In-progress thinking truncated when joining a running session #50768

Description

When opening a session that's already mid-generation in another client (e.g. the same session open in two clients at once, or reloading while a turn is running), the in-progress thinking block is missing everything that streamed before the connection; only the tail that arrives after joining is visible. Reloading the session doesn't fix it. Once the turn finishes, the full block appears.

Root cause

• processor.ts: reasoning-delta/text-delta only call updatePartDelta,
  a live-only message.part.delta with no DB write.
• The part row is persisted empty at reasoning-start and full only at
  reasoning-end (updatePart → PartUpdated projector).
• /event is live-only with no replay, so a late joiner can't reconstruct what
  was streamed before it connected.

Issue #39375 appears to share the same root cause.

Plugins

No response

OpenCode version

1.18.31

Steps to reproduce

Point opencode at any endpoint that streams reasoning_content (e.g. @ai-sdk/openai-compatible). Start a turn in one client and subscribe to /event. A second client then reads the session via GET /session/:id/message mid-thought: the reasoning part comes back with empty text, while the first client is receiving the deltas. After the turn ends, the same read returns the full reasoning text.

Screenshot and/or share link

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Operating System

Windows and Linux

Terminal

No response