CLAUDE_SLOW_FIRST_BYTE_MS

Updated October 13, 2026 by Alex Sorokoletov

CLAUDE_SLOW_FIRST_BYTE_MS is a Claude Code environment variable: Default 30,000ms. Logs a "Slow first byte: no stream chunk" observability event when the connection opens but produces no chunks within the window. Complements v2.1.107's byte watchdog. Added between v2.1.108 and v2.1.114 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_SLOW_FIRST_BYTE_MS=<value>
claude

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

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

Release notes that mention it

• Claude Code v2.1.118 — 10 New Environment Variables: Forked Subagents Ship, Subscription Tiering Takes Shape
• Claude Code v2.1.114 — Native Binary Launcher, /fewer-permission-prompts, and 20 New Environment Variables

Related variables

• CLAUDE_CODE_EXIT_AFTER_FIRST_RENDER: Exit after first UI render (testing).
• CLAUDE_CODE_INVESTIGATE_FIRST: Instructs the agent to read and investigate before acting — enforcing "read before write." Converts an agent that might immediately start editing into one that first gathers context.
• CLAUDE_CODE_SLOW_OPERATION_THRESHOLD_MS: Threshold for slow filesystem operation warnings.
• CLAUDE_CODE_CCR_FOLD_FIRST_TURN_RESCAN
• ANTHROPIC_BETAS: Comma-separated beta feature headers.
• ANTHROPIC_CUSTOM_HEADERS: Custom HTTP headers (newline-separated Key: Value).
• ANTHROPIC_FEDERATION_RULE_ID
• ANTHROPIC_ORGANIZATION_ID

Frequently asked questions

All variables, searchable: Claude Code environment variables.