TRANSIENT_HTTP_ERROR_CODES in src/agents/pi-embedded-helpers/errors.ts is missing 499 (Client Closed Request) and 504 (Gateway Timeout).

• 499: Commonly returned by Anthropic during overload. Currently doesn't trigger model fallback unless the error message happens to match a pattern.
• 504: Standard gateway timeout. Should obviously be transient.

Both should be in the set so they trigger fallback to the next model.