Custom GPT Actions not triggered on GPT-5.5 Instant, but work on GPT-5.3 Instant and GPT-5.5 Thinking

Hi, I’m seeing what looks like a Custom GPT Actions regression in GPT-5.5 Instant. I have a Custom GPT that must call a database action before answering any stock or security analysis request. The action is configured through an OpenAPI schema and points to my backend. The same Custom GPT, same instructions, same OpenAPI schema, same backend, same authentication, and same prompt behave differently depending on the selected model. Observed behavior: * GPT-5.3 Instant: works correctly. The GPT calls the action before answering. * GPT-5.5 Thinking: works correctly. The GPT calls the action before answering. * GPT-5.5 Instant: does not call the action. It answers with a fallback response instead. Example prompt: Analyze MSFT Expected behavior: The GPT should call the Custom GPT Action before producing any visible answer. Actual behavior on GPT-5.5 Instant: The model does not call the HTTP endpoint and returns the configured fallback response. Actual behavior on GPT-5.5 Thinking: The GPT Builder Preview debug panel shows: [debug] Calling HTTP endpoint [debug] Response received The backend is reachable and working, because the exact same action is called successfully from GPT-5.5 Thinking and GPT-5.3 Instant. I have already hardened both the GPT instructions and the OpenAPI schema to make the action requirement explicit. The operation summary is: MANDATORY first DB fetch for any security analysis The operation description starts with: ALWAYS call this action before answering any user request about a stock, ETF, bond, REIT, security, ticker, company, or security number. Do not answer from memory. Do not produce visible text before this action. The OpenAPI JSON validates successfully, the action imports correctly, and the backend works when called by other models. This appears to be a Custom GPT Actions tool-selection regression in GPT-5.5 Instant specifically, not an API, backend, authentication, or OpenAPI schema issue. Questions: 1. Is there a known change in GPT-5.5 Instant tool/action selection behavior for Custom GPT Actions? 2. Is there a recommended way to force or strongly bias an action call in GPT-5.5 Instant when the action is mandatory for the task? 3. Are there any known differences between GPT-5.3 Instant and GPT-5.5 Instant regarding Custom GPT Actions? I can provide redacted screenshots from GPT Builder Preview showing that GPT-5.5 Thinking calls the endpoint while GPT-5.5 Instant does not.