Developer Encounters 403 Error with GPT-5 Nano API Despite Proper Authorization

A software developer working on a personal mobile application has encountered a perplexing API error while attempting to integrate OpenAI’s newly rumored GPT-5 Nano model, despite flawless performance with higher-tier GPT-5 variants. The developer, who requested anonymity and emphasized the app’s private, non-commercial nature, shared their experience on Reddit’s r/OpenAI forum, sparking a wave of technical speculation among fellow developers and AI engineers.

The application, built using Cursor—a popular AI-assisted development tool—relies on the OpenAI API to process a single user input, generate two supplemental data points through reasoning, and then trigger a secondary, non-OpenAI API to retrieve detailed contextual information. The system functions without issue using GPT-5.2, GPT-5, and GPT-5 Mini. However, when switching to the purportedly lower-cost GPT-5 Nano model, the API consistently returns a 403 Forbidden error, stating the model is not found. The developer has confirmed their API key is valid, has enabled all GPT-5 models in their project’s usage limits, and has verified that the key works with other models in the same environment.

According to multiple responses on the Reddit thread, the root cause may not lie in configuration but in the model’s actual availability. As of now, OpenAI has not officially documented or released a model named "GPT-5 Nano" in its public API documentation. While OpenAI has introduced tiered models such as GPT-4o Mini and GPT-4 Turbo, no official "GPT-5 Nano" variant exists in the OpenAI API model list as of the latest public release (June 2024). Cursor, the IDE used by the developer, may be misinterpreting or auto-suggesting a non-existent model name based on internal beta terminology or user-generated aliases.

Furthermore, OpenAI’s API endpoints are strictly version-controlled. Attempting to call a model that is not officially registered in the API’s model registry—even if it exists internally or in a closed beta—will trigger a 403 error, not a 404. This distinction is critical: a 404 would indicate the endpoint doesn’t exist; a 403 indicates the model is forbidden, often because it’s either unauthorized, deprecated, or non-existent in the public API. The developer’s assumption that "GPT-5 Nano" is a legitimate model may stem from Cursor’s AI suggesting it as a "cost-effective alternative," a common behavior in AI-assisted coding tools that sometimes hallucinate plausible-sounding but non-existent options.

Technical experts on Reddit and in developer communities have advised verifying the exact model name by calling OpenAI’s /v1/models endpoint directly via cURL or Postman. This will return a definitive list of available models under the user’s API key. If "gpt-5-nano" does not appear in the response, it confirms the model is not accessible. Additionally, OpenAI’s web search functionality (via the "search" tool in the API) is only supported on specific models like GPT-4o and GPT-4 Turbo, and may not be compatible with any "nano"-tier models even if they existed.

The developer’s experience highlights a growing challenge in the AI development ecosystem: the proliferation of unofficial model names and the difficulty in distinguishing between OpenAI’s official offerings and third-party tooling interpretations. As AI-assisted development tools become more prevalent, developers—especially those new to APIs—may inadvertently rely on hallucinated model names, leading to frustrating debugging cycles. OpenAI has yet to issue a public statement on the rumored "GPT-5 Nano," but industry insiders suggest any such model would likely be released under a different naming convention, such as "gpt-5-turbo-nano" or "gpt-5-small," if it exists at all.

For developers facing similar issues, the recommendation remains: always validate model names against OpenAI’s official documentation and use the /v1/models endpoint to confirm availability. Until OpenAI officially releases a GPT-5 Nano variant, developers should continue using GPT-5 Mini or GPT-4o Mini for cost-efficient, supported inference.