Could you share a couple more details so we can look into this properly?
When the request “comes back to prompt,” do you see any error message (such as a banner or inline error), or does it silently reset as if nothing happened?
Could you grab a request ID from one of the failing attempts? You can find it via the three dots menu in chat > Copy Request ID. More info here.
Does this happen only with Composer 2.5-fast, or with other models too (e.g., Auto mode)?
If the issue is specific to Composer 2.5-fast, try switching to Auto mode in the meantime — it selects the best available model automatically.
Request ID: 520b60a9-5097-493e-bac8-669c2014dfc6
{“error”:“ERROR_BAD_REQUEST”,“details”:{“title”:“Bad Request”,“detail”:“This model does not support custom API keys.”,“isRetryable”:false,“additionalInfo”:{},“buttons”:,“planChoices”:,“analyticsMetadata”:{“actionRequired”:“config”}},“isExpected”:true}
Bad Request This model does not support custom API keys.
Iyn: Bad Request This model does not support custom API keys.
at Fpk (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:35852:26435)
at NDw (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:35852:25580)
at Gpk (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:35853:6852)
at hmf.run (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:35853:11731)
at async qZs.runAgentLoop (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:48220:22008)
at async OKf.streamFromAgentBackend (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:48288:12345)
at async OKf.getAgentStreamResponse (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:48288:19576)
at async Vyn.submitChatMaybeAbortCurrent (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:35985:17397)
at async Object.zh [as onSubmit] (vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:47271:3889)
at async vscode-file://vscode-app/Applications/Cursor.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:47270:101645
Thanks for the error details and request ID. That error message tells us exactly what’s going on.
You have a custom API key configured in Cursor Settings > Models (for OpenAI, Anthropic, or another provider). Composer 2.5-fast is a Cursor-proprietary model, so it can’t route through custom API keys. When any custom key is active in settings, explicitly selecting a Composer model triggers this error.
Quick fix: Go to Cursor Settings > Models and disable (uncheck) any custom API keys you have configured, then try Composer 2.5-fast again. It should work immediately.
If you need both your custom API key (for models like GPT-5.5) and Composer 2.5-fast, here are your options:
Use Auto mode - it automatically ignores custom keys and routes through Cursor’s backend, so it works regardless of your API key settings
Toggle the API key when switching between models - disable it for Composer, re-enable it for OpenAI models. You can quickly open settings with Cmd+Shift+,
This is a known limitation that affects all Composer versions. There’s an existing thread tracking this with more context. Our team is aware, but there’s no ETA for a fix yet.