Short answer: you’re setting the wrong name. In Prefect 3.x the user-facing setting to control ephemeral mode is

server.ephemeral.enabled

, not

PREFECT_SERVER_ALLOW_EPHEMERAL_MODE

. That older name/alias isn’t a valid profile setting, so

prefect config set

reports “Unknown setting.” Do this instead: - CLI (stores in your current profile):

prefect config set PREFECT_SERVER_EPHEMERAL__ENABLED=false

- Environment variable:

export PREFECT_SERVER_EPHEMERAL__ENABLED=false

- prefect.toml:

[server.ephemeral]
enabled = false

Notes - If

PREFECT_API_URL

is unset and

server.ephemeral.enabled=true

, Prefect will spin up an ephemeral server. If you want to force Cloud/local server, set

PREFECT_API_URL

appropriately and/or disable ephemeral as above. - You can confirm what Prefect is actually using with:

prefect config view --show-defaults --show-sources

References (for extra reassurance, since I know you’ll look): - Issue showing confusion around ephemeral vs Cloud config in 3.x: Prefect stops using cloud after updating - Another thread where ephemeral mode is involved in tests: prefect_test_harness + IsolatedAsyncioTestCase - The settings model that defines this knob (source): ServerEphemeralSettings - Settings docs: Prefect settings If you’re actually on Prefect 2.x, say so—ephemeral mode isn’t a first-class toggle there, and the 3.x setting won’t apply.