In tabletop gaming, I often find that uniformity feels like a wargame, while inconsistency feels like an RPG. When you've got 2 units of 10 soldiers, and 1 captain leading them, you usually think of the soldiers as generically "the army" but you're likely to give that captain a name, a backstory, a reason to fight, and so on. When you've got 4 captains and no soldiers, you've got a skirmish wargame, and that's exactly how D&D got invented in the first place. That kind of configuration is also common in the Agents of the Imperium faction in Warhammer 40,000. I bought the Code Imperial Agents book last year and have been playing the army in a campaign as well as some one-off games. This is my review.
A codex, in Warhammer 40,000 terminology, defines the way a specific faction plays in the game. Each army's codex contains special rules that apply just to that faction. The formula for a codex publication is revealed in its table of contents. There's a section for lore, a photo spread displaying models and armies, a starter army list ("combat patrol"), game stats for each miniature, and a Crusade section to use in campaigns. This codex keeps to that formula, but in every other way it deviates from what you'd normally expect from a codex. In fact, what you actually get is a subtle reinvention of the first edition of Warhammer 40,000, subtitled Rogue Trader.
People often talk about Warhammer 40,000: Rogue Trader like it was more a roleplaying game than a wargame. I'm not convinced that this is accurate, but admittedly the book encouraged the use of a games master, and it did seem to leave some rules just vague enough to allow a games master to make stuff up as the story demanded. What I think people actually are detecting about Rogue Trader is the lack of consistency in the way you could build an army.
By the rules of Rogue Trader, you could just grab a handful of blister pack miniatures, appoint a rogue trader or an inquisitor or a space marine as leader, and call it an army. I assume this was done because Citadel miniatures didn't yet have full armies of any faction wholly available yet, but also to save people from having to invest in hundreds of miniatures just to play an introductory game. Every army for Rogue Trader could be a motley crew of misfits, just like in Star Wars. There would likely be no in-world narrative cohesion, so it was up to you to come up with a story about how this seemingly random assortment of heroes found each other and decided to team up.
In modern Warhammer 40,000, that's exactly the role of the Agents of the Imperium faction. While most factions in 40k consist of a few model kits of infantry, a few vehicles, and a few special character models, Codex Imperial Agents contains data about:
The premise is that you're an Inquisitor, or an agent of an Inquisitor, or a Rogue Trader, and you either own or have requisitioned a specialist force. You might also requisition a whole military detachment (by buying a different codex and buying the models to make up an army), but you don't have to (more about that later.)
Liberating, is the word for it. If you've ever felt constrained by a codex because it doesn't have an army slot for that random free miniature of the month you got from your local Warhammer store, then the spirit of Codex Imperial Agents (if not the letter of its law) makes you feel like you've just discovered a cheat code. Many of the most significant models in the codex are available as single blister pack models. It's the army you can mostly build one model at a time.
To be clear, I'm really only praising the spirit of the book. The codex itself does have specific requirements for an army. You can't go buy literally any single model and add it to your force.
Frankly, an army of only Imperial Agents isn't likely to be terribly effective, at least not against big armies designed to raze entire cities. You'll have a lot of models with 3 Toughness and probably 4+ BS, with fairly weak abilities. It's a fun army, but not an especially lethal one.
The Agents of Imperium as a faction is clearly intended, as portrayed in the codex, as a "roleplay", and probably skirmish, army. You're supposed to take this army out onto the mean streets of a hive city, or into the houses of the nobility. It's almost like an upper-class Necromunda, and I think for it to really work effectively on the tabletop you'd have to pit 2 Imperial Agent armies against each other, or else add Imperial Agents to a more "serious" army.
That's not necessarily a bad thing, but it's important to know before you field an army of Imperial Agents against a powerful force of Chaos Space Marines or xenos. I've tried it, and it's not fun for anybody. The Imperial Agents player feels overwhelmed by the enemy, and the other player gets bored from always winning every fight. I use build points as reference only, but boy do I reference them when building an army of Imperial Agents.
The sample army provided in the Combat Patrol section is designed for a small game (500 points instead of 1000) of Warhammer 40,000. It does a reasonable job of setting the tone, even though I suspect that this is not the recipe for a winning force. The army consists of a preacher (Pious Vorne from Blackstone Fortress in all but name), an Eversor Assassin, 2 Vigilant Squads of Adeptus Arbites, and a unit of Inquisitorial Agents. All but the Assassin are Toughness 3 and the best Save among them is 4+. Imagine playing against the Chaos Space Marine's Zarkan's Daemonkin combat patrol, with its 5 Possessed at Toughness 6, 3+ Save, and 3+ WS, plus Legionaries with Toughness 4 and 3+ Save and BS. I think it would be a fun game, but both players would know which force had the advantage.
Even so, the template for building a fun army is there. Preacher Teguen is off to fight the evils of the galaxy, and has hired an Assassin, and requisitioned some Adeptus Arbites, and borrowed some staff from the local Inquisitor. That's exactly the kind of campaign I want to play.
The army rule for Agents of the Imperium is that you can include models from this codex in any other army that has the Imperium keyword. The number of units you can use depends on the points level of the game. At 1000 points (or whatever the Incursion level is) you can bring in 1 Retinue unit, 1 character, and 1 Requisitioned unit. I have no idea what a Retinue is mechanically, and can't find any mention of it in any of the rules. I'm guessing it's the Rogue Trader's Entourage or the Inquisitorial Agents model kits?
Anyway, there is no army rule aside from that, so if you're determined to play an army comprised only of Imperial Agents, then you get nousable army rule. Surely it must be an oversight. I guess you could use the army rule from the Combat Patrol, which allows you to choose a unit at the start of your turn and grant it Lethal Hits and Precision, as an example.
The detachments are themed around some of the common functions of imperial agents. There are just 4 detachments.
One-off games are fun, but my true passion for Warhammer 40,000 on the tabletop is for campaigns. The Crusade rules for Codex Imperial Agents features a minigame about Shadow Operations. If you have an Imperial Agent character in your Crusade Force, then you can choose to start a Shadow Operation. There are 3 difficulty levels to choose from, and they each have a Threat Score and a Covert Level. As you play campaign battles, you earn points you can use to improve your roll against the Covert Level. If you meet the Covert Level, then you've uncovered the threat, and you roll on a random table to find out what the problem is.
Now that you know what the problem is, you must earn Influence or Intrigue points by completing one of the Imperial Agent Agendas (also provided in the Crusade section). You can use these points for a roll to solve the problem, but if you fail you must roll on a Plot Twist random table, which has some fun results.
You track your Shadow Operations successes and failures on a pie chart tracker. When you decide your Shadow Operations are over, you can cash in whatever reward (or penalty) your clandestine activities have earned.
How fun is that? It's very fun.
The Crusade subsystem isn't exactly a victory of narrative wargaming, but it is a victory in how mechanics can encode narrative concepts. If you're the type of person who enjoys an the evening of recording and updating the state of your game, then the Crusade rules are for you. It's the best kind of busy work, because it's fun busy work.
Codex Imperial Agents is a fun and inspiring book. What it's supposed to do is embed a sub-faction into your main army. What it actually does, at least from my point of view, is implicitly grant you permission to treat your wargaming miniatures like toys. Make unlikely alliances, combine incompatible forces, tell interesting stories.
Or not.
I concede that there's a very good reason for the way 40k structures its armies. Points are important clues to what's likely to be a fun game and what's likely to be a boring massacre.
Somewhere between those 2 extremes, however, there's a happy medium where your army is crafted with loving care, a sincere fascination with lore and cool models, and an immersive gaming experience.
I don't think Codex Imperial Agents necessarily knows what it is. Maybe it's an army, or maybe it's just an enclave within your existing army, or maybe it's an accidental skirmish army. What it is for me is the perpetuation of Rogue Trader, and it has become one of my very favourite codexes.