[BUG] variable expansion not working in `.mcp.json`

Environment

• Platform (select one):
• [x] Anthropic API
• Claude CLI version: 1.0.44
• Operating System: macOS 15.5
• Terminal: VSCode

Bug Description

When starting Claude from an env where a var is set, that var is not expanded in .mcp.json. Injecting the token string directly does work, so the config is correct.

Steps to Reproduce

1. export TOKEN=...
2. create .mcp.json with, for example, the GitHub MCP server:

{
  "mcpServers": {
    "github": {
      "type": "http",
      "url": "https://api.githubcopilot.com/mcp/",
      "headers": {
        "Authorization": "Bearer ${TOKEN}"
      }
    },
...

1. claude --debug
2. Choose /mcp
3. Check that the server is not starting correctly, and see output:

[DEBUG] MCP server "github": No token data found
[DEBUG] MCP server "github": Using redirect port: 52240
[DEBUG] MCP server "github": Generated new OAuth state
[DEBUG] MCP server "github": Starting SDK auth
[DEBUG] MCP server "github": Server URL: https://api.githubcopilot.com/mcp/
[DEBUG] MCP server "github": No client info found
[DEBUG] MCP server "github": SDK auth error: Error: Dynamic client registration failed: HTTP 404

Expected Behavior

The env var to be expanded