If you're looking for free API access, Google offers access to Gemini for free, including for gemini-2.5-pro with thinking turned on. The limit is... quite high, as I'm running some benchmarking and haven't hit the limit yet.
Open weight models like DeepSeek R1 and GPT-OSS are also made available with free API access from various inference providers and hardware manufacturers.
I'm assuming it isn't sensitive for your purposes, but note that Google will train on these interactions, but not if you pay.
Gemini 2.5 pro free limit is 100 requests per day.
https://ai.google.dev/gemini-api/docs/rate-limits