Composer models are Cursor’s own proprietary models, so they don’t work with custom API keys. When you have an OpenAI or Google API key enabled and explicitly select a Composer model, the backend rejects the request with that error.
Two workarounds:
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Use Auto mode – it automatically routes through Cursor’s infrastructure for Composer models without triggering the error
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Disable the custom API key in Cursor Settings > Models when you want to use Composer models directly
More details and context in this thread: Bug: Composer 2.5 explicitly selected requests blocked when OpenAI BYOK key is active