUS ASTARTES keyword (excluding AGENT OF THE IMPERIUM and UNALIGNED units), this unit gains a bonus (see below)...
Premise 5: War of the Spider,
pg. 50. Agent of the Imperium
The inclusion of an AGENT OF THE IMPERIUM unit does not prevent other units from their Detachment from benefiting from Detachment abilities (e.g. Chapter Tactics, Defenders of Humanity etc.)...
The first part of this sentence was necessary in 8th edition because Codex: Space Marines (2019),
pg 174, did not give exemptions for Space Marine detachments having
AGENT OF THE IMPERIUMunits. It allowed them to benefit from Chaper Tactics and Defenders of Humanity. In the 9th edition Codex: Space Marines (2020),
pg. 93, the exemption for an
AGENT OF THE IMPERIUM being included in an
ADEPTUS ASTARTES detachment is already part of the rule. So this first part of this rule gives a concrete example that "benefiting from" includes both gaining and using an ability. If "benefiting from", didn't include gaining that ability, then the units in the Space Marine detachment in 8th wouldn't have been able to gain the Chapter Tactics or Defenders of Humanity abilities.
Codex: Space Marines (2019),
pg. 174. Space Marine Units and Detachments
A Space Marines Detachment is a Detachment that only includes Space Marines units.
Codex Space Marines (2020),
pg. 93. Detachment Abilities
An ADEPTUS ASTARTES Detachment is one that only includes models with the ADEPTUS ASTARTES keyword (excluding models with the AGENT OF THE IMPERIUM or UNALIGNED keyword).
Premise 6: Codex: Space Marines (2019),
pg. 174. Chapter Tactics
If your army is Battle-forged, units (other than SERVITORS) in a Space Marines Detachment (other than a Super-heavy Auxiliary Detachment) gain the Chapter Tactics ability, so long as every unit in that Detachment is drawn from the same Chapter.
Again when War of the Spider came out, Codex: Space Marines (2019) was the current book and its rules were given as examples in the Agent of the Imperium rule, so it is the most relevant to show how the rule works. The Vindicare Assassin is part of the detachment, but was not intended to receive Chapter Tactics or any other similar detachment abilities from other Imperium codices. The rule quoted on Premise 5 uses the phrase "other units from their Detachment". Knowing the intention based on an explicit example "(e.g. Chapter Tactics", "other units from their detachment" excludes the Agent of the Imperium when determing which units in the detachment benefit from the rule. The chapter tactics ability also has the requirement that every unit in the detachment is from the same chapter, of which an Agent of the Imperium would not be. So this also shows that the Agent of the Imperium is ignored as being part of the detachment for the purpose of determining if additional conditions presented in the detachment ability are met. What is still lacking is if it can be ignored as being part of the army, not just detachment, for determining if additional conditions present in the detachment ability are met. But setting aside that question for the moment, the premises given so far show codex supplements that came out in 8th edition have parity with those that came out in 9th, just 8th edition ones gain their exemptions through the Agent of the Imperium rule and 9th edition ones include keyword exclusions for
AGENT OF THE IMPERIUM right in their rules.
Premise 7: Codex Supplement: Dark Angels,
pg. 42. Detachment Abilities
If every unit in your army (except AGENT OF THE IMPERIUM or UNALIGNED units) has the DARK ANGELS keyword, then every unit in a DARK ANGELS Detachment that has the Combat Doctrines ability gains the Sons of the Lion ability.
Codex supplements from 9th edition, all benefited from being able to have their rules written with Agents of the Imperium in mind. While the rules for actually including the
AGENT OF THE IMPERIUM in the detachment are not part of the codex supplements, each rule and ability takes them into account. Codex Supplement: Dark Angels is unique amongst Space Marine codex supplements published so far, including the new Codex Supplement: Black Templars, in that it gives a keyword exclusion for
AGENT OF THE IMPERIUM in its rule that can give its "Super Doctrine" ability, Sons of the Lion, to every unit in a
DARK ANGELS Detachment that has the Combat Doctrines ability. It may be worth noting that a
DARK ANGELS Detachment is defined in exactly the same way as a
SPACE WOLVES Detachment was in Premise 1, except the
DARK ANGELS keyword is substituted for every instance of the
SPACE WOLVES keyword. So the previous premises have not set any precedent that an
AGENT OF THE IMPERIUM can be ignored when determing if every unit in an army has a shared keyword, only when determining if a detachment has a shared keyword. The explicit inclusion of the
AGENT OF THE IMPERIUM keyword in the Dark Angels' rule in Premise 7 and the lack of the keyword in the Space Wolves' rule in Premise 2 indicate that Agent of the Imperium rules from War of the Spider do not grant the exemption of units with the
AGENT OF THE IMPERIUM keyword from being counted against the gaining of the Savage Fury, or any other "Super Doctrine" abilities; and only by giving the Dark Angels an explicit exemption, may they gain their Sons of the Lion ability, with an Agent of the Imperium included as part of their army.
Conclusion:In
40K 9th Edition, a Space Wolves army
cannot include a Vindicare Assassin in a patrol detachment and still benefit from the Savage Fury ability.