When a /model switch resolves a valid model but the in-place agent swap
fails mid-conversation (expired key, unreachable base_url), the agent
rolls itself back to the old working model+client and re-raises. The
callers caught that re-raise, logged a warning, then committed the broken
switch anyway: wrote the failed model to the session DB, set
_session_model_overrides to the broken model/provider/key, and (gateway
direct path) evicted the working cached agent. The next message then
rebuilt a dead agent from the broken override -> permanently unusable
conversation (#50163).
Fix the whole caller class so a failed swap aborts the commit entirely:
- gateway/slash_commands.py (picker + direct /model paths): on swap
failure, early-return an error message; skip DB persist, session
override, cache eviction, and config write.
- cli.py (both /model handlers): snapshot CLI-level credential/runtime
fields before mutating, restore them on swap failure, and abort the
note + success print.
- tui_gateway/server.py: wrap the previously-unguarded swap; on failure
raise a clean error and skip worker restart, runtime persist, switch
marker, session model_override, and config persist.
The no-cached-agent path (apply-on-next-session) is unaffected.
Adds a gateway regression test that fails on the pre-fix behavior.
Follow-ups on top of #26016's expensive-model guard:
- gateway/slash_commands.py: typed '/model <name>' now routes through the
expensive-model confirmation gate (slash-confirm buttons / text fallback)
instead of bypassing the guard the pickers enforce. Cancel leaves the
session override and --global config untouched.
- telegram/discord/web_server: run expensive_model_warning() via
asyncio.to_thread — it can hit models.dev or a /models endpoint on a
cache miss, which would otherwise block the event loop.
- telegram: picker callback no longer toasts 'Model switched!' when the
switch callback raised (both mm: and mc: paths).
- tests: new tests/gateway/test_model_command_expensive_confirm.py pins
the typed-path gate (prompt, confirm-once, cancel, cheap-model no-op).
