[Error] unhandlederror-[LanguageService._onDidChange] potential listener LEAK detected, popular — textModel / modelService

[vs-code-engineering]

Error Message

unhandlederror-[LanguageService._onDidChange] potential listener LEAK detected, popular

Stack Trace

    at a.create (./src/vs/base/common/event.ts:1068:14)
    at oR._event [as event] (./src/vs/base/common/event.ts:1272:33)
    at oR.onFirstObserverAdded (./src/vs/base/common/observableInternal/observables/observableFromEvent.ts:95:28)
    at oR.addObserver (./src/vs/base/common/observableInternal/observables/baseObservable.ts:161:8)
    at Object.onWillAddFirstListener (./src/vs/base/common/event.ts:804:17)
    at vXe._event [as onDidChange] (./src/vs/base/common/event.ts:1281:19)
    at new hp (./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 yet._createModelData (./src/vs/editor/common/services/modelService.ts:310:54)
    at yet.createModel (./src/vs/editor/common/services/modelService.ts:444:20)
    at G0.doCreateTextEditorModel (./src/vs/workbench/common/editor/textEditorModel.ts:172:0)
    at G0.createTextEditorModel (./src/vs/workbench/common/editor/textEditorModel.ts:166:0)
    at G0.doCreateTextModel (./src/vs/workbench/services/textfile/common/textFileEditorModel.ts:565:0)
    at G0.resolveFromContent (./src/vs/workbench/services/textfile/common/textFileEditorModel.ts:547:0)
    at G0.resolveFromFile (./src/vs/workbench/services/textfile/common/textFileEditorModel.ts:471:0)
    at async G0.resolve (./src/vs/workbench/services/textfile/common/textFileEditorModel.ts:307:0)
    at async PGe.doResolve (./src/vs/workbench/services/textfile/common/textFileEditorModelManager.ts:446:0)
    at async get.doCreateReferencedObject (./src/vs/workbench/services/textmodelResolver/common/textModelResolverService.ts:57:17)
    at async sIe.acquire (./src/vs/base/common/lifecycle.ts:727:18)

Raw Stack Trace (minified)

Error     at a.create (out/vs/workbench/workbench.desktop.main.js:403:18436)     at oR._event [as event] (out/vs/workbench/workbench.desktop.main.js:405:2009)     at oR.onFirstObserverAdded (out/vs/workbench/workbench.desktop.main.js:407:19701)     at oR.addObserver (out/vs/workbench/workbench.desktop.main.js:407:1590)     at Object.onWillAddFirstListener (out/vs/workbench/workbench.desktop.main.js:403:16116)     at vXe._event [as onDidChange] (out/vs/workbench/workbench.desktop.main.js:405:2276)     at new hp (out/vs/workbench/workbench.desktop.main.js:668:36700)     at a._createInstance (out/vs/workbench/workbench.desktop.main.js:1904:2000)     at a.createInstance (out/vs/workbench/workbench.desktop.main.js:1904:1440)     at yet._createModelData (out/vs/workbench/workbench.desktop.main.js:1809:2490)     at yet.createModel (out/vs/workbench/workbench.desktop.main.js:1810:898)     at G0.doCreateTextEditorModel (out/vs/workbench/workbench.desktop.main.js:936:15249)     at G0.createTextEditorModel (out/vs/workbench/workbench.desktop.main.js:936:15075)     at G0.doCreateTextModel (out/vs/workbench/workbench.desktop.main.js:1785:17780)     at G0.resolveFromContent (out/vs/workbench/workbench.desktop.main.js:1785:17617)     at G0.resolveFromFile (out/vs/workbench/workbench.desktop.main.js:1785:16719)     at async G0.resolve (out/vs/workbench/workbench.desktop.main.js:1785:14593)     at async PGe.doResolve (out/vs/workbench/workbench.desktop.main.js:4910:8863)     at async get.doCreateReferencedObject (out/vs/workbench/workbench.desktop.main.js:1785:27870)     at async sIe.acquire (out/vs/workbench/workbench.desktop.main.js:403:3866)     at async fet.createModelReference (out/vs/workbench/workbench.desktop.main.js:1785:30166)     at async ZTe._handleAsResourceInput (out/vs/workbench/workbench.desktop.main.js:969:11608)     at async ZTe.$tryOpenDocument (out/vs/workbench/workbench.desktop.main.js:969:10864)

Details

Property	Value
Bucket ID	cc6cf24d-f55f-96d1-c890-7679576b48af
Version	1.118.0-insider
Commit	659d444a
Last Seen	Thu Apr 23 2026 23:58:17 GMT+0000 (Coordinated Universal Time)
Platforms	Windows, Mac, Linux
Product	VSCode

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

Regression Details

• Product: VSCode
• Regression type: new
• Last release without this error: 1.117.0-insider (7f0183b, Apr 21)
• First release with this error: 1.118.0-insider (102df5b, Apr 22)
• Commit range: 7f0183b5...102df5b5
• Users on target build: 73

Coalesced Buckets

This issue also tracks bucket 80eaae65 (34 users), which shares the same component path (textModel / modelService) but enters via chatEditingSession.ts instead of textFileEditorModel.ts.

Deminified Stack Summary

The emitter is LanguageService._onDidChange (textModel.ts:354). The subscription chain is:

1. event.ts → observableInternal → leak monitoring fires
2. textModel.ts → constructor subscribes to LanguageService.onDidChange
3. modelService.ts → _createModelData / createModel
4. textEditorModel.ts → textFileEditorModel.ts → file resolve path

This is a popular leak — multiple subscribers on the LanguageService.onDidChange emitter.

Generated by errors-regression-scan

Generated by errors-regression-scan · ● 10.1M · ◷

[vs-code-engineering]

🔍 Error Investigation

Summary

The [LanguageService._onDidChange] potential listener LEAK detected error fires when more than 200 LanguageSelection objects simultaneously subscribe to the shared LanguageService._onDidChange emitter (threshold: 200). This is a pre-existing listener accumulation issue that appeared as a new telemetry bucket because PR #311728 changed the error message format by adding the [LanguageService._onDidChange] prefix, creating a new bucket ID. The leak is driven primarily by chat editing sessions creating original-snapshot models with languageService.createById(), each adding a listener that doesn't need to track language changes.

Culprit Commit

Field	Value
Commit	4dabc9bb
Author	@bryanchen-d
PR	#311728
Message	feat: add leakWarningName to emitters for better leak error telemetry grouping
Why	Added leakWarningName: 'LanguageService._onDidChange' to the emitter, changing the error message from "potential listener LEAK detected, popular" to "[LanguageService._onDidChange] potential listener LEAK detected, popular". This created a new telemetry bucket for a pre-existing leak, making it appear as a recent regression.

Note: This commit did not introduce the leak itself — it made an existing leak visible as a new bucket by changing the error message format.

Code Flow

sequenceDiagram
    participant CES as ChatEditingSession<br/>_initEntries()
    participant CMDE as ChatEditingModified<br/>DocumentEntry
    participant LS as LanguageService<br/>createById()
    participant MS as ModelService<br/>createModel()
    participant TM as TextModel<br/>constructor
    participant Emitter as LanguageService<br/>._onDidChange

    CES->>CMDE: create entry per modified file
    CMDE->>LS: createById(languageId)
    Note over LS: Creates new LanguageSelection<br/>with observableFromEvent
    LS-->>CMDE: languageSelection
    CMDE->>MS: createModel(snapshot, languageSelection)
    MS->>TM: new TextModel(value, languageSelection)
    TM->>Emitter: subscribe to onDidChange
    Note over Emitter: ⚠️ Listener count++<br/>When ≥200: LEAK warning
    Note over Emitter: 💥 ListenerLeakError thrown

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Affected Files

File	Role
src/vs/base/common/event.ts:1068	Crash site — Stacktrace.create() called during leak detection
src/vs/editor/common/services/languageService.ts:26	Root cause — _onDidChange emitter with leakWarningThreshold: 200
src/vs/editor/common/model/textModel.ts:354	Subscription site — each TextModel subscribes to languageSelection.onDidChange
src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingModifiedDocumentEntry.ts:126-130	Primary leak contributor — creates original models with languageService.createById() for snapshots that don't need language-change tracking
src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingSession.ts:977-1017	_initEntries() — creates models for all session entries during restore
src/vs/editor/common/services/modelService.ts:310	_createModelData() — instantiates TextModel with language selection

Related Buckets

This leak manifests across 5 separate buckets (same emitter, different call paths):

Bucket	Hits	Path
cc6cf24d	5,675	Chat editing _createModifiedFileEntry → _initEntries
723457ca	2,369	Regular document opening via textFileEditorModel (this issue)
e1c27ce3	157	Chat content rendering renderTextEdit
f7135491	24	Resource labels in list views
17a023da	19	Agent tool invocation _fillInResourceGroup

Repro Steps

1. Open VS Code Insiders (1.118.0+)
2. Start a Copilot chat editing session that modifies many files (50+)
3. The chat editing session creates an "original" snapshot model per file, each subscribing to LanguageService._onDidChange
4. Combined with regular open text models and other subscribers, the total exceeds 200
5. The leak warning fires on the next model creation

The error is more likely with large workspaces or multiple active chat editing sessions.

Fix Proposal

Fix location 1 (primary): src/vs/workbench/contrib/chat/browser/chatEditing/chatEditingModifiedDocumentEntry.ts → constructor (line ~126-130)

Approach: The original-snapshot model is read-only and does not need to react to language registry changes. Instead of calling languageService.createById(languageId) (which creates a LanguageSelection that subscribes to _onDidChange), pass null as the language selection and immediately set the language on the created model via model.setLanguage(languageId). This avoids adding a listener to the shared emitter. Alternatively, modify IModelService.createModel to accept a plain string language ID.

Fix location 2 (secondary): src/vs/editor/common/services/languageService.ts:26

Approach: Consider increasing the leakWarningThreshold from 200 to a higher value (e.g., 400) to accommodate the legitimate listener count from chat editing sessions, or make it configurable. The current threshold of 200 was set per #119968 before chat editing existed.

Recommended Owner

@bryanchen-d — culprit commit author (PR #311728 that surfaced this bucket). The underlying fix for the leak's largest contributor (chat editing sessions) may require coordination with the chat editing code owners.

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