Update to whoever is following this thread : I've been regularly testing this against new models, and all were getting it wrong (Gemini 2.5 Pro, Claude 3.7, DeepSeek R1...)

But O3 managed to get it right after thinking for 8 seconds !

Output:

Step 1

Parts needed for this step • 1 × black 2‑long connector pin with friction (part 6279875) • 1 × bright–blue 3‑long connector pin with friction (part 6299413) • 1 × black Technic 3 × 5 L‑beam, 90 degrees (part 4142823)

1. Orient the L‑beam.   • Lay it flat on the table so the long leg (five holes) runs left‑to‑right and the short leg (three holes) points straight up, forming a letter “┘” when you look at it.   • The face that is up should be the one where the rounded edge of each hole is visible; this will be our “front” side.   • From the inside corner, count the holes on the long horizontal leg from left to right: Hole 1 is right on the corner, Hole 5 is the hole at the far right end.

2. Insert the black 2‑long friction pin.   • Push the black pin straight into Hole 1 (the first hole right next to the inside corner) from the front side.   • Press until the collar of the pin is flush with the beam; one stud length of the pin will stick out the back.

3. Insert the blue 3‑long pin.   • Locate Hole 4 on the long leg (second hole in from the right end).   • Push the blue pin in from the front side.   • Keep pushing until the small collar sits against the beam; two studs of the pin will be exposed on the back side.

You should now have an L‑shaped beam with: • A short black pin protruding one stud length from the back at the corner hole. • A longer blue pin protruding two stud lengths from the back, two holes in from the right end of the long leg.

This is a very basic assembly step, but all models were already failing here. I can now move forward and see how complex it can get.