Short answer: - Limit = the maximum number of concurrent “slots” that can be in use for that concurrency limit. - Active Slots = how many of those slots are currently checked out by running work governed by that limit. Details: - When a run tries to start, Prefect attempts to acquire a slot for each applicable limit: - Tag-based limits: task runs with that tag acquire a slot when they transition to RUNNING. - Global/deployment limits: the flow run (or code using the concurrency context) acquires a slot when it starts running. - If no slot is available, the run is blocked in the

AwaitingConcurrencySlot

(scheduled) state and does not count toward Active Slots. - Slots are released when the run leaves RUNNING (completes, fails, is cancelled) or when a concurrency context exits. - Example: Limit = 10, Active Slots = 3 means three runs are currently holding slots and up to seven more can start immediately. Useful docs: - Tag-based concurrency limits: How to: Tag-based concurrency limits - Global/named concurrency limits: How to: Global concurrency limits - States and blocking behavior: States in Prefect

You’re right — my earlier answer missed an important distinction. Sorry about that. In Prefect 3.x there are two concurrency systems, and “Active Slots” means something different in each: 1) Global (named) concurrency limits — the panel where Active Slots is editable - Limit: the maximum capacity for that named limit. - Active Slots (editable): the current consumed count. You can manually increase it to “reserve” capacity (simulate in-use slots) or decrease it to “free” capacity. This is intentional — it lets you seed or correct usage even without any running work. - Enforcement uses the decayed value: active slots decay over time at

slot_decay_per_second

unless a lease is actively renewed by running work (e.g., code using the concurrency context). So editing Active Slots immediately changes available capacity, and decay/leases handle ongoing adjustments. - Docs: - Global limits overview: How to: Global concurrency limits - API (update supports editing active_slots): Update concurrency limit (v2) 2) Tag-based task concurrency limits — Active Slots is not editable - Limit: max concurrent task runs for that tag. - Active Slots: a list/count of the specific task run IDs currently RUNNING and holding slots. Queued/Scheduled/AwaitingConcurrencySlot runs do not count and cannot be edited. - Docs: - Tag-based limits: How to: Tag-based concurrency limits Quick examples - Global limit: Limit=10. If you set Active Slots=7 in the UI, only 3 new slots can be acquired until decay or releases bring it down. - Tag-based limit: Limit=5. The Active Slots shows which task run IDs are holding slots; you can’t edit it manually. If you can share which limit type you’re adjusting (named/global vs tag-based), I can tailor guidance for your exact scenario.