Windows: Entra broker token cache stays 0 bytes after MCP sign-in, so every new process re-prompts #5236

Describe the bug

After I sign in through the Microsoft Entra account broker for an HTTP MCP server, the token cache file stays empty. Every new process shows the account picker again. A /restart in the same process does not prompt.

~\.copilot\entra-broker-accounts\<id>.bin is written (1,974 bytes) when I pick the account. ~\.copilot\entra-broker-cache\<id>.bin stays at 0 bytes. I did not read the file contents.

Affected version

1.0.103

Steps to reproduce the behavior

1. Add an HTTP MCP server that uses Entra sign-in to ~\.copilot\mcp-config.json: {"mcpServers": {"EngHub": {"type": "http", "url": "https://mcp.eng.ms", "tools": ["*"]}}}
2. Start a session. The CLI shows Authenticating: EngHub and the Windows "Pick an account" dialog.
3. Pick the signed-in account. Authentication succeeds.
4. Check the file sizes in ~\.copilot\entra-broker-cache\ and ~\.copilot\entra-broker-accounts\. The cache file is 0 bytes.
5. Exit and start a new session. The account picker shows again.
6. Run /restart in a running session. No prompt shows.

Expected behavior

The token is saved in entra-broker-cache. A new process signs in silently without the account picker.

Additional context

• OS: Windows, x64
• Shell: PowerShell
• Launched with agency copilot (Agency 2026.10.20.6)
• I deleted the empty cache file and signed in again. It was recreated at 0 bytes.
• Related but different: Remote MCP OAuth: silent refresh fails with AADSTS70011 (refresh reuses granted scope mixing .default + resource-specific scopes), forcing repeated interactive sign-in #4464 (repeated interactive sign-in from refresh scope errors), Windows: MCP Entra sign-in fails with "this server's advertised scopes could not be safely validated for the account broker" #5068 and Windows: CLI crashes (0xc0000005 in msalruntime.dll) on first Entra account-broker sign-in for MCP server - interactive WAM called with no parent window #5088 (Windows account broker failures).