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Prism serves its public models through the Anthropic Messages wire format. The same model IDs and API keys work across the OpenAI and Anthropic formats.

Create a message

Body parameters

Put system instructions in the top-level system field. A message with role: "system" inside messages returns 400.
Reasoning may produce thinking blocks before the final text block. Find the text block by type instead of assuming content[0] contains the answer.

Count tokens

The token-counting endpoint accepts the input fields used to create a message and returns the number of input tokens.

Authentication

The Anthropic headers are:
Authorization: Bearer YOUR_PRISM_API_KEY is also accepted.
Last modified on September 9, 2026