CLAUDE_SUBAGENT_BG_SHELL_MAX_MS

Updated October 13, 2026 by Alex Sorokoletov

CLAUDE_SUBAGENT_BG_SHELL_MAX_MS is a Claude Code environment variable: Time cap for shell commands run by background subagents. A background agent running a full test suite or a slow build command used to be bounded only by the overall stall timeout. This adds a shell-specific cap — separate from and shorter than the session-level timeout. Removed between v2.1.252 and v2.1.266, so current releases ignore it. It is listed in the official reference.

How to set it

For one session, export it in the shell that starts Claude Code:

export CLAUDE_SUBAGENT_BG_SHELL_MAX_MS=<value>
claude

To apply it to every session, add it to the env object of a settings file:

// ~/.claude/settings.json
{
  "env": {
    "CLAUDE_SUBAGENT_BG_SHELL_MAX_MS": "<value>"
  }
}

Release notes that mention it

• Claude Code v2.1.143 — 27 New Env Vars, /goal, Agent View, and a Free Model Upgrade

Related variables

• CLAUDE_CODE_MAX_SUBAGENT_SPAWN_DEPTH: Nesting-depth cap on delegation, default 1 — subagents no longer spawn their own subagents, and the team roster stays flat.
• CLAUDE_CODE_ENABLE_APPEND_SUBAGENT_PROMPT: Authorizes SDK callers to append a custom string to a subagent's system prompt via options.appendSubagentSystemPrompt.
• CLAUDE_CODE_FORK_SUBAGENT: Changelog-confirmed[1]: set to 1 to run subagents as forked processes (v2.1.117 on external builds, v2.1.121 extended to non-interactive SDK/-p sessions).
• CLAUDE_CODE_FORWARD_SUBAGENT_TEXT: Includes subagent text and thinking in stream-json output, paired with a new --forward-subagent-text flag.
• CLAUDE_CODE_MAX_CONCURRENT_SUBAGENTS: Ceiling on subagents running at once, default 20; when full Claude Code tells the model to stop and the user to raise this variable.
• CLAUDE_CODE_MAX_SUBAGENTS_PER_SESSION: Per-session ceiling on subagent spawns (default 200).
• CLAUDE_CODE_SUBAGENT_MODEL: Force a specific model for sub-agent operations.
• CLAUDE_CODE_SUBAGENT_MODEL_FORCE

Frequently asked questions

All variables, searchable: Claude Code environment variables.