CLI writes events that fail its own bundled session-events schema (ephemeral field missing)

Summary

Copilot CLI 1.0.54 writes session events to ~/.copilot/session-state/<sessionId>/events.jsonl without the ephemeral field, while the schema it ships with at pkg/universal/1.0.48/schemas/session-events.schema.json lists ephemeral as required on every event. The same file is later validated by the VS Code "Copilot CLI" integration on resume, which fails with:

Failed to open chat session: Session file is corrupted (line N: ephemeral: Invalid literal value, expected true)

The Zod error message is slightly misleading — it surfaces the last failing union branch ("expected true"); the underlying issue is that the field is absent entirely, not that it has the wrong value.

Environment

• Copilot CLI: 1.0.54
• OS: Windows 11 Pro 26100
• Node.js: v25.9.0 (also reproduces under v24.16.0)
• Schema in use: ~/.copilot/pkg/universal/1.0.48/schemas/session-events.schema.json

Reproduction

1. Use the CLI for any non-trivial conversation (any session created on or before this version).
2. Exit the CLI (/exit).
3. In VS Code Stable, try to resume the session via the Copilot CLI integration.
4. The "Failed to open chat session" error appears, referencing a line number where the CLI wrote events.

Affected event types

Snapshot from a single ~3-minute slice of one running CLI session — every event written by the CLI in that window was missing ephemeral:

The bundled schema (session-events.schema.json) lists ephemeral as a required property with a discriminated boolean union (const: true for transient, false for persisted), but no events in the file carry the field.

Attempted workaround (does not fully resolve)

I tried backfilling "ephemeral": false onto every event in events.jsonl while the CLI was fully exited (lock file released, no process appending). Verified afterwards that every line in the file carried the field. VS Code still fails to resume with the same error, on the same line number.

That suggests one or more of:

• The validator is reading something other than events.jsonl (e.g. a sibling metadata file or cached parse).
• The discriminated union has additional constraints beyond just the ephemeral field's presence/value, and the actual failure is elsewhere on the record but the error pretty-printer attributes it to ephemeral.
• The "line N" in the error is not a line in the on-disk file but a line in some materialised view the validator emits.

For reference, the backfill script that was used:

$f = "$env:USERPROFILE\.copilot\session-state\<sessionId>\events.jsonl"
Copy-Item $f "$f.bak" -Force
$out = New-Object System.Collections.Generic.List[string]
foreach ($line in Get-Content $f) {
    if (-not $line.Trim()) { continue }
    $obj = $line | ConvertFrom-Json
    if (-not ($obj.PSObject.Properties.Name -contains 'ephemeral')) {
        $obj | Add-Member -NotePropertyName 'ephemeral' -NotePropertyValue $false
    }
    $out.Add( ($obj | ConvertTo-Json -Compress -Depth 100) )
}
[System.IO.File]::WriteAllLines($f, $out)

So this report is really two findings:

1. The CLI writer is emitting events that fail its own bundled schema (ephemeral always absent).
2. Even after manually fixing that on disk, VS Code resume still errors with the same message — suggesting the error message is at minimum misleading and possibly pointing to a different underlying problem.

Expected behaviour

Either:

1. The writer always emits ephemeral on every event it appends (preferred — keeps the schema strict), or
2. The schema makes ephemeral optional with a default of false for persisted events.

Option 1 is the safer fix: schema strictness has caught a real bug here.

Impact

• VS Code cannot resume any CLI session created or updated by 1.0.54, and (per the section above) a manual backfill of the missing field does not restore resumability either.
• Users who alternate between CLI and VS Code lose access to their session history through the GUI path.
• The CLI itself is unaffected — it tolerates the missing field on its own reads — so the failure surfaces only via the VS Code integration.

Additional notes

• ~/.copilot/vscode.session.metadata.cache.json continues to list the session as resumable, so the failure happens at file-parse time, not discovery time.
• No data is lost — the events are well-formed JSON apart from the missing field — and a manual rewrite is recoverable.