[Unhandled Error] Cannot read properties of undefined (reading 'currentModeObs')

[vs-code-engineering]

Error Message

Cannot read properties of undefined (reading 'currentModeObs')

Stack Trace

    at Ib._computeFn (./src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts:2575:0)
    at Ib._recompute (./src/vs/base/common/observableInternal/observables/derivedImpl.ts:201:23)
    at Ib.get (./src/vs/base/common/observableInternal/observables/derivedImpl.ts:159:10)
    at Ib.reportChanges (./src/vs/base/common/observableInternal/observables/baseObservable.ts:45:7)
    at OV.endUpdate (./src/vs/base/common/observableInternal/reactions/autorunImpl.ts:157:9)
    at Ib.endUpdate (./src/vs/base/common/observableInternal/observables/derivedImpl.ts:281:6)
    at Ib.endUpdate (./src/vs/base/common/observableInternal/observables/derivedImpl.ts:281:6)
    at Ib.endUpdate (./src/vs/base/common/observableInternal/observables/derivedImpl.ts:281:6)
    at _O.finish (./src/vs/base/common/observableInternal/transaction.ts:102:12)
    at VP.set (./src/vs/base/common/observableInternal/observables/observableValue.ts:77:8)
    at O7.delete (./src/vs/base/common/observableInternal/set.ts:37:13)
    at _ei._fn (./src/vs/workbench/contrib/chat/common/tools/languageModelToolsService.ts:434:0)
    at _ei.dispose (./src/vs/base/common/lifecycle.ts:396:7)
    at je (./src/vs/base/common/lifecycle.ts:339:7)
    at dispose (./src/vs/base/common/lifecycle.ts:458:3)
    at a.dispose (./src/vs/base/common/lifecycle.ts:439:7)
    at (anonymous) (./src/vs/workbench/contrib/mcp/common/mcpLanguageModelToolContribution.ts:130:0)
    at OV._runFn (./src/vs/workbench/contrib/mcp/common/mcpLanguageModelToolContribution.ts:130:0)
    at OV._run (./src/vs/base/common/observableInternal/reactions/autorunImpl.ts:110:10)
    at OV.endUpdate (./src/vs/base/common/observableInternal/reactions/autorunImpl.ts:167:11)

Raw Stack Trace (minified)

TypeError: Cannot read properties of undefined (reading 'currentModeObs')     at Ib._computeFn (out/vs/sessions/sessions.desktop.main.js:2371:77651)     at Ib._recompute (out/vs/sessions/sessions.desktop.main.js:407:12496)     at Ib.get (out/vs/sessions/sessions.desktop.main.js:407:11830)     at Ib.reportChanges (out/vs/sessions/sessions.desktop.main.js:407:497)     at OV.endUpdate (out/vs/sessions/sessions.desktop.main.js:407:7247)     at Ib.endUpdate (out/vs/sessions/sessions.desktop.main.js:407:13497)     at Ib.endUpdate (out/vs/sessions/sessions.desktop.main.js:407:13497)     at Ib.endUpdate (out/vs/sessions/sessions.desktop.main.js:407:13497)     at _O.finish (out/vs/sessions/sessions.desktop.main.js:406:109976)     at VP.set (out/vs/sessions/sessions.desktop.main.js:407:2882)     at O7.delete (out/vs/sessions/sessions.desktop.main.js:407:25843)     at _ei._fn (out/vs/sessions/sessions.desktop.main.js:944:2044)     at _ei.dispose (out/vs/sessions/sessions.desktop.main.js:403:1319)     at je (out/vs/sessions/sessions.desktop.main.js:403:747)     at a.clear (out/vs/sessions/sessions.desktop.main.js:403:1634)     at a.dispose (out/vs/sessions/sessions.desktop.main.js:403:1544)     at out/vs/sessions/sessions.desktop.main.js:3157:55860     at Map.forEach (<anonymous>)     at OV._runFn (out/vs/sessions/sessions.desktop.main.js:3157:55841)     at OV._run (out/vs/sessions/sessions.desktop.main.js:407:6694)     at OV.endUpdate (out/vs/sessions/sessions.desktop.main.js:407:7325)     at Ib.endUpdate (out/vs/sessions/sessions.desktop.main.js:407:13497)     at Ib.endUpdate (out/vs/sessions/sessions.desktop.main.js:407:13497)     at _O.finish (out/vs/sessions/sessions.desktop.main.js:406:109976)     at VP.set (out/vs/sessions/sessions.desktop.main.js:407:2882)     at Object.dispose (out/vs/sessions/sessions.desktop.main.js:3157:82360)     at Object.dispose (out/vs/sessions/sessions.desktop.main.js:3157:47427)     at Ot.deleteAndDispose (out/vs/sessions/sessions.desktop.main.js:403:4618)     at ZBe.sync (out/vs/sessions/sessions.desktop.main.js:3157:46796)     at out/vs/sessions/sessions.desktop.main.js:3157:45479     at up.queue (out/vs/sessions/sessions.desktop.main.js:406:73482)     at yye.value (out/vs/sessions/sessions.desktop.main.js:3157:45464)     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 CHe.onDidUninstallMcpServer (out/vs/sessions/sessions.desktop.main.js:3167:10508)     at yye.value (out/vs/sessions/sessions.desktop.main.js:3167:9269)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3066)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3385)     at yye.value (out/vs/sessions/sessions.desktop.main.js:2452:59748)     at E._deliver (out/vs/sessions/sessions.desktop.main.js:405:3066)     at E.fire (out/vs/sessions/sessions.desktop.main.js:405:3385)     at uat.removeWorkspaceService (out/vs/sessions/sessions.desktop.main.js:2452:71847)     at async Promise.allSettled (index 0)     at async uat.onDidChangeWorkspaceFolders (out/vs/sessions/sessions.desktop.main.js:2452:70017)

Details

Property	Value
Bucket ID	1a34f352-a2e8-1714-955d-0dd763ab3baa
Version	1.116.0-insider
Commit	47add6df
Last Seen	Sun Apr 12 2026 22:25:35 GMT+0000 (Coordinated Universal Time)
Platforms	Mac, Windows, Linux
Product	Agents

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

Regression Details

• Product: Agents
• Regression type: New error
• Last release without this error: 1.115.0 (41dd792b)
• First release with this error: 1.116.0-insider (fc6d18c3)
• Commit range: 41dd792b...fc6d18c3

Deminified Stack (top frames)

1. (anonymous) — src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts:2575
2. _recompute — src/vs/base/common/observableInternal/observables/derivedImpl.ts:201
3. get — src/vs/base/common/observableInternal/observables/derivedImpl.ts:159
4. reportChanges — src/vs/base/common/observableInternal/observables/baseObservable.ts:45
5. endUpdate — src/vs/base/common/observableInternal/reactions/autorunImpl.ts:157
6. ...
7. _fn — src/vs/workbench/contrib/chat/common/tools/languageModelToolsService.ts:434
8. dispose — src/vs/base/common/lifecycle.ts:396

The error occurs during observable recomputation in chatWidget.ts when accessing currentModeObs on an already-disposed object. The disposal is triggered by languageModelToolsService.ts cleaning up tool registrations, which cascades through observable updates. This is a disposal race condition — a derived observable reads a property that was nullified during cleanup. 24 users affected across Mac, Windows, Linux.

Related Bucket

Bucket b0ec104e has the same error message and top frame (chatWidget.ts:2575) but a different trigger path (tool registration flush vs disposal). 14+ users on the same build.

Generated by errors-agents-regression-scan · ● 6.5M · ◷

[vs-code-engineering]

🔍 Error Investigation

Summary

The error Cannot read properties of undefined (reading 'currentModeObs') occurs at chatWidget.ts:2575 inside a derived observable (scopedTools) that is created by getModeRequestOptions() and passed to the chat service. When MCP tools are unregistered (e.g., due to a workspace folder change removing an MCP server), the observable re-evaluates on an already-disposed ChatWidget, where this.input returns undefined because inputPartDisposable has been cleared. This is a use-after-dispose race condition affecting 24+ users across all platforms in the Agents product.

Culprit Commit

No single culprit commit could be definitively identified. The scopedTools derived observable was introduced in PR #301381 (1.112.0, by @roblourens) and the autorun that keeps it subscribed in chatServiceImpl.ts:1227-1239 was part of the same PR. The bug is latent — it surfaces when a ChatWidget is disposed while the chat service still holds an active subscription to scopedTools via the autorun, and an MCP tool change triggers re-evaluation. Possible triggers within the regression range (41dd792b..fc6d18c3) include:

• bda49a1b — Enabled chat.customizations.providerApi.enabled by default, potentially activating new MCP tool management code paths
• Multiple sessions/workspace management changes that could alter widget disposal timing relative to MCP server lifecycle

Code Flow

sequenceDiagram
    participant WF as WorkspaceFolders
    participant MCP as McpWorkbenchService
    participant Contrib as McpLMToolContribution
    participant ToolSvc as LanguageModelToolsService
    participant ChatSvc as ChatServiceImpl (autorun)
    participant Widget as ChatWidget (disposed)

    WF->>MCP: onDidChangeWorkspaceFolders
    MCP->>MCP: removeWorkspaceService
    MCP->>Contrib: onDidUninstallMcpServer → sync()
    Note over Contrib: collection is null →<br/>tools.forEach(t ⇒ t.store.dispose())
    Contrib->>ToolSvc: _tools.delete(data) [line 434]
    Note over ToolSvc: ⚠️ Observable set change<br/>propagates through dependency graph
    ToolSvc->>ChatSvc: autorun triggered [line 1227]
    ChatSvc->>Widget: scopedTools derived recomputes
    Note over Widget: 💥 this.input is undefined<br/>(inputPartDisposable cleared)<br/>Cannot read 'currentModeObs'

Loading

Affected Files

File	Role
src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts	Crash site — scopedTools derived at line 2575 accesses this.input.currentModeObs without null check
src/vs/workbench/contrib/chat/common/chatService/chatServiceImpl.ts	Subscription holder — autorun at line 1227 keeps scopedTools subscribed after widget disposal
src/vs/workbench/contrib/chat/browser/widget/input/chatSelectedTools.ts	Data flow — userSelectedTools derived depends on tool registry
src/vs/workbench/contrib/chat/common/tools/languageModelToolsService.ts	Data flow — _tools.delete() at line 434 triggers observable propagation
src/vs/workbench/contrib/mcp/common/mcpLanguageModelToolContribution.ts	Trigger — autorun at line 112 disposes tool stores when MCP server is removed

Repro Steps

1. Open VS Code Agents with a multi-folder workspace that has MCP servers configured
2. Start a chat session and send a request (this creates scopedTools via getModeRequestOptions())
3. While the request is active (or the service still holds the autorun), change workspace folders (e.g., remove a folder that has an MCP server configured)
4. The MCP server uninstall triggers tool disposal, cascading through observables to the scopedTools derived on the disposed widget
5. Error is non-deterministic — requires the widget to be disposed (e.g., navigated away from) while the service still holds the autorun subscription

Fix Proposal

Fix location: src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts → getModeRequestOptions(), the scopedTools derived computation

Approach: Add a guard clause at the top of the scopedTools derived computation (line 2573-2582) to check whether this.input is still available before accessing currentModeObs. If this.input is undefined (widget disposed), return lastToolsSnapshot immediately. This prevents the crash while preserving the last known tools state:

const scopedTools = derived(reader => {
    const input = this.input;          // ← add null check
    if (!input) { return lastToolsSnapshot; }
    const activeSession = this._viewModelObs.read(reader)?.sessionResource;
    const currentModeId = input.currentModeObs.read(reader).id;
    // ... rest unchanged
});

Additionally, consider disposing the autorun in chatServiceImpl.ts:1227 when the widget is disposed, so the scopedTools derived is no longer subscribed.

Recommended Owner

@roblourens — author of the scopedTools derived observable (PR #301381) and primary maintainer of the chat widget + chat service request flow. The fix is in chatWidget.ts which is core chat widget infrastructure.

Generated by errors-investigate · ● 15.7M · ◷