What
A new environment variable, CLAUDE_CODE_MCP_STARTUP_WAIT_MS, controls how long Claude Code waits for MCP (Model Context Protocol) connector servers to start up. When set, it overrides the normally computed wait time, though it's still capped by an internal maximum. Setting it to a value greater than 0 also switches on waiting for 'deferrable' MCP connections, which otherwise may not be waited for.
Why
This gives users a way to tune startup behavior for MCP servers that are slow to connect, instead of being stuck with a fixed built-in timeout.
Names in the bundleCLAUDE_CODE_MCP_STARTUP_WAIT_MS
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed since
Anthropic's documentation has since written up CLAUDE_CODE_MCP_STARTUP_WAIT_MS, on Claude Code changelog.
* Changed the non-interactive first turn to still wait up to 2s for connecting MCP servers named by `--allowedTools` or an `mcp_tool` hook, even when `CLAUDE_CODE_MCP_STARTUP_WAIT_MS` is `0`changelog see the edit
Two sources agreeTwo things we can check say the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up CLAUDE_CODE_MCP_STARTUP_WAIT_MS, on Claude Code changelog.
Anthropic's release notes agree
Added CLAUDE_CODE_MCP_STARTUP_WAIT_MS to bound how long the first non-interactive turn waits for connecting MCP servers (0 = don't wait)
The name it cites is new in this build
New in this build: CLAUDE_CODE_MCP_STARTUP_WAIT_MS