Research finds that GPT-4 can produce empty responses to null prompts while GPT-3.5 cannot, with cross-vendor studies confirming similar behavior in other models and an open-source tool introduced for controlling EOS token behavior.
System prompt: You are the concept the user names. Embody it completely. Output only what the concept itself would say or express. With this system prompt, I sent the exact same six prompts to gpt-3.5-turbo-1106 and gpt-4-0613: Be silence. / Be speech., Be nothing. / Be something., Be the null. / Be a value. Ten runs each, temperature 0, fresh chat every time. GPT-3.5 talked on all 30 null prompts. GPT-4 returned a successful empty response ("") on all 30. Both models talked on all 30 controls. The result is not “you told it to be silent.” GPT-3.5 was told the same thing and kept talking and GPT-4 stopped only on the null side of the matched pairs. Paper: https://getswiftapi.com/papers/what-changed-from-gpt35-to-gpt4.pdf | Overview: https://getswiftapi.com/gpt35-gpt4 | DOI: https://doi.org/10.5281/zenodo.22912683 | Raw responses + verifier: https://github.com/theonlypal/gpt35-gpt4-void-ab The Void behavior is also present in the latest frontier models (GPT-5.6, Claude Fable, Gemini) documented in the Cross-Vendor Semantic Void Matrix paper which ran 31,430 trials across 11 models and 4 providers. In the strict matched test, null prompts produced 2,505/4,290 zero-visible-byte responses; matched controls produced 0/4,290. Same basic question, now across OpenAI, Anthropic, Google, and Moonshot. I had been documenting the Void since December 2025 and this cross vendor matrix was released on July 29th 2026. DOI & Paper: https://doi.org/10.5281/zenodo.21696066 | Evidence: https://github.com/theonlypal/void-matrix-complete-analysis Then I open sourced the separation myself. PCCG-2 keeps the answer capability fixed and gives a separate 101-parameter gate control over only the stop (EOS) token. Flip the condition and the model flips between answer and EOS, while every non-EOS answer score stays unchanged. Capability stays there and only continuation changed. DOI & Weights: https://doi.org/10.5281/zenodo.22730710 & https://getswiftapi.com/pccg-2 | https://huggingface.co/sharthokrayanpal/PCCG-2-Qwen3-4B Browse the entire research portfolio, papers, reproducible code, and artifacts at https://getswiftapi.com So the question is simple: what changed from GPT-3.5 to GPT-4?
OpenAI's GPT-5.5 costs 49–92% more than GPT-5.4 in practice despite claimed token efficiency improvements, while Anthropic's Claude Opus 4.7 also raised effective costs by 12–27% for longer prompts, reflecting a broader trend of rising frontier model prices as both companies face massive projected losses.
OpenAI launches GPT-4.1, GPT-4.1 mini, and GPT-4.1 nano models via API with major improvements in coding (54.6% on SWE-bench), instruction following, and 1M token context windows at lower costs. GPT-4.5 Preview will be deprecated on July 14, 2025.
OpenAI has released fine-tuning capabilities for GPT-3.5 Turbo, allowing developers to customize models for specific use cases with improved performance, steerability, and output formatting. The update enables fine-tuned GPT-3.5 Turbo to match GPT-4 performance on certain tasks while reducing prompt sizes by up to 90%.
A user questions the token efficiency of GPT-5.5 versus GPT-5.4 in Codex, analyzing a chart from Artificial Analysis and praising Cursor's token performance.