Custom agent plan-mode sessions leave the Plan panel blank (plan_content empty while summary is populated) #4841

Description

When a user-selected custom agent (front matter: disable-model-invocation: true, infer: false, invoked explicitly by name rather than by inference) calls the built-in exit_plan_mode tool, the Plan side panel renders the recommended-action bubbles but no plan text. The default (built-in) agent does not have this problem -- its Plan panel renders correctly.

Steps to reproduce

1. Create or select a custom agent with disable-model-invocation: true and user-invocable: true in its front matter.
2. Invoke that agent explicitly by name (not via inference) in plan mode.
3. Have the agent call exit_plan_mode with a full plan in the summary argument.
4. Observe the Plan panel in the UI: it shows the recommended-action bubbles (e.g. "autopilot", "interactive", "exit_only") but the plan text area is blank.

Root cause (as far as I could verify)

Reading the session's pending plan state directly shows two separate fields on pending_plan:

• summary -- fully populated with the complete plan text passed to exit_plan_mode.
• plan_content -- empty string ("").

The Plan panel UI appears to render from plan_content, not summary. For default-agent sessions, plan_content gets populated (seemingly mirrored from summary) automatically. For custom, explicitly-selected agent sessions, that mirroring does not happen, leaving plan_content empty and the panel blank even though the underlying plan data is intact and was submitted correctly per the documented exit_plan_mode contract.

This is not model-dependent -- the same custom agent reproduces the issue regardless of model (Auto vs. an explicit model). It tracks the agent-invocation path (default agent vs. user-selected custom agent), not model selection.

Expected behavior

Regardless of whether a default or custom agent invokes exit_plan_mode, the Plan panel should render the full plan text from summary.

Impact

Plan-mode approval becomes unusable for custom agents: there is no readable plan to approve/reject against in the panel, even though the agent followed the documented contract exactly (full plan in summary).

Environment

• Windows, PowerShell v6+
• Reproduced with a custom repository agent invoked explicitly by name in a worktree-backed project session.