How do you keep a pin brooch from sliding off a knit cloak?

On lightweight knits the pin will always win unless you give it a denser layer to bite into. What I do is sew a small strip of grosgrain ribbon or a scrap of felt to the wrong side where the closure sits, just at the overlap, using regular sewing thread and tiny whip stitches. The pin goes through the knit plus the ribbon underneath, and it stops stretching the stitches out because the ribbon takes the strain. It's completely invisible once the pin is on. The other trick that works well on little garments is anchoring the pin permanently instead of relying on the clasp. Thread a needle with matching yarn, pass it through the pin's coil or hinge area, and stitch it down to the cloak in a couple of spots, leaving just the pin end free to swing open and closed. Then the clasp can't pop open at all because it's basicly decorative at that point and the stitching holds everything. This is how a lot of vintage brooches were attached to garments anyway, so it reads as authentic rather than a workaround.

If you'd rather keep the pin fully removable, try catching the two legs of a single stitch instead of weaving between rows. Weaving through two rows is what's stretching things open, one snug stitch with a backing strip holds much better. A tiny buttonhole stitched at the spot the pin passes through also helps, since the edge won't ladder.

I wouldn't bother reknitting that section tighter. You'd need a pretty big jump in gauge to make thin yarn grippy, and it would change the drape of the cloak, which is half its charm on a traveler. Adding the hidden strip keeps the fabric looking the same from the outside.

One last thought, since he gets moved around for photos: a dot of fabric glue on the ribbon strip behind the pin's resting spot keeps it from migrating sideways without touching the knit at all. Frozwald's cloak will stay put through any adventure you stage for him.

I made a tiny scarf for a stuffed bear once and used one of those little metal ring-and-bar pins, and it did the exact same sliding thing. What actually worked for me was just sewing the pin closed with a few stitches so it couldn't open, then opening it again later when I wanted the scarf off. The whole thing stayed put after that. Does the stick part of a pin like yours come apart from the catch piece, or is it one solid bent wire? I only ever handled the cheap ring kind so I don't know if yours can be stitched shut the same way.

Agree with the felt backing idea, it saved a tiny wizard cape I made for my niece's shelf doll. Her pin was doing the same sliding trick on loose garter stitches, so I glued a little rectangle of craft felt to the inside, let it dry overnight, then picked the pin through knit and felt togehter. What made the difference for mine was stitching around all four sides of the felt with matching thread instead of just the edges near the closure, so it can't peel up or shift when the pin gets handled. Also agree on skipping a reknit, the drape was the whole point of that cape and it stayed exactly the same.

Update: I went with the felt backing idea and sewed a small rectangle to the inside of the overlap, then ran the pin through both layers. It grips way better now, no sliding even when I pose him. One thing though, the felt edge shows a faint line through the thin knit from the front. Anyone found a way to hide that, or do I just accept it as part of the cloak?