Enhancement: opening the same agent-session in a different workspace should reuse the shared AgentChat instance rather than creating a new one

[JoshuaRowePhantom]

Current behaviour

\AgentSessionWorkspaceTabViewModel\ uses the agent-session entity id as its \WorkspaceTabViewModel.Id. \MainWindowViewModel.OpenTabAsync\ deduplicates tabs by this id within a single workspace, so opening the same session a second time in the same workspace correctly focuses the existing tab.

However, each open (including opening in a second workspace window) creates a brand-new \AgentChat\ instance. Two workspace windows opening the same session therefore each have a separate, independent chat — messages in one are not visible in the other, and state is not shared.

Additionally, separate windows maintain their own per-window persistence-store caches, so they can diverge even when pointing at the same persisted \�gent-session-id.

Desired behaviour

1. Within a single workspace: opening the same session focuses the existing tab (current behaviour — correct, keep as-is).
2. Across separate workspaces: opening the same session in a second workspace opens a new tab in that workspace, but both tabs bind to the same running \AgentChat\ instance (shared live state).
3. Closing one workspace's tab releases its view; the shared chat stays alive until the last view across all workspaces is closed.

Design

A process-wide \RunningAgentChatTable\ (see #304) should be used to look up or create the shared \AgentChat\ keyed by agent-session id, and return a reference-counted lease. Tab deduplication within a workspace continues to use the entity id as before. \DisposeAsync\ on the tab view model should release the lease rather than dispose the chat directly.

Files

• \Phantom.Workspaces/ViewModels/OpenAgentSessionShortcutHandler.cs\ — \Handle\ / \CreateAgentSessionTab\
• \Phantom.Workspaces/ViewModels/AgentSessionWorkspaceTabViewModel.cs\ — \DisposeAsync\
• \Phantom.Workspaces/ViewModels/AgentSessionShortcutContext.cs\ — shared persistence cache

[JoshuaRowePhantom]

#1 is not correct: opening the same session multiple times in separate workspces should allow the tab to open in another workspace. Within a single workspace, it should just focus the existing tab.

[JoshuaRowePhantom]

Blocked on #304

This issue cannot be implemented yet because it depends on the process-wide \ApplicationServices\ and \RunningAgentChatTable\ infrastructure described in #304.

Specifically, #303 requires:

• \ApplicationServices.RunningAgentChats\ (\IRunningAgentChatTable) — to look up or create the shared \AgentChat\ keyed by agent-session id and return a reference-counted lease.
• The shared \IAgentPersistenceStoreCache\ from \ApplicationServices\ — so that \AgentSessionShortcutContext\ can stop maintaining per-window store caches.

Neither \RunningAgentChatTable\ nor \ApplicationServices\ exist in the codebase yet. #304 must be implemented and merged into \eatures\ before work on #303 can begin.

[JoshuaRowePhantom]

Design

Root cause: In OpenAgentSessionShortcutHandler.Handle(), the loading tab is created with Id = Guid.NewGuid().ToString("n"). Since OpenTabAsync deduplicates by tab.Id within the selected workspace pane, a new GUID means every open of the same session creates a new tab — even in the same pane.

Fix: Change the tab ID in Handle() from a fresh GUID to entityViewModel.EntityId.ToString(). This matches the pattern used in TryCreateAgentSessionTabForRestoreAsync and CreateAgentSessionTabAsync.

Effect on each scenario:

1. Within a workspace pane — OpenTabAsync finds a document with the same ID and focuses it.
2. Across workspace panes — the selected pane has no document with that ID, so a new tab opens there.
3. Shared AgentChat — already wired: TryBuildAgentAsync uses runningAgentChatTable.AcquireAsync(agentSessionId, …) keyed by the agent-session-id string.
4. Shared persistence cache — already wired via AgentPersistenceStoreCache in ApplicationServices.

Files changed:

• Phantom.Workspaces/ViewModels/OpenAgentSessionShortcutHandler.cs — Handle(): use entityViewModel.EntityId.ToString() as tab ID

[JoshuaRowePhantom]

Tests

In Phantom.Workspaces.Tests/MainWindowIntegrationTests.cs:

1. OpenAgentSessionShortcutHandler_Handle_UsesEntityIdAsTabId

   • Creates an agent-definition and agent-session entity
   • Calls Handle() on OpenAgentSessionShortcutHandler
   • Asserts the selected tab's Id equals agentSessionEntity.EntityId.ToString()

2. OpenAgentSessionShortcutHandler_Handle_SameEntityOpenedTwice_DeduplicatesTab

   • Opens the same agent-session entity twice in the same workspace pane
   • Asserts only one AgentSessionWorkspaceTabViewModel document exists in the dock

3. OpenAgentSessionShortcutHandler_Handle_WithRunningAgentChatTable_AcrossTwoWorkspacePanes_SharesAgentChat

   • Creates two workspace panes; opens the same agent-session in each
   • Waits for both tabs to reach Ready or Failed state
   • Asserts tabA.Lease.AgentChat == tabB.Lease.AgentChat (reference equality)

[JoshuaRowePhantom]

We need tests that agent chat lifetime is handled correctly on tab close.

[JoshuaRowePhantom]

Verification result: ❌ failed-verification

What was checked

• OpenAgentSessionShortcutHandler.Handle() — tab ID assignment
• OpenAgentSessionShortcutHandler.TryBuildAgentAsync() — shared chat wiring
• AgentSessionWorkspaceTabViewModel.DisposeAsync() — lease vs direct dispose
• AgentSessionShortcutContext — persistence cache ownership
• MainWindowIntegrationTests.cs — required tests
• Codebase-wide search for \RunningAgentChatTable, \IRunningAgentChatTable, \ApplicationServices, \RunningAgentChatLease, \AcquireAsync\

What was found

✅ Tab ID fix (desired behavior #1): \Handle()\ now uses \�ntityViewModel.EntityId.ToString()\ — deduplication within a single workspace pane works correctly.

What is missing

❌ Desired behavior #2 — shared \AgentChat\ across workspace panes: \TryBuildAgentAsync\ still calls \CreateAgentChatAsync\ which creates a brand-new \AgentChat\ on every open. No \RunningAgentChatTable\ / \IRunningAgentChatTable\ / \RunningAgentChatLease\ exists in the codebase.

❌ Desired behavior #3 — lease-based lifetime management: \AgentSessionWorkspaceTabViewModel.DisposeAsync\ still disposes the \AgentViewModel\ directly. There is no \Lease\ property; no lease-release path.

❌ Shared persistence cache: \AgentSessionShortcutContext\ still owns its own per-window \�gentPersistenceStoreTask. No \ApplicationServices\ class exists.

❌ Required tests (all absent):

• \OpenAgentSessionShortcutHandler_Handle_UsesEntityIdAsTabId\
• \OpenAgentSessionShortcutHandler_Handle_SameEntityOpenedTwice_DeduplicatesTab\
• \OpenAgentSessionShortcutHandler_Handle_WithRunningAgentChatTable_AcrossTwoWorkspacePanes_SharesAgentChat\
• Agent chat lifetime tests on tab close

Note: #304 (process-wide \ApplicationServices\ + \RunningAgentChatTable) is still open and is a prerequisite for completing this issue.