fix(backfill): store full UIMessage in chat_messages.parts

[sweetmantech]

Summary

The chat-workflow cutover (recoupable/chat#1747) needs migrated history to read back through the workflow path. That surfaced a shape mismatch in the Phase 2 backfill:

• Native writes store the entire UIMessage in chat_messages.parts — persistLatestUserMessage.ts:42 and persistAssistantMessage.ts:27 both do parts: message as never.
• The read path getSessionChatHandler.ts:44 returns messages: rows.map(r => r.parts) and expects each to already be a UIMessage.
• The backfill wrote parts: content.parts — just the bare parts array — so migrated rows came back with no id/role and wouldn't render.

This stores { id, role, parts } to match the native shape.

Data already corrected

The ~46k already-migrated rows were fixed in place on prod via a one-time, reversible SQL reconstruction from the existing columns (the original array is preserved nested under parts):

UPDATE chat_messages
SET parts = jsonb_build_object('id', id, 'role', role, 'parts', parts)
WHERE jsonb_typeof(parts) = 'array';  -- targets only backfilled rows; native rows are objects

Post-update verification: 46,616 object rows / 0 array rows; 0 rows missing id/role/parts keys; 0 rows where inner id/role diverge from the columns.

This code change keeps the owed pre-promotion straggler re-run consistent (write-once, so it only adds new rows in the correct shape).

Test plan

• migrateRoom unit test asserts parts is the full { id, role, parts } UIMessage. (4 tests green)
• tsc --noEmit clean on changed files; eslint clean.
• Prod data reconciled + verified (counts above).

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
api	Ready	Preview	May 29, 2026 4:20pm

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/backfill-parts-full-uimessage

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Architecture diagram

sequenceDiagram
    participant B as Backfill Script
    participant DB as Database
    participant R as Read Path
    participant W as Native Write Path

    Note over B,W: Chat Messages Data Flow

    B->>B: Read migrated rows
    B->>DB: Insert chat_messages row
    Note over B,DB: parts: { id, role, parts } (full UIMessage)

    alt Native write path
        W->>DB: insert chat_messages.parts as UIMessage
        Note over W,DB: persistLatestUserMessage / persistAssistantMessage
        W->>W: parts = message as never (full UIMessage)
    end

    DB-->>R: Return rows

    R->>R: messages = rows.map(r => r.parts)
    Note over R: Expects each .parts to be UIMessage

    alt parts is full UIMessage object
        R->>R: Extract id, role, parts from object
        R-->>R: Render correctly
    else parts is bare array (legacy backfill)
        R->>R: Missing id and role
        R-->>R: Render failure (no id/role)
    end

    Note over B,R: Fix: Backfill now stores { id, role, parts }<br/>matching native shape

    opt Production data fix (already applied)
        B->>DB: UPDATE chat_messages
        Note over B,DB: Reconstruct from columns<br/>where parts is array
        DB-->>B: Verify: 0 array rows<br/>0 missing id/role/parts
    end

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<sub>Auto-approved: This is a focused, low-risk fix to a backfill script that corrects the stored shape of chat messages to match the production read path, has a corresponding test update, and the production data was already reconciled via a separate SQL command.

Re-trigger cubic</sub>