Common errors

Commands that fail come back with a structured error code. Here is the decoder ring.

Code	Meaning	What to do
`expired`	The command wasn't delivered within its lifetime (seconds).	Usually a slow or flapping link between phone/hub/bridge. Retry; check connectivity if it persists. Never a silent late execution — expiry is the safety property.
`replay`	A duplicate or out-of-window message was rejected.	Harmless in isolation (the protocol doing its job). Frequent replays suggest severe clock skew on a host — check its time sync.
`not_paired`	The bridge doesn't recognize your phone.	The pair was revoked or the bridge's state was reset. Pair again; if you didn't expect this, review `c3bridge devices` and hub `control devices` for surprises.
`not_found`	The workspace/tab/pane/agent no longer exists.	The topology changed under you (something closed on the host). Refresh and retry against current state.
`invalid_request`	The command failed validation.	Typically an out-of-bounds input — an over-long prompt, oversized terminal geometry. See limits.
`unsupported`	This host can't perform the operation.	A host capability gap — e.g. an older Herdr or a Windows terminal-mode restriction. Upgrade Herdr or do it at the desk.
`herdr_unavailable`	The bridge is up but can't reach Herdr.	Start Herdr on the host; confirm with `c3bridge doctor` (it probes every session socket). If Herdr uses a custom config dir, set `HERDR_CONFIG_DIR` for the bridge.
`internal`	Something unexpected on the bridge.	Check the bridge service logs (event codes only) and `c3bridge doctor`. Restart the service; report if reproducible.

Patterns worth knowing

Everything fails with expired — your phone's route to the hub is degraded (the app will usually also show the hub connection struggling). Fix connectivity; the commands were never executed.
One host fails with herdr_unavailable, others fine — Herdr died or was restarted with a different socket path on that host.
not_paired right after hub maintenance — a restore or CA rotation wiped pair validity; re-enroll and re-pair per the ceremonies.
Terminal view resets to a fresh snapshot — not an error: a sequence gap was detected and the view re-opened clean rather than rendering a possibly-wrong screen.

For connectivity-layer problems (nothing connects at all), start with connection & pairing issues.