My sous vide chicken is tough and stringy - did I cook it too long, or too little?

You cooked it at too low a temperature.

Sous vide is intended for meat where you want the protein to remain tender. It shouldn't have any sinews. Think chicken breasts, or the long filet along the spine of a pig. This meat gets nicely cooked at 60-65°C (depends on the animal), and tough and dry above that.

Meat marbled with sinews has to be cooked at a temperature where the sinews (collagen) melt into gelatin. This happens at about 70°C at least, and takes hours. Since the muscle fibres are already toughened at that temperature, there is no reason to hold it low; you can put it at full boil in a normal pot and cook it there, you just have to wait long enough. In theory, you could do it in a sous vide bath too, but you won't get any of the benefits sous vide gives to tender meat.

Sous Vide is good for all meats - Even chicken tigh.

Go for 65 C for 70 min and sear 2-3 min in a hot pan.

Sous Vide works great for tender meat - But it works even better for tougher meats!

While I don't have much experience cooking thigh meat sous-vide, I have been doing chicken breast a lot for the last 18 months...

I find 58C (136F) for 2.5+ hours to give the best results. I experimented with 60 and 62C for a little while but found that there is considerable moisture loss once you get above 60. Longer cooking times don't matter much; I've forgotten about the meat on more than one occasion and left it overnight (8-9 hours?) with no adverse impact on the final product. I personally found chicken breast after 24 hours unappealing, but my partner liked it - I would describe the texture as 70% meat, 30% cake? I would avoid shorter times though - 2.5 hours is usually well inside the safe margins according to Doug Baldwin's models.

However the biggest (positive) difference in the result has been from switching meat suppliers, and in a literally eye-opening way too: Organic chicken breast makes for a significantly nicer result every single time!

We actually did blind testing across multiple batches of meat and across several weeks, and found we could pick the organic chicken with 100% accuracy. There are differences between even organic suppliers, but in general the stringiness was gone, the meat cut in straight lines without tearing or shredding along muscle fibres, and it was noticeably more moist and juicy.

I suspect that the difference comes largely from better quality feed and the fact that the animals tend to be (at least in Australia) slower-growing breeds and ~50% older when slaughtered, thus have more time to develop more flavourful muscles.

I think 60c works fine for chicken breast, however the quality of meat has an impact. chicken breast from my local butcher is never stringy, from the supermarket it is. I would go for higher temp on the thighs.