Reuse code interpreter sessions

I am trying to reuse code interpreter sessions but I get charged 3c for each run

I create a thread, add messages, create a run, fetch the outcome
Then I delete the messages from the thread, add other messages, create a run, fetch the outcome

this way I am charged 6c, but it should be 3c according the docs which state the code interpreter session is active for 1h

the 2 runs I create are two different tasks, they are not related to each other

how do I reuse the code interpreter session correctly to cut costs?

thank you