CLAUDE_INTERNAL_FC_OVERRIDES

Updated October 13, 2026 by Alex Sorokoletov

CLAUDE_INTERNAL_FC_OVERRIDES is a Claude Code environment variable: Internal overrides for function-call (tool-use) behavior. Injects overrides that bypass normal routing — for testing new tool schemas or shadowing production tool behavior in staging. Added between v2.1.118 and v2.1.123 and still present in v2.1.294. It is not in the official reference.

How to set it

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

export CLAUDE_INTERNAL_FC_OVERRIDES=<value>
claude

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

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

Release notes that mention it

• Claude Code v2.1.123 — Background Daemons, Agent Fleets, and CoWork Memory

Related variables

• CLAUDE_CODE_EVAL_ALLOW_FLAG_OVERRIDES
• CLAUDE_AX_PREPARK_MS
• CLAUDE_AX_SCREEN_READER
• CLAUDE_AX_STARTUP_QUIET_MS: Startup quiet window (milliseconds) for screen-reader mode.
• CLAUDE_CODE_ACCESSIBILITY: Enable accessibility mode; disables cursor hiding.
• CLAUDE_CODE_ALT_SCREEN_FULL_REPAINT
• CLAUDE_CODE_BLOCKING_LIMIT_OVERRIDE: Override token limit for input blocking.
• CLAUDE_CODE_DISABLE_ALTERNATE_SCREEN: Prevents the TUI from using the terminal alternate screen buffer.

Frequently asked questions

All variables, searchable: Claude Code environment variables.