What happened?

The MCP server discovery process fails when the configuration headers contain non-ASCII characters (like 'ą').
It appears that Gemini CLI attempts to convert these header values into a ByteString (restricted to 255), causing an overflow error when a Unicode character is encountered.

Steps to Reproduce:

1. Configure an MCP server in settings.json

"mcpServers": {
		"mcp": {
			"httpUrl": "https://mcp",
			"headers": {
				"X-Custom": "mąka",
			},
			"timeout": 60000
		},

2. Trigger the discovery process for the server.

What did you expect to happen?

convert non-ascii characters automatically

Client information

• CLI Version: 0.38.2
• Git Commit: b0ed611
• Session ID: 977679c3-feec-4261-98ad-bf1baf1720d6
• Operating System: win32 v25.0.0
• Sandbox Environment: no sandbox
• Model Version: gemini-3.1-flash-lite-preview
• Auth Type: oauth-personal
• Memory Usage: 332.6 MB
• Terminal Name: Unknown
• Terminal Background: #0c0c0c
• Kitty Keyboard Protocol: Unsupported

Login information

No response

Anything else we need to know?

No response