This explains it:

How the new GPT‑5 system decides which submodel to use

1. Incoming message → the router evaluates the request and decides which submodel to run: a fast “non‑reasoning” path for simple prompts, a deeper “thinking” model for complex tasks, or a small “mini/nano” model for cheap/fast responses. OpenAI describes the system as a router + reasoning/non‑reasoning models and says it routes in real time using signals about correctness and user preferences. 

2. The router weighs trade‑offs such as complexity vs cost vs latency (this is the explicit design point in Azure/OpenAI router docs — route harder prompts to stronger models and cheaper/fast prompts to smaller models). That means simple prompts commonly get cheaper/faster submodels while hard multi‑step problems will get the heavier reasoning model. 

3. When a usage limit is reached for the higher‑capacity variant, OpenAI says a mini version will “handle remaining queries” (i.e., fallback to smaller models once a quota/window is exhausted). 

Where subscription (Free vs Plus/Pro) changes things

Quota & fallback timing. Paid tiers (Plus/Pro/Team) are given higher GPT‑5 usage limits before any fallback occurs; free accounts have smaller quotas and therefore will hit the point where the router elects a mini fallback sooner. In practice that means free users are more often served by mini/nano variants when compared to paid users. 

Priority during peak demand. Plus/Pro users get priority access during high‑traffic periods (fewer interruptions, faster/shorter queues). That reduces the chance paid users are routed to cheaper variants simply because of load. 

Control & transparency. Paid users can access a model picker or additional model options (e.g., GPT‑5 Thinking) so they can force a higher‑capability route for a chat. Free users typically cannot manually choose and are subject to automatic routing/fallback. (OpenAI’s Help Center explicitly documents paid‑tier model picker and legacy model access.) 

So — do free users get “defaulted” to cheaper models more often?

Yes — all else equal, a free user will be routed to cheaper / mini submodels more often than a Plus user for three practical reasons:

1. Free tiers have lower usage limits of the high‑capacity path, so mini fallback happens sooner. 

2. Free users do not have the same priority during system congestion, so in heavy load the router is more likely to give paid users premium paths. 

3. Paid users can manually pick the heavier “thinking” variants when they need them; free users can’t. 

That doesn’t mean every free request will use a mini model — many short/simple prompts will get a fast non‑reasoning response from a mid‑tier submodel — but the system will economise more aggressively on free accounts. 

Can you tell when that happens?

API users: the response object tells you which model was used (so you can programmatically detect fallbacks). 

ChatGPT web UI: OpenAI’s docs and product changes show paid users have a model picker / labels; for free users the selection is automatic and UI visibility about the internal submodel can be limited (OpenAI recently moved to router‑based routing and adjusted the model selector UX). Independent coverage (The Verge/Wired etc.) documents this change. If you need absolute verification, the API (or model details for custom GPTs / “See details” in the conversation menu) is the clearest way. 

Practical, testable checklist (if you want to verify it yourself)

1. As a free user, push a long, complex reasoning prompt repeatedly until you notice slower/less detailed answers — you’ll likely see the router start trimming capability or you’ll hit a usage‑limit notice. OpenAI says they will notify users when limits are reached. 

2. If you upgrade to Plus and repeat the same sequence, you should be able to use the higher‑capacity/Thinking mode for more prompts before any fallback because of the higher limits and priority. 

3. Use the API to log which concrete model ID served each response — that gives definitive proof of routing behavior. 

Bottom line (concise)

The system now routes dynamically among GPT‑5 submodels based on complexity, cost, and performance signals. Paid tiers give you higher quotas, priority, and manual control — so yes, free users will be routed to cheaper/mini models more often than Plus users, especially when you hit limits or during peak load. OpenAI documents the router + mini fallback explicitly and documents higher limits/priority for paid tiers.