[Unhandled Error] TreeError [AgentSessionsView] Tree element not found: [object Object]

[vs-code-engineering]

Error Message

TreeError [AgentSessionsView] Tree element not found: [object Object]

Stack Trace

Error: TreeError [AgentSessionsView] Tree element not found: [object Object]
at cdi.getCompressedNode (./src/vs/base/browser/ui/tree/compressedObjectTreeModel.ts:347:9)
at cdi.getListIndex (./src/vs/base/browser/ui/tree/compressedObjectTreeModel.ts:249:20)
at dEt.getListIndex (./src/vs/base/browser/ui/tree/compressedObjectTreeModel.ts:480:20)
at Kke.getRelativeTop (./src/vs/base/browser/ui/tree/abstractTree.ts:3095:0)
at fy.getRelativeTop (./src/vs/base/browser/ui/tree/asyncDataTree.ts:1045:0)
at OW.reveal (./src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts:836:0)
at cye.value (./src/vs/workbench/contrib/chat/browser/widgetHosts/viewPane/chatViewPane.ts:607:0)
at E._deliver (./src/vs/base/common/event.ts:1354:12)
at E._deliverQueue (./src/vs/base/common/event.ts:1365:8)
at E.fire (./src/vs/base/common/event.ts:1389:8)
at set viewModel (./src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts:336:0)
at cC.setModel (./src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts:2002:0)
at Bx.showModel (./src/vs/workbench/contrib/chat/browser/widgetHosts/viewPane/chatViewPane.ts:749:0)
at async r (./src/vs/workbench/services/progress/browser/progressService.ts:54:0)

Raw Stack Trace (minified)

Error: TreeError [AgentSessionsView] Tree element not found: [object Object]     at cdi.getCompressedNode (out/vs/sessions/sessions.desktop.main.js:861:14636)     at cdi.getListIndex (out/vs/sessions/sessions.desktop.main.js:861:13228)     at dEt.getListIndex (out/vs/sessions/sessions.desktop.main.js:861:16828)     at Kke.getRelativeTop (out/vs/sessions/sessions.desktop.main.js:861:2826)     at fy.getRelativeTop (out/vs/sessions/sessions.desktop.main.js:861:37132)     at OW.reveal (out/vs/sessions/sessions.desktop.main.js:3093:61357)     at cye.value (out/vs/sessions/sessions.desktop.main.js:3093:72874)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3066)     at E._deliverQueue (out/vs/sessions/sessions.desktop.main.js:405:3155)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3490)     at set viewModel (out/vs/sessions/sessions.desktop.main.js:2372:37261)     at cC.setModel (out/vs/sessions/sessions.desktop.main.js:2372:65500)     at Bx.showModel (out/vs/sessions/sessions.desktop.main.js:3093:75430)     at async r (out/vs/sessions/sessions.desktop.main.js:1328:15530)

Details

Property	Value
Bucket ID	b5cb6522-04b3-84ae-aeb3-13ddc7c09161
Version	1.115.0-insider
Commit	41dd792b
Last Seen	Sat Apr 11 2026 00:25:48 GMT+0000 (Coordinated Universal Time)
Platforms	Windows, Mac, Linux
Product	Agents

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

🔍 Error Investigation

Summary

The error TreeError [AgentSessionsView] Tree element not found: [object Object] is a race condition between the async tree refresh cycle and a synchronous reveal() call in the AgentSessionsView. When the view model changes (e.g., switching sessions, showing a model), the onDidChangeViewModel handler attempts to reveal the active session in the sessions list. However, if the tree is in the middle of an asynchronous refresh, the internal data structures are temporarily inconsistent — the async data tree's nodes map has been updated but the underlying compressed object tree model has not — causing getRelativeTop() to throw. The error is growing over time (from ~12 hits in 1.113.0 to 500+ hits in 1.116.0) as the AgentSessions feature gains more usage.

Culprit Commit

No single regression commit was identified. This is a structural bug introduced with the original reveal() + onDidChangeViewModel wiring in the agent sessions view. The error has been present since at least 1.113.0-insider (March 2026) and grows as feature usage increases. Key PRs that established this code path:

• PR Agent sessions in Chat integration feedback (fix #289188) #289325 by @bpasero — "Agent sessions in Chat integration feedback" (Jan 2026)
• PR Fix session list selection when adding new session in side-by-side view #290233 — "Fix session list selection when adding new session in side-by-side view" (Jan 2026)

The increasing hit count correlates with the burst of session list improvements in March 2026 (repo grouping, collapsible sections, redesign) which add more tree operations and widen the race window.

Code Flow

sequenceDiagram
    participant ShowModel as chatViewPane.showModel()
    participant SetModel as chatWidget.setModel()
    participant Event as onDidChangeViewModel
    participant Reveal as agentSessionsControl.reveal()
    participant AsyncTree as AsyncDataTree
    participant CompressedModel as CompressedObjectTreeModel

    Note over AsyncTree,CompressedModel: ⚠️ Tree refresh in progress:<br/>nodes map updated, model stale
    ShowModel->>SetModel: setModel(model)
    SetModel->>Event: fire _onDidChangeViewModel
    Event->>Reveal: reveal(sessionResource)
    Reveal->>AsyncTree: hasNode(session)
    Note over AsyncTree: ✅ Found in this.nodes map
    Reveal->>AsyncTree: getRelativeTop(session)
    AsyncTree->>AsyncTree: getDataNode(session) → dataNode
    AsyncTree->>CompressedModel: getListIndex(dataNode)
    CompressedModel->>CompressedModel: getCompressedNode(dataNode)
    Note over CompressedModel: 💥 dataNode not in this.nodes!<br/>TreeError thrown

Loading

Root cause detail: In AsyncDataTree.refreshAndRenderNode(), there is a two-phase update:

1. await this.refreshNode(...) — calls setChildren() which updates this.nodes (the async data tree's element → data-node map)
2. this.render(...) — calls tree.setChildren() which updates the compressed model's nodes map

Between steps 1 and 2, there is a microtask boundary (await). During this window, hasNode() returns true (element exists in async tree's map) but the compressed model's getCompressedNode() throws because it hasn't been updated yet.

Affected Files

File	Role
src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts	Crash site — reveal() at line 836 calls getRelativeTop() without guarding against inconsistent tree state
src/vs/workbench/contrib/chat/browser/widgetHosts/viewPane/chatViewPane.ts	Trigger — onDidChangeViewModel handler at line 607-609 calls reveal() during model switch
src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts	Event source — setModel() fires _onDidChangeViewModel at line 336
src/vs/base/browser/ui/tree/asyncDataTree.ts	Data flow — refreshAndRenderNode() has async gap between nodes map update and compressed model update
src/vs/base/browser/ui/tree/compressedObjectTreeModel.ts	Throw site — getCompressedNode() at line 347 throws when data node not found

Repro Steps

1. Open VS Code Insiders with the AgentSessions view in side-by-side mode
2. Have multiple agent sessions active
3. Rapidly switch between sessions, or trigger a session list update while switching models (e.g., create a new session and immediately navigate to it)
4. The error is a race condition, so it's non-deterministic. It's more likely when:
   • The session list is refreshing (e.g., after onDidChangeSessions triggers update())
   • A model change happens simultaneously (e.g., showModel() is called)
   • The tree data source has latency (widens the async gap)

Fix Proposal

Fix location: src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts → reveal()

Approach: Wrap the getRelativeTop() call in a try-catch to gracefully handle the case where the tree's internal state is temporarily inconsistent during an async refresh. When the error is caught, treat it the same as getRelativeTop() returning null (i.e., the element is not visible, so reveal it):

reveal(sessionResource: URI): boolean {
    if (!this.sessionsList) {
        return false;
    }

    const session = this.agentSessionsService.model.getSession(sessionResource);
    if (!session || !this.sessionsList.hasNode(session)) {
        return false;
    }

    try {
        if (this.sessionsList.getRelativeTop(session) === null) {
            this.sessionsList.reveal(session, 0.5);
        }
    } catch {
        // Tree model may be temporarily inconsistent during async refresh.
        // Silently skip — the next onDidChangeSessions will re-trigger reveal.
        return false;
    }

    this.sessionsList.setFocus([session]);
    this.sessionsList.setSelection([session]);

    return true;
}

Alternatively, a deeper fix could ensure refreshAndRenderNode() updates both maps atomically, but that would require changes to the shared tree infrastructure (asyncDataTree.ts) which could have wider impact.

Recommended Owner

@bpasero — Primary author of the agent sessions control code, the reveal() method, and the onDidChangeViewModel wiring in chatViewPane.ts. The current assignee @bryanchen-d may also be appropriate if they are now responsible for this area.

Generated by errors-investigate · ● 10.5M · ◷

[vs-code-engineering]

Agents Triage — Additional Error Bucket

These additional error buckets are tracked by this issue (commit 98b4d3e2):

Bucket	Error Message	Version
01aabbf8	TreeError [AgentSessionsView] Tree element not found	98b4d3e2

28 users, 51 hits on this build.

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Generated by errors-agents-triage · ● 19.2M · ◷