Agent Host "Allow remote session access" hangs and triggers Unexpected end of JSON input on VS Code 1.133.0

Does this issue occur when all extensions are disabled?: Not tested yet. The failing UI/action appears to be part of VS Code's built-in Agent Host / Remote Connections functionality.

- VS Code Version: 1.133.0
- Commit: a5b500951314efd502d07465bd138dfbd714a960
- OS Version: Windows 10 x64

Steps to Reproduce:

1. Open VS Code and start/open a session using the **Copilot** agent harness.
2. Confirm that the Agent Host session itself works normally.
3. Click the **Allow remote session access** radio-tower icon shown at the bottom of the Copilot session.
4. Observe the `Remote Connections` output.
5. Observe the `Main` log at the same time.

## Actual Result

`Remote Connections` immediately logs:

```text
2026-08-15 15:30:35.082 [info] Starting tunnel hosting...
```

At essentially the same timestamp, `Main` logs the following from `AgentHost:stderr`:

```text
2026-08-15 15:30:35.083 [error] [AgentHost:stderr] SyntaxError: Unexpected end of JSON input
at JSON.parse ()
at fl (file:///C:/Users//AppData/Local/Programs/Microsoft%20VS%20Code//resources/app/out/vs/platform/agentHost/node/agentHostMain.js:433:119096)
at fl (file:///C:/Users//AppData/Local/Programs/Microsoft%20VS%20Code//resources/app/out/vs/platform/agentHost/node/agentHostMain.js:433:119061)
at Du.onRawMessage (file:///C:/Users//AppData/Local/Programs/Microsoft%20VS%20Code//resources/app/out/vs/platform/agentHost/node/agentHostMain.js:433:120000)
at el.value (file:///C:/Users//AppData/Local/Programs/Microsoft%20VS%20Code//resources/app/out/vs/platform/agentHost/node/agentHostMain.js:433:119386)
at k._deliver (file:///C:/Users//AppData/Local/Programs/Microsoft%20VS%20Code//resources/app/out/vs/platform/agentHost/node/agentHostMain.js:433:3617)
at k._deliverQueue (file:///C:/Users//AppData/Local/Programs/Microsoft%20VS%20Code//resources/app/out/vs/platform/agentHost/node/agentHostMain.js:433:3706)
at k.fire (file:///C:/Users//AppData/Local/Programs/Microsoft%20VS%20Code//resources/app/out/vs/platform/agentHost/node/agentHostMain.js:433:4040)
at MessagePortMain.P (file:///C:/Users//AppData/Local/Programs/Microsoft%20VS%20Code//resources/app/out/vs/platform/agentHost/node/agentHostMain.js:431:13792)
at MessagePortMain.emit (node:events:509:28)
at Object.emit (node:electron/js2c/utility_init:2:2934)
```

A second related stack is logged at the same timestamp through `onBuffer`:

```text
SyntaxError: Unexpected end of JSON input
at JSON.parse ()
...
at pl.onBuffer (...)
...
at MessagePortMain.P (...)
```

The remote connection never finishes establishing. The radio-tower icon remains in the **Establishing tunnel connection...** state.

I can reproduce this consistently: clicking **Allow Remote Connections** immediately triggers the JSON parsing error.

## Expected Result

Clicking **Allow Remote Connections** should successfully enable remote session access and eventually report that the tunnel/remote connection is ready, instead of remaining stuck at:

```text
Starting tunnel hosting...
```

## Additional Information

The Agent Host itself otherwise starts and works normally. For example:

```text
Agent Host process started successfully
AgentService initialized
Registering agent provider: copilotcli
[ProtocolServer] Initialize: clientId=..., protocolVersions=[0.8.0, 0.7.0, 0.6.0, 0.5.2, 0.5.1]
[WebSocketProtocol] Server listening on socket \\.\pipe\vscode-agent-host-...
[Copilot] CopilotClient started successfully
[Copilot] Found 1 sessions
[AgentService] Restored session ...
[Copilot] Found multiple available models
```

Normal Copilot Agent Host sessions continue to work. The failure appears specifically when enabling **Allow remote session access**.

I also tested the browser Agents window. Since the remote connection never finishes establishing, it continues to show:

```text
No Host
```

At the time of the repro, the standard VS Code Remote Tunnel CLI was not running:

```powershell
code tunnel status
```

returned:

```json
{"tunnel":null,"service_installed":false}
```

The `Remote Tunnel Service` log also reports:

```text
[info] No other tunnel running
```

This may be related to other recent Agent Host `Unexpected end of JSON input` / MessagePort deserialization reports, but I cannot confirm that the root cause is the same.

I can provide additional trace logs or test an Insiders/candidate build if useful.

Reproduce the failure by using the Agent Host's Allow remote session access action, then compare the Remote Connections and Main logs. Start with the AgentHost:stderr and onBuffer paths shown in agentHostMain.js, and use code tunnel status as a baseline; done means the tunnel reaches a ready state without Unexpected end of JSON input.

Written by the indexing model from the issue text.