Google Gemini 3.5 Flash vs OpenAI GPT-4o mini: Agent Cost Fight
Verdict first: OpenAI GPT-4o mini wins the pure cost fight by a clean knockout, while Google Gemini 3.5 Flash stays in the bout if you’re building around Gemini’s new Interactions API. For high-volume agent chatter, the price gap is too wide to ignore.
Price: GPT-4o mini lands the heavy shots
Here are the numbers, and they don’t whisper. Google Gemini 3.5 Flash costs $1.50/1M input tokens and $9.00/1M output tokens. OpenAI GPT-4o mini costs $0.15/1M input tokens and $0.60/1M output tokens.
That makes GPT-4o mini 10x cheaper on input and 15x cheaper on output. On a 100M input / 20M output token month, Gemini 3.5 Flash comes to $330. GPT-4o mini comes to $27. That’s not a split decision; that’s a budget manager throwing in the towel.
Agent workflows: Gemini has the fresher angle
The timing matters. Google’s Gemini Interactions API is the current hook, and it points straight at agent-style apps: multi-turn interaction, tool-ish flows, and stateful product experiences. If your app is already sitting inside Google’s AI stack, Gemini 3.5 Flash may save engineering friction even while it costs more per token.
GPT-4o mini, though, is the grinder’s pick. For routing, summarizing, tagging, support drafts, low-risk coding help, and background agent loops, the model’s tiny $0.15 input and $0.60 output pricing lets you run more retries, more evals, and more monitoring without flinching.
Benchmarks: don’t crown what we can’t score
No fake belts here. The supplied current benchmark roundup talks about frontier leaders like Claude Opus 4.8 at 67.9 overall, GPT-5.5 at 62.9, and Claude Opus 4.7 at 60.5. It doesn’t give direct benchmark scores for Gemini 3.5 Flash or GPT-4o mini, so this matchup should be judged mainly on price and product fit.