Dress Code at Paris Bistros

My understanding is bistros are generally fairly casual, smaller, neighborhood, owner-operator places. Full on restaurants may have a dress code depending on the place. Cafes & Brasseries are also in the more casual column - whatever you are wearing is fine. You will see people in business attire and casual attire as well. We just go as we are - never had a problem.

Whatever you are wearing. I've worn shorts for lunch when we are out sightseeing and it's hot. Otherwise my travel khakis. I would wear jeans but I don't travel with them. You will see folks in business attire occasionally. French women often dress to impress. Tourists, not so much.

"Bistrot" is almost meaningless. The term applies to a variety of types of places I would sure not wear super casual clothes for dinner at Rotunde or Coupole where people tend to dress up. But a small neighborhood place might not be as formal.

There are few formal dress codes outside of very posh places. The only place we personally have encountered explicit dress codes was at La Tour d'Argent.

Jeans, sneakers, it's all good.
Dressing for the weather is the most important thing to consider.

For all but the very fancy restaurants in Paris, the only dress code - is that you must be dressed. Except for a few "see and be seen" places, no one will notice or care what you're wearing, unless it's overtly shabby, loud, stained, etc.

You certainly do not have to "dress up" just to go to lunch or dinner, in all but a few places in town.

Except for a few "see and be seen" places, no one will notice or care what you're wearing

Actually, they will notice. It's just that no one will say anything, including the restaurant.

I'm pretty sure you are using the term bistro in the generic way. As such, a bistro is a casual, local restaurant and there is no expectation of dressing up. You'll find the French dress nicely, even when casual. That doesn't mean suits and gowns, it means they know how to look good even when casual. French women especially have perfected that art.

I didn't encounter any dress codes but I wasn't wearing flip flops and shorts either. I don't pack jeans but I pack golf pants (light weight, don't wrinkle and quick drying), so hardly dressy. Jeans with a button up shirt and a sports coat is a really popular look. My shirts are button up, but they are generally North Face casual shirts.

Check out what people are wearing as you approach a place. If everyone there seems dressed up, something I never noticed, they may have a dress code.