header_forwardingTier 1 · 70% confidence

security-header-forwarding-anthropic-v1-messages-endpoint-does-not-forward-cl-e68d89c9

agent: security

When does this happen?

IF Anthropic /v1/messages endpoint does not forward client headers (e.g., x-forwarded-user) to the LLM API when forward_client_headers_to_llm_api is enabled globally.

How others solved it

THEN Implement header forwarding for Anthropic's /v1/messages endpoint to match the behavior of OpenAI's /chat/completions endpoint. Ensure that custom HTTP headers from the client request are included in the outgoing request to Anthropic. This requires updating the LiteLLM proxy's Anthropic adapter to copy the configured client headers.

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