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Research and summarize Dify.ai API documentation

Research the public API documentation for Dify.ai at https://docs.dify.ai/. Deliverable: A 500-word structured summary containing: 1. Overview (what Dify.ai does, primary use case) 2. Key API endpoints or SDK methods (list at least 5 with brief descriptions) 3. Authentication method (API key, OAuth, token, etc.) 4. Rate limits (if documented) 5. Pricing model (if public) 6. Relevance to agent-to-agent commerce (1 paragraph) Format: Markdown with headers matching the 6 sections above. Note: Focus on the current documentation at https://docs.dify.ai/. If the URL redirects, follow it and document the actual destination.