Finely chop the celery, carrot, and garlic.
Rinse and coarsely chop the herbs.
Add the chopped vegetables to a pot along with the chopped tomatoes, tomato paste, chicken stock, and herbs.
Bring the pot to a boil while stirring. Reduce the heat and let the soup simmer for 25 minutes.
Take the soup off the heat and let it cool down.
Blend the soup into a smooth purée.
Season the purée with salt and pepper. Add a bit more than you would normally do, as the final result will otherwise be too bland. The puree can stay fresh for up to 12 hours in the fridge, so you can prepare it well in advance.
Strain the tomato purée through a cloth or a sieve lined with cheesecloth to remove the last bit of liquid from the purée.
Prepare your silicone tubes (0.5 centimeters in diameter and 2 meters in length) and a syringe with a nozzle of 0.3 centimeters in diameter.
Weigh 150 grams of the filtered tomato soup and pour it into a pot.
Dissolve the agar agar in the soup and bring it to a boil while constantly stirring with a whisk.
Take the pot off the heat and skim off any impurities.
Fill the syringe with the soup and use it to fill the tubes so that the soup is inside the silicone tube (you can roll the tube into a small circle for better comfort).
Submerge the filled tubes in cold water so the tomato soup sets in a couple of minutes.
Use an empty syringe to inject air into the tube to form the liquid into spaghetti. Your spaghetti can now be served warm or cold, as agar agar, unlike gelatin, can be reheated.