Developer tests Claude Code MCP env variable expansion, finds key gaps in docs

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A developer ran controlled experiments on Claude Code version 2.1.278 to test how environment variables are expanded in .mcp.json configuration files. The official documentation states that two syntaxes — ${VAR} and ${VAR:-default} — are supported across five fields: command, args, env, url, and headers. Testing revealed that ${VAR} and ${VAR:-default} expanded correctly in all supported fields, while bare $VAR syntax never expanded anywhere. An unset variable without a default arrived as literal text, and nested defaults like ${A:-${B}} only expanded in the headers field, which processes expansion twice. The findings highlight several undocumented edge cases that developers relying on MCP configuration may encounter in practice.

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