OpenAI to Claude Migration Guide

Migrating from OpenAI to Claude is not just a model swap — the prompting idioms are different enough that naively copying prompts leads to degraded performance. This guide closes that gap. **What's covered:** **System Prompt Conversion (5 patterns)** — Claude responds differently to authority framing, constraint specification, and persona assignment. Includes a conversion checklist and common mistakes that cause Claude to over-hedge or under-follow instructions. **XML Tag Structure** — Claude's training strongly favours XML tags for structured inputs (`<document>`, `<instructions>`, `<examples>`). The guide shows when to use tags vs plain text and why it matters for long prompts. **Tool Use / Function Calling** — The OpenAI `functions` array maps to Claude's `tools` array with differences in schema format, parallel tool calls, and error handling. Side-by-side SDK comparison in TypeScript. **Streaming Differences** — Claude's SSE stream format differs from OpenAI's. Includes drop-in adapter code for the Vercel AI SDK, LangChain, and raw fetch. **30 Before/After Prompt Examples** across these domains: - Code generation and refactoring (6) - Summarisation and extraction (6) - Classification and routing (5) - Creative writing and tone (4) - Data transformation (5) - System prompt hardening (4) Each example includes a quality note explaining why the Claude version performs better or differently. Compatible with Claude 3.5 Sonnet, Claude 3.7 Sonnet, and the claude-3-5-haiku model family.