CLAUDE_CODE_MCP_ALLOWLIST_ENV
A Claude Code environment variable, read out of the shipped bundle. It has been in the build since v2.1.138, including the newest one read.
What it is
Set to 1 to spawn stdio MCP servers with only a safe baseline environment plus the server's configured env, instead of inheriting your shell environment
Anthropic's own wording. Read off Environment variables, captured 2026-08-28.
Presence across releases
| Releases | Reading | Declared type | Default |
|---|---|---|---|
| 2.1.245 – 2.1.250 5 releases | never mined | — | — |
| 2.1.160 – 2.1.243 75 releases | in the build | str | — |
| 2.1.138 – 2.1.159 20 releases | in the build | — | — |
A row reading never mined is a release with no archived bundle behind it. Nothing was read out of it, so nothing here says whether the name was in it. A declared type or a default is only ever what that release's own bundle stated.
Changelog entries naming it
No changelog entry here has ever named it. That is a statement about what has been written, not about the name: it has been in the build since v2.1.138 without anybody writing a line about it.
Documentation pages
- Environment variables reference table Claude Code CLI
A reference table page is one whose own table defines this name, and it is where the description above came from. A page that mentions it carries the name somewhere in its text and may say nothing about it at all. Every page carrying it.
Names beside it
Every environment variable in the inventory starting claude_code_mcp, up to twelve of them.
CLAUDE_CODE_MCP_AUTO_BACKGROUND_MSElapsed time in milliseconds before a still-running MCP tool call [moves to a background task](/docs/en/mcp#automatic-backgrounding-of-long-tool-calls) (default: 120000, or 2 minutes). Set to 0 to turn automatic backgrounding off. Requires Claude Code v2.1.212 or later 2.1.198
CLAUDE_CODE_MCP_MEMORY_CGROUPOn Linux you can force MCP servers into their own memory cgroup with an environment variable. 2.1.239
CLAUDE_CODE_MCP_TOOL_IDLE_TIMEOUTIdle timeout in milliseconds for MCP tool calls. When a stdio, HTTP, SSE, WebSocket, or [claude.ai connector](/docs/en/mcp#use-mcp-servers-from-claude-ai) MCP server sends no response and no progress notification for this long, the tool call aborts with an error instead of waiting for the overall MCP_TOOL_TIMEOUT. Overrides the per-transport defaults of 300000 (5 minutes) for network servers and 1800000 (30 minutes) for stdio servers. Set to 0 to disable the idle check. Values below 1000 are raised to one second, and the value is capped at the effective MCP_TOOL_TIMEOUT. A per-server timeout in .mcp.json of at least 1000 raises that server's idle window to at least the timeout value. Doesn't apply to IDE servers or SDK in-process servers. Requires Claude Code v2.1.187 or later. Before v2.1.203, stdio servers were exempt from the idle timeout 2.1.187
Read out of the published npm bundle release by release, and out of Anthropic's own documentation as this site captured it. Nothing on this page is a description anybody here wrote about what the environment variable does. All environment variables.