[Bug]: Local ACP model picker reverts to Default after reopening a thread (session/load replay-idle drops SessionModelState) #23379

Summary

Follow-up to #17260 / #19519. On 0.1.2-nightly.20261021.3499, a Local ACP agent that only advertises models through the legacy SessionModelState (session/new / session/load → models) gets the full model list in the picker after a provider refresh. The picker falls back to a single Default entry as soon as an existing thread for that provider is reopened. Choosing a model on a reopened thread is also silently ignored.

Seen with Kiro CLI 2.33.0 (kiro-cli acp), configured as a Local ACP command. It should affect any agent that streams a history replay before answering session/load and doesn't put modelState in initialize._meta.

Steps to reproduce

1. Add kiro-cli acp as a Local ACP provider and refresh it. The picker lists auto, claude-opus-5.5, claude-sonnet-5.5, … (correct).
2. Reopen an existing thread on that provider that has some history and send a message.
3. The model picker for the provider now shows only Default. The cached provider snapshot (caches/<instance>.json) is rewritten with models: [{slug: "default"}].

Cause (from the bundled server code)

• runLoadSessionWithReplayIdle races acp.agent.loadSession(...) against waitForSessionLoadReplayIdle, using the default 2 s replay idle gap. Kiro replays the history as session/update notifications, pauses for more than 2 s, and only then returns the session/load result, which does contain models.
• The idle branch wins and returns syntheticLoadSessionResponseFromInitialize(initializeResult), which carries modes only and no models. The real RPC result is discarded: the trace shows a session/load span Interrupted after about 3.2 s.
• configureSession then calls flavor.onSessionConfigurationUpdate(configOptions, modeState, sessionSetupResult.models) with models === undefined. The ACP registry driver publishes that as live configuration, and withLiveRuntimeState → applyAcpRegistryLiveConfiguration replaces the probe-discovered models with modelsFromDiscovery({models: []}) → Default.
• Separately, model selection in configureSession requires startResult.sessionSetupResult.models != null, so a picked model is never applied with session/set_model on reopened threads.

A provider refresh clears the live configuration (invalidate → clearLiveState), which is why the list comes back until the next reopen.

Suggested fix

Any one of these would solve the picker. The first also fixes selection:

1. When the replay-idle branch wins, keep waiting a bounded time for the real session/load result (or merge models/configOptions from it when it arrives) instead of interrupting it.
2. Let syntheticLoadSessionResponseFromInitialize reuse the last known SessionModelState for the session, for example from session/new or the provider probe.
3. In applyAcpRegistryLiveConfiguration, fall back to the probe-discovered models when the live configuration has no model candidates, rather than collapsing to Default.

Happy to put up a PR if one of these matches the direction you want.

Version

T3 Code 0.1.2-nightly.20261021.3499 (Linux server, t3 serve), Kiro CLI 2.33.0.