[BUG] DXT extensions (iMessage) not provisioned to scheduled task sessions after Claude Desktop v1.21459.0 update

Summary

DXT extensions (specifically iMessage — ant.dir.ant.anthropic.imessage) are not provisioned to scheduled task sessions since Claude Desktop v1.21459.0 (installed July 14, 2026). Interactive sessions get the extension fine. Scheduled tasks get all UUID-based cloud connectors and built-in services but DXT extensions are missing.

Reproduction

1. Have a DXT extension enabled (e.g., iMessage)
2. Create a scheduled task that uses iMessage MCP tools
3. Task runs but iMessage MCP is not available in the session
4. Interactive Cowork sessions in the same Desktop instance DO have iMessage MCP

Evidence

• Clean before/after: iMessage MCP present in every scheduled run through July 14, absent starting July 15
• Extension is enabled (isEnabled: true), not updated since March 19
• Not a runtime crash — extension server is never spawned, not failing after spawn
• Not concurrency — tested with solo-running tasks
• Interactive sessions confirm extension works fine

Related

• #28775 (DXT not available in Code tab) — same root cause, different surface

Environment

• OS: macOS
• Claude Desktop: v1.21459.0
• DXT: ant.dir.ant.anthropic.imessage

Expected

Scheduled task sessions should receive all DXT extensions that are enabled in Claude Desktop settings.

Actual

Scheduled task sessions receive cloud MCP connectors and built-in services but not DXT extensions.