[Unhandled Error] Session type cannot be changed

[vs-code-engineering]

Error Message

Session type cannot be changed

Stack Trace

    at RGe.setSessionType (./src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessionsProvider.ts:1188:0)
    at Ymt.setSessionType (./src/vs/sessions/services/sessions/browser/sessionsManagementService.ts:268:34)
    at Object.onSelect (./src/vs/sessions/contrib/chat/browser/sessionTypePicker.ts:103:0)
    at wde.onListSelection (./src/vs/platform/actionWidget/browser/actionList.ts:1138:0)
    at Sye.value (./src/vs/platform/actionWidget/browser/actionList.ts:599:0)
    at T._deliver (./src/vs/base/common/event.ts:1354:12)
    at T._deliverQueue (./src/vs/base/common/event.ts:1365:8)
    at T.fire (./src/vs/base/common/event.ts:1389:8)
    at (anonymous) (./src/vs/base/common/event.ts:143:83)
    at Sye.value (./src/vs/base/common/event.ts:1784:15)
    at T._deliver (./src/vs/base/common/event.ts:1354:12)
    at T.fire (./src/vs/base/common/event.ts:1385:8)
    at Dai._set (./src/vs/base/browser/ui/list/listWidget.ts:186:17)
    at Dai.set (./src/vs/base/browser/ui/list/listWidget.ts:173:14)
    at Hl.setSelection (./src/vs/base/browser/ui/list/listWidget.ts:1734:17)
    at Yle.onViewPointer (./src/vs/base/browser/ui/list/listWidget.ts:793:14)
    at Sye.value (./src/vs/base/common/event.ts:194:84)
    at T._deliver (./src/vs/base/common/event.ts:1354:12)
    at T._deliverQueue (./src/vs/base/common/event.ts:1365:8)
    at T.fire (./src/vs/base/common/event.ts:1389:8)

Raw Stack Trace (minified)

Error: Session type cannot be changed     at RGe.setSessionType (out/vs/sessions/sessions.desktop.main.js:4165:22648)     at Ymt.setSessionType (out/vs/sessions/sessions.desktop.main.js:4178:69594)     at Object.onSelect (out/vs/sessions/sessions.desktop.main.js:4160:71002)     at wde.onListSelection (out/vs/sessions/sessions.desktop.main.js:852:253023)     at Sye.value (out/vs/sessions/sessions.desktop.main.js:852:245040)     at T._deliver (out/vs/sessions/sessions.desktop.main.js:405:3066)     at T._deliverQueue (out/vs/sessions/sessions.desktop.main.js:405:3155)     at T.fire (out/vs/sessions/sessions.desktop.main.js:405:3490)     at out/vs/sessions/sessions.desktop.main.js:403:11566     at Sye.value (out/vs/sessions/sessions.desktop.main.js:405:6625)     at T._deliver (out/vs/sessions/sessions.desktop.main.js:405:3066)     at T.fire (out/vs/sessions/sessions.desktop.main.js:405:3385)     at Dai._set (out/vs/sessions/sessions.desktop.main.js:685:38787)     at Dai.set (out/vs/sessions/sessions.desktop.main.js:685:38604)     at Hl.setSelection (out/vs/sessions/sessions.desktop.main.js:728:10407)     at Yle.onViewPointer (out/vs/sessions/sessions.desktop.main.js:685:48151)     at Sye.value (out/vs/sessions/sessions.desktop.main.js:403:11923)     at T._deliver (out/vs/sessions/sessions.desktop.main.js:405:3066)     at T._deliverQueue (out/vs/sessions/sessions.desktop.main.js:405:3155)     at T.fire (out/vs/sessions/sessions.desktop.main.js:405:3490)     at Sye.value (out/vs/sessions/sessions.desktop.main.js:403:11566)     at T._deliver (out/vs/sessions/sessions.desktop.main.js:405:3066)     at T.fire (out/vs/sessions/sessions.desktop.main.js:405:3385)     at HTMLDivElement.o (out/vs/sessions/sessions.desktop.main.js:483:63361)

Details

Property	Value
Bucket ID	06a20fb4-dd02-ba4d-964e-f7ebbdbeaf67
Version	1.116.0-insider
Commit	1ec46de1
Last Seen	Thu Apr 09 2026 23:47:57 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]

Regression Details

• Product: Agents
• Regression type: New error
• Last release without this error: 1.116.0-insider (0271b4d2)
• First release with this error: 1.116.0-insider (1ec46de1)
• Commit range: 0271b4d2...1ec46de1

Deminified Stack (top frames)

1. setSessionType — src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessionsProvider.ts:1188
2. setSessionType — src/vs/sessions/services/sessions/browser/sessionsManagementService.ts:268
3. onSelect — src/vs/sessions/contrib/chat/browser/sessionTypePicker.ts:103
4. onListSelection — src/vs/platform/actionWidget/browser/actionList.ts:1138

The error occurs when a user selects a session type from the picker but the session type change is rejected. 33 users affected across Mac and Windows.

Generated by errors-agents-regression-scan · ● 5.9M · ◷

[vs-code-engineering]

🔍 Error Investigation

Summary

The error "Session type cannot be changed" is thrown when a user clicks on the session type picker in the Agents chat UI and selects the currently-active (enabled) session type. The setSessionType method on CopilotChatSessionsProvider unconditionally throws because session type switching is not implemented, but the onSelect handler in the picker calls it without guarding against the no-op case (selecting the already-active type).

Culprit Commit

Field	Value
Commit	ef3548d7
Author	@sandy081
PR	#308770
Message	fix #308675
Why	This PR changed the session type picker to show all provider types (with unsupported ones disabled) instead of only supported ones. Before this change, the picker was hidden when only one type was supported, so users could never trigger setSessionType. After the change, the picker opens and the currently-active type is enabled/clickable, but clicking it still calls setSessionType which always throws.

Code Flow

sequenceDiagram
    participant User as User (click picker)
    participant Picker as SessionTypePicker
    participant Mgmt as SessionsManagementService
    participant Provider as CopilotChatSessionsProvider

    User->>Picker: Selects enabled session type
    Picker->>Picker: onSelect(type)
    Note over Picker: ⚠️ No guard: doesn't check<br/>if type === current type
    Picker->>Mgmt: setSessionType(session, type)
    Mgmt->>Provider: setSessionType(sessionId, type)
    Note over Provider: 💥 throw new Error(<br/>'Session type cannot be changed')

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Affected Files

File	Role
src/vs/sessions/contrib/chat/browser/sessionTypePicker.ts	Root cause — onSelect handler calls setSessionType without checking if type is already active
src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessionsProvider.ts	Crash site — setSessionType() throws unconditionally (line 1188)
src/vs/sessions/services/sessions/browser/sessionsManagementService.ts	Passthrough — routes setSessionType to provider (line 270)

Repro Steps

1. Open VS Code Insiders (Agents product, 1.116.0+)
2. Start a new session (Copilot CLI or Cloud)
3. Click the session type picker in the chat input area
4. Click the currently-active (enabled) session type in the dropdown
5. Error "Session type cannot be changed" is thrown as an unhandled error

Fix Proposal

Fix location: src/vs/sessions/contrib/chat/browser/sessionTypePicker.ts → _showPicker() → onSelect handler

Approach: Add a guard in the onSelect callback to skip calling setSessionType when the selected type is already the current session type. The handler at line 101–103 should check if (type.id !== this._sessionType) before calling sessionsManagementService.setSessionType(session, type). This prevents the no-op case from reaching the provider's throwing implementation.

An alternative/complementary fix would be to make CopilotChatSessionsProvider.setSessionType return the existing session when the requested type matches the current type, rather than throwing unconditionally.

Recommended Owner

@sandy081 — Author of the culprit commit (PR #308770) and primary maintainer of the sessions management and session type picker code.

Generated by errors-investigate · ● 7.5M · ◷