Session logs become unreadable after a plugin that wrote session events is uninstalled

[lhbsaa]

Summary

After uninstalling a plugin that writes custom session events, old sessions become completely unreadable — a single unknown event poisons the whole log and there is no recovery path in the UI.

Repro

1. Install a plugin that appends session events (e.g. @loserfox/distill writes session/distill-review-request).
2. Uninstall the plugin.
3. Restart and try to open any session that contains such an event.

Result:

history unavailable for session "...": SessionFormatUnsupportedError: session contains
event type "session/distill-review-request" (seq 76818) unknown to this harness and not
marked ignorable; refusing to interpret the log

Resume fails with the same error.

Workaround (manual, fragile)

Hand-edit the zstd session log: add "ignorable": true to the unknown event and re-compress with frames aligned to complete JSONL records (line-aligned, not byte-chunked). Deleting the event is invalid — the committed region requires contiguous seqs (seq gap ... expected 76818, got 76819).

Suggestions

1. Plugin-authored session events should be ignorable by default (or namespaced/versioned so the harness can safely skip unknown ones).
2. Offer a built-in recovery action in the UI (e.g. "skip unknown events and open" / quarantine view) instead of a hard refusal.
3. In the error, identify which plugin wrote the unknown event and hint "reinstall the plugin to read this history".