What it is
How long in milliseconds the first turn of a [non-interactive](/docs/en/headless) session waits for MCP servers that are still connecting, in place of the default [first-turn wait](/docs/en/agent-sdk/mcp#connection-timing). When set, the wait covers every pending server. Set to 0 to skip the wait. A [--permission-prompt-tool](/docs/en/cli-reference#cli-flags) server keeps its own MCP_TIMEOUT wait regardless of the value. Requires Claude Code v2.1.274 or later
Anthropic's own wording. Read off Environment variables, captured 2026-09-22.
Presence across releases
1Build by build
5One row per release since the first build this name was read out of. Absent means the build was read and the name was not in it, never mined means no bundle for that release was ever archived, and a declared type or a default is only ever what that release's own bundle stated.
First cited
1The earliest release whose published evidence quoted CLAUDE_CODE_MCP_STARTUP_WAIT_MS was v2.1.274, 16 Sep 2026. That is the oldest release this project wrote about it, so it is a floor on the name's age and not the release that introduced it. It is not a presence reading: which builds carried the name is the table above.
Names beside it
Every environment variable in the inventory starting claude_code_mcp, up to twelve of them.
CLAUDE_CODE_MCP_ALLOWLIST_ENVSet 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 v2.1.138
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 v2.1.198
CLAUDE_CODE_MCP_CONNECTOR_PREWAIT_MSNew CLAUDE_CODE_MCP_CONNECTOR_PREWAIT_MS environment variable overrides how long Claude Code waits for remote MCP servers to start v2.1.271
CLAUDE_CODE_MCP_MEMORY_CGROUPOn Linux you can force MCP servers into their own memory cgroup with an environment variable. v2.1.239
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 v2.1.187
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.