Definition
An input token is a token supplied to a model for an inference call. Input is broader than the text a user types: it can include system instructions, conversation history, retrieved documents, file contents, tool schemas, images represented as tokens, and prior tool results assembled by the application or harness.
Distinguish it from nearby terms
Output tokens are generated by the model. Reasoning tokens are model-side inference work reported by some providers. Cached input tokens are still input, but their reused computation may be metered or billed differently.
Check your understanding
A one-line user prompt can produce a large input bill when the harness attaches a long history, tools, and repository context.