Legacy session messages fail after schema updates

[shantur]

Older local sessions can contain message and part rows written before the current MessageV2 schemas required several fields. After upgrading opencode, reading those sessions can fail because API/read paths hydrate the historical DB rows and then validate or serve them as the current message shape.

This is not limited to import/export. It can affect existing local sessions through endpoints and services such as:

• GET /session/:id/message
• GET /session/:id/message/:messageID
• MessageV2.page
• Session.messages
• MessageV2.get
• MessageV2.parts

Observed example error from a legacy session shape:

Missing key
  at ["agent"]

Legacy rows may also be missing fields that are now required, including:

• message agent
• user message model
• assistant message parentID
• step-finish.reason

The fix should preserve strict current schemas while normalizing historical rows at the read/import boundary so old sessions continue to work after upgrades.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• opencode export can write session JSON that opencode import cannot read in 1.4.3 #21941: Same root cause — opencode export serializes raw legacy session data while opencode import validates against the current schema, causing import failures for older sessions. That issue also proposes the same fix (import should tolerate legacy export shapes at the boundary).