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usage-metadata — stable

Optional extra fields recording economics and provenance:

  • Message-level: tokens (object, e.g. { "input": 12, "output": 340 }), cost (number, writer's currency), provider (string).
  • Readers MUST preserve these verbatim on round-trip; they never affect validation.
  • Status: stable. Emitted by porter, curtis-web, and curtis-ai-tech.

Like every extension, these fields are unknown fields as far as core parsing is concerned: a reader that doesn't implement usage-metadata ignores the names and reads only the fields it knows. See SPEC, "Unknown fields".