The Koumei and the Fives Fates update for Warframe will be introducing all manner of changes to companions, so this is a breakdown of everything you can currently expect on October 2, when the update drops.

Warframe companion rework details

First, I want to be clear that, according to the developers, things might change between now and the release. It should also be noted that the aim of this rework is to improve the quality of life and balance problems that have been an issue for some time. 

Beast companions (Kavats, Kubrow, etc.) are gaining a moddable weapon slot

Yes, you will no longer need to mod your pet's weapon from the main mod screen for the pet, freeing up some room in your main build. 

• When you start the game, you will automatically receive a Beast-specific weapon for each of your Beast companions. This does not take inventory space or cost you any resources.
• Beast weapons will gain affinity and rank up, but they will not contribute toward your overall Mastery.
• Posture Mod slot for Beast melee weapon alters overall behavior, gives extra capacity for modding.

This also means that some changes are coming to the stat lines for pet weapons.

Kubrows:

• Previously ALL types of Kubrows: Damage 304, Status 5%, Critical 10%/3x
• Chesa Kubrow: Damage 275, Status 15%, Critical 15%/1.5x
• Huras Kubrow: Damage 350, Status 5%, Critical 20%/3.5x
• Raksa Kubrow: Damage 250, Status 20%, Critical 7.5%/2.5x
• Sahasa Kubrow: Damage 300, Status 5%, Critical 10%/3x
• Sunika Kubrow: Damage 550, Status 7.5%, Critical 7.5%/3.5x

Predasites:

• Previously ALL types of Predasites: Damage 304, Status 5%, Critical 10%/3x
• Medjay Predasite: Damage 350, Status 20%, Critical 10%/3x
• Pharaoh Predasite: Damage 325, Status 25%, Critical 7.5%/2.5x
• Vizier Predasite: Damage 300, Status 30%, Critical 5%/2x

Kavats:

• Previously ALL types of Kavats: Damage 80, Status 7.5%, Critical 20%/2x
• Adarza Kavat: Damage 90, Status 5%, Critical 30%/2.5x
• Smeeta Kavat: Damage 80, Status 7.5%, Critical 20%/2x
• Vasca Kavat: Damage 110, Status 25%, Critical 15%/2x

Vulpaphyla:

• Previously ALL types of Vulpaphyla: Damage 80, Status 7.5%, Critical 20%/2x
• Crescent Vulphaphyla: Damage 100, Status 17.5%, Critical 22.5%/2x
• Panzer Vulpaphyla: Damage 90, Status 12.5%, Critical 25%/2x
• Sly Vulpaphyla: Damage 80, Status 20%, Critical 20%/2x

Other:

• Helminth Charger: Previously 350 Damage, now 200 Slash damage plus an additional non-combining 50 Toxin damage
• Hounds: Hound companions already had a moddable weapon, but we rebalanced these from a base of 152 Damage to a base of 300 Damage.

There are currently no plans for Rivens for pet weapons. 

Changes to Companion AI behavior

Companion AI is also being changed to solve a couple of problems, such as them falling behind or just being hard to spot in the fog of war.

• Pets would previously follow behind you while moving, which made it harder to notice them around and put them further away from enemies when combat started. Pets will now try to lead you instead, staying a few meters ahead while you are moving so they are ready to attack when combat starts.
• The base speed of companions with legs has been significantly increased
• Pets would previously choose targets in a complete circle around you. This means that when using their abilities or making attacks, they weren't even on screen, so it was hard to appreciate their usefulness. This has been tuned so that pets will prefer to choose targets that are near you and in your field of view.
• When selecting a new target to attack, pets now gain a speed boost on their way to the target while off-screen (to reduce the time it takes them to engage).
  Improved pet animations while following the player through bullet jumps and double jumps so that they animate more quickly and fluidly.

Changes are being made to Companion Precepts

The developers (correctly) feel that some Precepts are too low-impact, with an obvious high-impact outlier that just takes over the meta. That means you, little Smeeta Charm. 

Charm (SMEETA Kavat): The resource and credit pickup multiplier has been removed from Charm and promoted to being its own separate precept Mod (Loyal Retriever,) which can be equipped on all Beasts. Charm's chance to provide a buff on activation has been increased to 40%. The available buffs are:

• 25% chance of Reinforced Shield
• 25% chance of Instant Reload (re-rolled if your current weapon doesn't need to reload)
• 10% chance of Bonus Critical Chance buff
• 10% chance of Free Ability Casts
• 10% chance of Bonus Affinity. (Affinity bonus has been increased to 3x)

So, essentially, the looting meta in the game has been freed up a bit, as you can now use the following mod to get Smeeta-like benefits on any Beast companion: 

• Loyal Retriever (Beast companions): Provides a passive 13% chance that each resource or credit pickup will be doubled. This effect is always active: there is no cooldown, no duration, no waiting for it to be ready. If you already own a Smeeta, then you will automatically receive a Loyal Retriever for free. In the future, this will be purchased with Conservation tags from the Earth Conservation vendor. 

And then there are all the more minor changes to all the other Precepts as well.

New Companions Mods and changes to existing mods

New Beast weapons means new mods for them, and changes to already existing mods.

Beast Melee Postures

• Standard Training: Every new Kubrow owner will gain this Mod by default on completing the quest "Howl of the Kubrow". This Posture has no special behaviors or benefits. If you've already completed this quest, it will be added to your account automatically.
• Protector Training: Your pet will prioritize attacking enemies that are within 15 meters of your Warframe. Also applies four stacks of Puncture on hit. Purchased with Conservation tags from the Venus Conservation vendor.
• Pacifist Training: Your pet will not attack enemies unless they strike it with a melee weapon first. It is useful if you value your pet's supporting features rather than its claws and teeth. It also gives a 50% Evasion bonus to your pet to keep it safe. Purchased with Conservation tags from the Earth Conservation vendor.
• Persistent Engagement: Your pet will prefer to keep attacking one target until it's down. It also increases the pet's damage. Purchased with Conservation tags from the Venus Conservation vendor.
• Bravado Assault: Your pet will prefer to attack Eximus and VIP enemies. Increases damage to Overguard done by your pet by 300%. Purchased with Conservation tags from the Venus Conservation vendor.
• Lunging Claws: Your pet will prefer to hit an enemy once and then switch targets to affect as many different enemies as possible. Increases Status Duration by 80%. Purchased with Conservation tags from the Deimos Conservation vendor.

New Damage Conversion Mods

• Icy Inclination: 300% Cold Damage, +300% Status Chance, elemental damage converted to Cold. Purchased with Conservation tags from the Venus Conservation vendor.
• Incendiary Inclination: 300% Heat Damage, +300% Status Chance, elemental damage converted to Heat. Purchased with Conservation tags from the Venus Conservation vendor.
• Infectious Inclination: 300% Toxin Damage, +300% Status Chance, elemental damage converted to Toxin. Purchased with Conservation tags from the Deimos Conservation vendor.
• Inductive Inclination: 300% Electric Damage, +300% Status Chance, elemental damage converted to Electric. Purchased with Conservation tags from the Venus Conservation vendor.
• Precision Conditioning:+140% Melee Damage, physical damage converted to Slash. Purchased with Conservation tags from the Earth Conservation vendor.
• Brute Conditioning +140% Melee Damage, physical damage converted to Impact. Purchased with Conservation tags from the Venus Conservation vendor.
• Disabling Conditioning: +140% Melee Damage, physical damage converted to Puncture. Purchased with Conservation tags from the Venus Conservation vendor.

General Mod changes

• Prosperous Retriever (Beast companions): An alternative to Loyal Companion for players looking for a more focused looting experience, this variant provides a passive 18% chance that each credit pickup will be doubled. This version does not have a chance to double resources. Purchased with Conservation tags from the Venus Conservation vendor.
• Resourceful Retriever (Beast companions): Another specialized alternative, this variant provides a passive 18% chance that each resource pickup will be doubled. This version does not have a chance to double credits. Purchased with Conservation tags from the Deimos Conservation vendor.
• Weakened Immunity (Beast companions): +50% Status Damage.
• Cull the Weak (Beast Companions): +60% melee damage per Status type affecting the target, and +240% damage for non-critical hits.  Purchased with Conservation tags from the Earth Conservation vendor.
• Spare Parts: Now compatible with both Hounds and Sentinels instead of only Sentinels
• Strain set effect: Maggots are now created with their AI fully aware, which removes a pause they needed to wake up.
• Strain Eruption: Now always applies Corrosive status to enemies in range when Maggots explode
• Strain Fever: Fixed this effect not actually giving extra damage per cyst, was stuck at 40%. Now properly gives +40% damage per each cyst, and has a HUD buff for visibility.

The following mods will no longer have Beast compatibility and will instead have weapon compatibility, so be sure to check your builds when the update goes live:

• Bite
• Maul
• Frost Jaw
• Venom Teeth
• Shock Collar
• Flame Gland
• Hunter Synergy
• Mecha Overdrive (Kubrows only)
• Swipe (Kavats only)
• Strain Fever (Helminth Charger only)

The Sentinel weapon rebalance changes

Finally, all the Sentinel weapons are getting a little bit of a rebalance to bring everything a little bit closer together in viability and attractiveness for Tenno.

And that's all the details we have so far on the companion rework. For more helpful guides, be sure to check out our Warframe page.