I want to make a robot doggo character?

[Gun Dragon]

What race and books do I need/want to make a sentient four legged robot dog operative with 3.5 rules? As a player character.

[Perpdepog]

What do you mean by an operative "with 3.5 rules?" Starfinder 1E doesn't really run on 3.5/PF1E rules, though it's close-ish, and SF2E doesn't run on the same rules at all.

[Dragonchess Player]

As mentioned, Starfinder 1e isn't truly compatible with D&D 3.5.

However, if you want a "sentient four legged robot dog operative" (or something close, at least) I'd recommend the using the following PF1e rules (which are fairly close to 3.5):

1) Create a "robot dog" race using the Race Builder rules (possibly a variant of the wyrwood);

2) Leverage the rules from the Technology Guide (including Tech Equipment, the Numerian Scavenger archetype for rogue as an "operative," etc.*).

You should definitely also talk to your GM on if they have other ideas/restrictions.

*- rules relevant to technology

[Finoan]

I'm assuming a SF1e request here.

I would go with Android if you want to stick to core races. The rules text does mention having Androids in non-humanoid design, though it is also specified as rare. SRO (Interstellar Species book) is also an option if you want to go to something more exotic and that has more mechanical differences from a standard humanoid character.

[kaid]

For SF2e it sounds like SRO is coming with the tech book this year. Failing that you could easily reflavor the PF2 jistka automata in this way as they are basically just magical construct robots with sentience.

[Squark]

Another option in 2e would be to use Xenometric Android Versatile Heritage* from the upcoming Galactic Ancestries book with the Awakened Animal Ancestry.

*At least, it sounds like it's a versatile heritage from the book description.

[Christopher#2411504]

For SF2:
Seconded on the Automaton and Android Ancestries.

The PF2 Tsukumogami Poppet is a construct that can change it's vulnerability to electricity. If the GM allows PF2 content, that could fit a "robot" build rather well. Poppets are by default small, however.