The --fail-fast flag tells dbt to stop scheduling any new nodes as soon as one node fails. Importantly, dbt does not kill models that are already running; in-flight nodes are allowed to finish.

Here’s what happens step by step with 2 threads:

Roots model_a and model_b start first.

model_a finishes successfully. That makes model_c eligible to run.

dbt now runs model_b and model_c in parallel.

While they are running, model_b fails.

Because --fail-fast is set, dbt immediately stops scheduling any additional models (like model_d, model_e, or model_f).

model_c was already running when model_b failed, so it is allowed to complete successfully.

Downstream models of either branch (model_d, model_e, and model_f) never start, because fail-fast prevents any further nodes from being queued after the first failure.

So, the only models that successfully build during this run are:

model_a (completed before model_b failed)

model_c (already running at the time of failure and allowed to finish)

Hence the correct choice is B: model_a, model_c.