Picking a primary LLM API vendor in 2026 is no longer a question of raw capability — both GPT-4.1 and Claude 4 are good enough for most tasks. The real decision comes down to developer experience, pricing mechanics, and a handful of sharp edges that documentation doesn't surface until you're already in production.
API Design: Messages vs. Chat Completions
Both APIs follow a messages-based architecture, but they diverge in structure.
OpenAI's chat completions endpoint is the older format and has the widest third-party library support. Requests look like:
from openai import OpenAI
