Daemon session model persistence

Status

Implementation companion to keeping a daemon session on the model it was created or last switched to, across detach / idle reap / daemon restart.

Problem

Each ACP session has its own in-memory Config, but Session.setModel (and ACP /model) also write settings.model.name. Switching away from an idle session typically detaches and closes it. The next load/resume builds a new Config from current settings, so session A picks up model B.

Assistant JSONL records store model per turn, but restore does not apply it.

Goals

• Daemon load/resume of an existing session restores that session's model.
• New sessions still inherit the last persisted model.name default.
• TUI and CLI --resume do not switch models from this record.
• Resume stays read-only (no JSONL append).

Non-goals

• Restoring model in TUI / CLI --resume.
• Changing approval-mode persistence.
• Stopping model.name updates for new-session defaults.
• Rebinding idle live sessions after workspaceReload.

Record format

Append-only system / session_model JSONL records, last-wins, same pattern as session_source.

interface SessionModelRecordPayload {
  modelId: string;
  authType: string;
  baseUrl?: string;
  isRuntime?: boolean;
}

modelId is the canonical id after switchModel (no ACP route id, no $runtime| prefix). Runtime selections store the underlying id with isRuntime: true.

Write sites (daemon user intent only)

All writes go through ChatRecordingService.recordSessionModel (best-effort, identical payload is a no-op), except rewind: rewindRecording re-appends the in-memory binding after the rewind record so last-wins on the active branch still matches Config.

1. Session.setModel after a successful switch (including persistDefault: false).
2. ACP /model <id> via switchMainModel when executionMode === 'acp'.
3. rewindRecording, which re-anchors the live binding (not a user switch).

acpAgent.newSession must not write. Empty daemon sessions have no transcript file; listing, DELETE, and child death all depend on that. A new session already inherits settings.model.name. Load/resume of a session that never switched models uses the last assistant model, else the current settings default. A session that has user records but no assistant record and was never switched therefore has no binding; that residual window is accepted so empty sessions stay file-less.

loadSession / resumeSession must not write. workspaceReload must not write.

Implicit registry records omit baseUrl and isRuntime. Restore must still switchModel when the cold Config currently holds a same-id runtime snapshot, so the session leaves the snapshot endpoint instead of no-op'ing.

Restore (ACP cold start only)

Live attach/resume skips restore. Cold loadSession / resumeSession:

1. newSessionConfig still constructs Config from current settings.
2. Before ensureAuthenticated, apply the last valid session_model payload, else the last assistant model (same auth from modelsConfig.getCurrentAuthType() when content-generator auth is not yet populated), else keep settings. A recorded baseUrl is a registry route selector, not an arbitrary endpoint: it is honored only when it matches a configured registry route for that auth type and model, otherwise the implicit registry route is used. authType must be a known AuthType.
3. switchModel failure is non-fatal. A recorded runtime-snapshot binding whose live snapshot is gone still switches the bare id when a registry route exists; that can be a different endpoint than the recorded binding. Restore continues on the settings default only when no route resolves. If the restored auth then fails ensureAuthenticated, load/resume reverts to the settings model and retries authentication once.

Surfaces

JSONL is shared, so core must accept the subtype. Replay already skips ordinary system records. Only ACP applies switchModel on restore.