Some poking around in the source code for https://github.com/mistralai/mistral-vibe got me to this:

  curl https://api.mistral.ai/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $(llm keys get mistral)" \
  -d '{
    "model": "mistral-medium-3.5",
    "messages": [
      {"role": "user", "content": "Generate an SVG of a pelican riding a bicycle"}
    ]
  }'

Which did work: https://gist.github.com/simonw/f3158919b18d2c47863b0a5dc257a... - it's pretty disappointing.

Weird that it doesn't show up in the model list:

  curl https://api.mistral.ai/v1/models \
    -H "Content-Type: application/json" \
    -H "Authorization: Bearer $(llm keys get mistral)" | jq