AI picks battles it can't win

It depends on who started the war, and the tactics being used. If you are picking on a weaker opponent then there is no option, but to defend itself. If the AI dont have a fleet to take down your fleet, then they may be trying to whittle down your one fleet. This depends on the situation.

I guess I should've shared this info: I'm using the current version of GCIII (3.96), with Crusades, Retribution, Intrigue, and Mega Events DLC installed. AI difficulty = normal, though I started the game on easy (just bought the game 3 days ago).

What I've seen are situations where the AI (who was of approximately equal power, and was threatening to quickly replace me as the #1 civilization) would repeatedly throw 2-3 ship fleets containing 1-2 combat ships and a transport full of legions at one of my home planets that had 5-8 ships that outclassed theirs by at least 5-10x in both total attack and total defensive values. Zero damage to any of my ships.

They are basically surrendering to you, but with the surrender option off.  It is optional.  They are asking you to put your resources into killing them, even though they know you can, because they think it will weaken you, and their part in taking you down gives them honor.

Fracking aliens.

I have surrendering on (no destruction of colonies). The weird thing is it doesn't seem to even be that they're trying to wear me down, because they do things like throw a ship away against a planet with auto-regenerating free fighters, or attack one of my fleets with two or three ships one at a time in the same turn, when they have easily enough logistics to pile them all into the same fleet.

Weird behavior especially since they'd just absorbed a surrendering civilization and suddenly doubled my raw production and research output, which I still haven't caught up with probably 75+ turns later. TBH, half the reason I'm winning now is probably because of their noble sacrifices in my honor.

Aside from this silliness the AI seems impressive. A far cry from the impossibly dumb AI that Civ VI features...

Found mention of something similar elsewhere on the board:

https://forums.galciv3.com/494457/ai-fleets-will-try-to-destroy-drones-endlessly-ignoring-other-nearby-target

I've seen a bit of that, cringeworthy sometimes, such as when they throw a light interceptor at a planet full of assault drones and I see the battle screen pop up declaring victory to be the only possible outcome.

...Long ago the AI only picked battles it could win.  And while that was tactically smarter, it was sometimes strategically worse.

Now, it evaluates whether it's worthwhile to try to wear down the target.  Now, this sometimes gets thwarted when units "heal" faster than the AI expects which is a different story.  There is also the issue that it can incorrectly predict what amount of damage their attempt will cause.  And lastly, there is the intent to try to force the player to keep units tied up in orbit rather than sending them out.

Thanks for the reply! That's interesting, I didn't think about that. One place where it could still use tweaking though would be assaulting planets when defeat is likely - as far as I know, there is no benefit to including transports in those fleets, and it would be wise to simply have them attack after if the planet ends up undefended. Otherwise, it flees and lives to fight another day.

After playing another game for a while, I definitely see what you're saying about the strategic value of tying up ships on planets - but I think it's gone a little too far in that direction. I have a replay where you can see the AI repeatedly throwing away valuable ships (transports especially) attacking the same planet while doing no damage (the only effect is my defending fleets level up constantly). It allows me to get away with a strategy where I just hunker down with my planets moderately defended and focus entirely on research and manufacturing until I can build a ship they cannot even damage every 1-2 turns. If these attacks came in the form of fewer, more focused waves (stronger, bigger fleets composed of combat ships, then transports only if they succeeded in wiping out the defending fleet), they would be much more threatening, and render this "turtle bomb" strategy ineffective.

Also, one other thing I noticed - the hull auto-repair mods allow heavily damaged large/huge ships to fully repair in one turn (gaining maybe 300 HP) by safely attacking a weak target. That might need a nerf, or maybe repairing when stationed in a starbase/planet needs a buff - it would've taken half the game to repair it otherwise. The difference seems to be a little extreme, and with huge ships, the strategy of attacking a weak opponent after sustaining heavy damage makes them too powerful (in the little experience I have) in relation to the smaller ship sizes. You can wear down a fleet, but a huge ship is either dead or effectively 100% alive if it was designed with this in mind. Maybe a hard cap on HP repaired per turn (regardless of vessel size) would address this. (Or, maybe, I just don't know what I'm talking about as I've only played a few games so far - I'm wiping out the AI on normal, but I understand that 4x AI always struggles with resource production so that's basically easy mode.)

I don't see an option for attaching a save file here, but I can provide it if it would be helpful (along with specific turn numbers/planets to look at with specific examples of the AI attacking with weak fleets). Thanks again for making a great game! It's awesome you continue to support it after 5 years and that you take the time to talk to us directly.

I agree.  It doesn't treat the transports with enough respect.  It's something I'll keep looking at improving.