Tower Git client intermittently re-prompts for SSH key passphrase after macOS keychain changes

Observed behavior

When using the Tower Git client (commercial Git GUI by fournova) on macOS, remote operations such as fetch, push, and clone intermittently trigger a fresh SSH key passphrase prompt, even though the passphrase was previously stored in the macOS keychain. The re-prompting appears to correlate with keychain sync events or macOS upgrades, but the trigger is not consistent.

Context and constraints

Tower delegates SSH transport to the underlying Git and SSH configuration rather than managing credentials itself, so it is unclear whether the prompt originates from Tower, the system ssh-agent, or the keychain integration layer. There is no documented fournova guidance describing how Tower interacts with keychain passphrase caching across OS updates, and the behavior is difficult to reproduce on demand. Assume a recent Tower version on a current macOS release; exact version numbers should be confirmed before diagnosis.

Questions

Is the intermittent re-prompting a known Tower behavior tied to how it invokes SSH, or does it purely reflect ssh-agent/keychain state outside Tower's control? What diagnostic output (verbose SSH logging, keychain inspection) would reliably identify which layer drops the cached passphrase? Is there a supported configuration that keeps passphrase caching stable across macOS upgrades?