[Unhandled Error] potential listener LEAK detected, popular

[vs-code-engineering]

Error Message

potential listener LEAK detected, popular

Stack Trace

    at a.create (./src/vs/base/common/event.ts:1062:14)
    at hR._event [as event] (./src/vs/base/common/event.ts:1262:33)
    at hR.onFirstObserverAdded (./src/vs/base/common/observableInternal/observables/observableFromEvent.ts:95:28)
    at hR.addObserver (./src/vs/base/common/observableInternal/observables/baseObservable.ts:161:8)
    at Object.onWillAddFirstListener (./src/vs/base/common/event.ts:804:17)
    at HJe._event [as onDidChange] (./src/vs/base/common/event.ts:1271:19)
    at new bp (./src/vs/editor/common/model/textModel.ts:354:65)
    at a._createInstance (./src/vs/platform/instantiation/common/instantiationService.ts:162:17)
    at a.createInstance (./src/vs/platform/instantiation/common/instantiationService.ts:128:17)
    at Gtt._createModelData (./src/vs/editor/common/services/modelService.ts:310:54)
    at Gtt.createModel (./src/vs/editor/common/services/modelService.ts:444:20)
    at y6.getSnapshotModel (./src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingSession.ts:401:0)
    at async Vtt.ensureResolvedTextModelContent (./src/vs/workbench/services/textmodelResolver/common/textModelResolverService.ts:192:7)
    at async Vtt.doCreateReferencedObject (./src/vs/workbench/services/textmodelResolver/common/textModelResolverService.ts:65:3)
    at async pye.acquire (./src/vs/base/common/lifecycle.ts:727:18)
    at async Btt.createModelReference (./src/vs/workbench/services/textmodelResolver/common/textModelResolverService.ts:242:9)

Raw Stack Trace (minified)

Error     at a.create (out/vs/sessions/sessions.desktop.main.js:403:18382)     at hR._event [as event] (out/vs/sessions/sessions.desktop.main.js:405:1807)     at hR.onFirstObserverAdded (out/vs/sessions/sessions.desktop.main.js:407:19307)     at hR.addObserver (out/vs/sessions/sessions.desktop.main.js:407:1590)     at Object.onWillAddFirstListener (out/vs/sessions/sessions.desktop.main.js:403:16116)     at HJe._event [as onDidChange] (out/vs/sessions/sessions.desktop.main.js:405:2074)     at new bp (out/vs/sessions/sessions.desktop.main.js:668:36702)     at a._createInstance (out/vs/sessions/sessions.desktop.main.js:1466:2000)     at a.createInstance (out/vs/sessions/sessions.desktop.main.js:1466:1440)     at Gtt._createModelData (out/vs/sessions/sessions.desktop.main.js:1371:2490)     at Gtt.createModel (out/vs/sessions/sessions.desktop.main.js:1372:898)     at y6.getSnapshotModel (out/vs/sessions/sessions.desktop.main.js:3047:26087)     at async Vtt.ensureResolvedTextModelContent (out/vs/sessions/sessions.desktop.main.js:1347:29274)     at async Vtt.doCreateReferencedObject (out/vs/sessions/sessions.desktop.main.js:1347:28023)     at async pye.acquire (out/vs/sessions/sessions.desktop.main.js:403:3866)     at async Btt.createModelReference (out/vs/sessions/sessions.desktop.main.js:1347:30166)     at async Promise.all (index 1)

Details

Property	Value
Bucket ID	245a5f54-7614-5f79-58c8-7675c4124e70
Version	1.116.0-insider
Commit	1dfb3b9d
Last Seen	Mon Apr 13 2026 22:09:42 GMT+0000 (Coordinated Universal Time)
Platforms	Mac, Windows
Product	Agents

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

🔍 Error Investigation

Summary

The LanguageService.onDidChange emitter (leak threshold: 200) is exceeded when the chat editing system creates many snapshot TextModel instances simultaneously. Each snapshot model subscribes to this shared event through the ILanguageSelection.onDidChange → observableFromEvent → LanguageService.onDidChange chain. In Agents sessions with many edited files and checkpoints, the cumulative listener count crosses the 200-listener warning threshold, triggering this "popular" listener leak error.

Culprit Commit

No single culprit commit was identified. This is a systemic issue that emerged with the growth of agent session file counts. The error bucket first appeared in version 1.114.0-insider (April 7) and has been steadily increasing:

• 1.114.0: ~6 hits
• 1.115.0: ~42 hits
• 1.116.0: ~119 hits (latest)

The escalation correlates with increased Agents product adoption and the agent host editing infrastructure introduced in PRs #304303, #305348, and #305861 by @connor4312 (merged late March, milestone 1.114.0), which expanded the number of snapshot models created per session.

Code Flow

sequenceDiagram
    participant Resolver as textModelResolverService
    participant Provider as ChatEditingSnapshotTextModelContentProvider
    participant Session as ChatEditingSession.getSnapshotModel
    participant ModelSvc as ModelService.createModel
    participant TextModel as TextModel constructor
    participant LangSel as LanguageSelection (observableFromEvent)
    participant LangSvc as LanguageService.onDidChange

    Resolver->>Provider: provideTextContent(snapshotUri)
    Provider->>Session: getSnapshotModel(requestId, undoStop, uri)
    Session->>ModelSvc: createModel(content, createByFilepathOrFirstLine(uri), uri)
    Note over Session: ⚠️ Root cause: creates ILanguageSelection<br/>which subscribes to shared event
    ModelSvc->>TextModel: new TextModel(value, languageSelection, ...)
    TextModel->>LangSel: languageSelection.onDidChange(callback)
    LangSel->>LangSvc: subscribe to onDidChange (threshold: 200)
    Note over LangSvc: 💥 ListenerLeakError thrown<br/>when listeners > 200

Loading

Affected Files

File	Role
src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingSession.ts	Root cause — getSnapshotModel() at line 401 creates models with ILanguageSelection
src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingTextModelContentProviders.ts	Entry point — ChatEditingSnapshotTextModelContentProvider.provideTextContent()
src/vs/editor/common/services/languageService.ts	Shared emitter — onDidChange has leakWarningThreshold: 200
src/vs/editor/common/model/textModel.ts	Subscription site — constructor subscribes to languageSelection.onDidChange (line 354)
src/vs/editor/common/services/modelService.ts	Model creation — _createModelData passes ILanguageSelection to TextModel

Repro Steps

1. Open VS Code Agents (Insiders)
2. Start an agent session that edits many files (20+ files across multiple checkpoints)
3. Navigate the chat editing timeline or view diffs that resolve snapshot URIs
4. Each snapshot model resolution adds a listener to LanguageService.onDidChange
5. When total live snapshot models exceed ~200, the listener leak error fires
6. This is more likely with longer agent sessions that accumulate many checkpoints across many files

Fix Proposal

Fix location: src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingSession.ts → getSnapshotModel()

Approach: On line 401, replace this._languageService.createByFilepathOrFirstLine(snapshotUri) with a resolved language ID string. The modelService.createModel → _createModelData method accepts string | ILanguageSelection as the languageIdOrSelection parameter. When a plain string is passed, the TextModel constructor at line 352 takes the string branch and does not subscribe to onDidChange, completely avoiding the listener accumulation.

Snapshot models are read-only representations of past content — they do not need to react to language registration changes. The change would be:

• Call this._languageService.createByFilepathOrFirstLine(snapshotUri).languageId to get the resolved language ID string
• Pass that string directly to createModel() instead of the ILanguageSelection object

This eliminates the per-model subscription to the shared emitter while preserving correct syntax highlighting.

Recommended Owner

@connor4312 — Primary author of the chat editing session and agent host editing infrastructure (PRs #304303, #305348, #305861). The fix is in the getSnapshotModel method of ChatEditingSession which is core to the snapshot model creation path.

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