[Error] unhandlederror-Unknown folder

[vs-code-engineering]

Error Message

unhandlederror-Unknown folder

Stack Trace

    at bqe.compareAndDeleteFolderConfiguration (./src/vs/platform/configuration/common/configurationModels.ts:946:9)
    at bqe.compareAndDeleteFolderConfiguration (./src/vs/workbench/services/configuration/common/configurationModels.ts:140:15)
    at dXt.onWorkspaceFoldersChanged (./src/vs/sessions/services/configuration/browser/configurationService.ts:262:38)
    at cye.value (./src/vs/sessions/services/configuration/browser/configurationService.ts:92:77)
    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 pXt._doUpdateFolders (./src/vs/sessions/services/workspace/browser/workspaceContextService.ts:163:36)

Raw Stack Trace (minified)

Error: Unknown folder     at bqe.compareAndDeleteFolderConfiguration (out/vs/sessions/sessions.desktop.main.js:4545:48374)     at bqe.compareAndDeleteFolderConfiguration (out/vs/sessions/sessions.desktop.main.js:4547:783)     at dXt.onWorkspaceFoldersChanged (out/vs/sessions/sessions.desktop.main.js:4547:33613)     at cye.value (out/vs/sessions/sessions.desktop.main.js:4547:29993)     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 pXt._doUpdateFolders (out/vs/sessions/sessions.desktop.main.js:4549:2992)

Details

Property	Value
Bucket ID	86be8097-e849-d0cf-44ca-970ddb183ce0
Version	1.115.0-insider
Commit	41dd792b
Last Seen	Tue Apr 14 2026 17:59:52 GMT+0000 (Coordinated Universal Time)
Platforms	Linux, Windows, Mac
Product	Agents

---

This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

🔍 Error Investigation

Summary

The sessions ConfigurationService throws "Unknown folder" when a workspace folder is removed before its configuration has been loaded into the internal folderConfigurations map. This is a race condition inherent to the sessions configuration service design — the onWorkspaceFoldersChanged handler iterates e.removed folders and calls compareAndDeleteFolderConfiguration(), which throws if the folder config was never loaded. The error affects the Agents product across all platforms (Linux, Windows, Mac) and has been present since version 1.114.0-insider (March 27, 2026), with increasing hit counts as adoption grows — currently ~600+ hits / 268 users on the latest 1.116 insider builds.

Culprit Commit

No single culprit commit — this is an inherent design bug in the sessions ConfigurationService introduced when the service was originally created. PR #299302 by @sandy081 ("fix reading configuration from workspace folders", merged March 5, 2026) is the closest related PR. The error has been present across versions 1.114 through 1.117, hitting consistently on every insider build.

Code Flow

sequenceDiagram
    participant Caller as Workspace Consumer
    participant WCS as SessionsWorkspaceContextService
    participant CS as Sessions ConfigurationService
    participant Cfg as Configuration (platform)

    Caller->>WCS: addFolders([A]) / updateFolders(...)
    WCS->>CS: onWillChangeWorkspaceFolders (added: [A])
    CS->>CS: loadFolderConfigurations([A]) (async)
    Note over CS: ⏳ Awaiting folder config load...
    WCS->>WCS: workspace.update(newWorkspace)
    WCS->>CS: onDidChangeWorkspaceFolders (removed: [])
    Note over CS: ✅ No removals, OK

    Caller->>WCS: removeFolders([A])
    WCS->>CS: onWillChangeWorkspaceFolders (added: [])
    Note over CS: Nothing to load
    WCS->>WCS: workspace.update(newWorkspace)
    WCS->>CS: onDidChangeWorkspaceFolders (removed: [A])
    CS->>Cfg: compareAndDeleteFolderConfiguration(A)
    Note over Cfg: ⚠️ folderConfigurations.get(A)<br/>returns undefined if config<br/>never loaded (race/init timing)
    Note over Cfg: 💥 throw new Error('Unknown folder')
```

### Affected Files

| File | Role |
|------|------|
| `src/vs/platform/configuration/common/configurationModels.ts` | 💥 Crash site — `compareAndDeleteFolderConfiguration()` throws at line 946 |
| `src/vs/workbench/services/configuration/common/configurationModels.ts` | Data flow — override delegates to super at line 140 |
| `src/vs/sessions/services/configuration/browser/configurationService.ts` | ⚠️ Root cause — `onWorkspaceFoldersChanged()` at line 262 iterates `e.removed` unsafely |
| `src/vs/sessions/services/workspace/browser/workspaceContextService.ts` | Trigger — `_doUpdateFolders()` fires events at line 163 |

### Repro Steps

1. Open VS Code Insiders with the Agents product (sessions enabled)
2. Create or enter a multi-folder workspace in a session
3. Rapidly add and remove workspace folders (or trigger a workspace update that removes a folder before its configuration has been fully loaded)
4. The error fires when a removed folder's configuration was never loaded into the `folderConfigurations` map — this can happen if:
   - `initialize()` hasn't completed when a folder removal event arrives
   - `loadFolderConfigurations()` fails or is still in-flight when a removal occurs
   - The `_configuration` object is replaced by `initialize()` (line 102-114) while folder changes are in progress

This is non-deterministic and depends on timing between async config loading and folder removal events.

### Fix Proposal

**Fix location**: `src/vs/sessions/services/configuration/browser/configurationService.ts` → `onWorkspaceFoldersChanged()`

**Approach**: Follow the pattern used by the workbench `ConfigurationService.onFoldersChanged()` (line 944-962 of `src/vs/workbench/services/configuration/browser/configurationService.ts`). Instead of iterating `e.removed` and calling `compareAndDeleteFolderConfiguration()` directly, iterate over `this.cachedFolderConfigs.keys()` and only delete folders that are no longer in the workspace. This ensures only folders whose configs were actually loaded are deleted:

```
// Current (unsafe) pattern in sessions ConfigurationService:
for (const folder of e.removed) {
    const change = this._configuration.compareAndDeleteFolderConfiguration(folder.uri);  // throws!
}

// Suggested fix (safe pattern from workbench ConfigurationService):
for (const key of this.cachedFolderConfigs.keys()) {
    if (!this.workspace.folders.some(f => f.uri.toString() === key.toString())) {
        this.cachedFolderConfigs.deleteAndDispose(key);
        changes.push(this._configuration.compareAndDeleteFolderConfiguration(key));
    }
}

Loading

This guarantees compareAndDeleteFolderConfiguration is only called for folders that have entries in the folderConfigurations map, eliminating the race condition.

Recommended Owner

@sandy081 — Original author of the sessions ConfigurationService and configuration infrastructure (PR #299302), and primary contributor to sessions workspace/configuration services.

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