Developed a robust billing-enabled OAuth flow for the Anthropic Messages API within the bytedance/deer-flow repository, focusing on backend reliability and maintainability. The work centered on injecting a billing header into OAuth requests, ensuring it was consistently prepended to the system prompt and linked to user metadata for accurate billing validation. Using Python, the developer implemented configurable environment variables and validated the integration across multiple Claude model tiers, achieving reliable 200 OK responses. The approach included comprehensive unit testing and code hygiene improvements, with attention to cross-platform compatibility, particularly for Windows environments, resulting in a maintainable and well-documented backend feature.