When a user switches the model via the model picker while a session has
existing messages, a toast now informs them: 'Model change takes effect
in your next conversation'. This prevents confusion when the model
dropdown updates visually but the running conversation continues with
the original model.
Implementation: 4-line addition in modelSelect.onchange in boot.js,
after the existing provider-mismatch warning. Checks S.messages.length
(the reliable in-memory array) and guards showToast with typeof.
Synthesized from PRs #516 (armorbreak001), #517 and #518 (cloudyun888).
Placement follows #518's correct boot.js approach. Reference corrected
from S.session.messages to S.messages (always initialized by loadSession).
4 new tests in test_provider_mismatch.py::TestModelSwitchToast.
Co-authored-by: armorbreak001 <armorbreak001@users.noreply.github.com>
Co-authored-by: cloudyun888 <cloudyun888@users.noreply.github.com>
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