Attach file by ID (OpenAI Responses, Anthropic, Mistral)

[ashatrov]

OpenAI API has functionality to upload a file and reuse it in future requests https://platform.openai.com/docs/api-reference/files/create (same functionality is available for Mistral and Claude)

This saves a lot of time and tokens when you need to use the same file in a few prompts. E.g., I use it once to get a summary and then one more time to validate this summary against the document.

Upload file:

curl https://api.openai.com/v1/files \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -F purpose="fine-tune" \
  -F file="@mydata.jsonl"
  -F expires_after[anchor]="created_at"
  -F expires_after[seconds]=2592000

Then this file can be used by ID in future requests to the API https://platform.openai.com/docs/guides/pdf-files?api-mode=responses#uploading-files

Use previously uploaded file by id "file_id": "file-6F2ksmvXxt4VdoqmHRw6kL":

curl "https://api.openai.com/v1/responses" \
    -H "Content-Type: application/json" \
    -H "Authorization: Bearer $OPENAI_API_KEY" \
    -d '{
        "model": "gpt-5",
        "input": [
            {
                "role": "user",
                "content": [
                    {
                        "type": "input_file",
                        "file_id": "file-6F2ksmvXxt4VdoqmHRw6kL"
                    },
                    {
                        "type": "input_text",
                        "text": "What is the first dragon in the book?"
                    }
                ]
            }
        ]
    }'