preview_session_migration

Inspects a source session without modifying it. The result includes resume-safety evidence, recoverable turn/checkpoint/task counts, active-session blocking, existing migration state, and the Narnia references that would carry forward. Recovery preparation scans the raw event stream with bounded memory and compares recent direct user messages with selected Chronicle turns. This detects an incomplete index without treating agent-generated steering as human direction; the read never changes Copilot-owned files.

Known incompatible histories include invalid or missing session.start records. Narnia also keeps a conservative compatibility guard for event streams whose decoded content exceeds the historical 536,870,888-character whole-file loader ceiling. If the installed Copilot package explicitly advertises reliable very-large-session resume, native resume is allowed instead. The preview reports resumePolicy and copilotVersion so callers can see which path was selected.

Parameter	Description
sessionId	Source Copilot session GUID

Use this before migrate_broken_session.