The Anthropic provider entry in PROVIDER_REGISTRY is the only standard
API-key provider missing a base_url_env_var. This causes the credential
pool to hardcode base_url to https://api.anthropic.com, ignoring
ANTHROPIC_BASE_URL from the environment.
When using a proxy (e.g. LiteLLM, custom gateway), subagent delegation
fails with 401 because:
1. _seed_from_env() creates pool entries with the hardcoded base_url
2. On error recovery, _swap_credential() overwrites the child agent's
proxy URL with the pool entry's api.anthropic.com
3. The proxy API key is sent to real Anthropic → authentication_error
Adding base_url_env_var="ANTHROPIC_BASE_URL" aligns Anthropic with the
20+ other providers that already have this field set (alibaba, gemini,
deepseek, xai, etc.).
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