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Cursor’s custom-model support lets the entire IDE — chat, agent mode, inline edits — run through the AI Reserve gateway, so every request is metered and billed on your key. The configuration lives in Cursor Settings → Models → API Keys:
  1. OpenAI API Key — paste your AI Reserve key (aireserve_api_…) and switch the toggle on.
  2. Override OpenAI Base URL — set to https://api.aireserve.com/v1 (note the /v1 suffix) and switch the toggle on.
  3. Leave the Anthropic key toggle off — Cursor has no Anthropic base-URL override, so Anthropic-slot traffic cannot reach the gateway.
  4. Under the model list, choose Add Custom Model and add the gateway model IDs you want, e.g. fable-5, opus-4-8, sonnet-4-6, gpt-5.4-mini.
  5. Pick your custom model in the chat model selector and send a message — the request appears in your AI Reserve usage dashboard within seconds.
Use the prefix-free Claude aliases. Cursor routes any model named claude-* to its built-in Anthropic provider slot on the client side — the request never reaches your base URL. The gateway therefore publishes alias IDs without the prefix: fable-5, opus-4-8, sonnet-4-6. Same upstream models, same pricing; only the name differs.
Start a fresh chat per task. Cursor does not trim conversation context for custom API models, so a long-running chat grows monotonically until it exceeds the model’s context window. Oversized requests fail fast with 413 and a clear message rather than hanging. New task, new chat — you also get better answers that way.