GPT-5.6 vs Gemini 3.1 Pro: identical list prices, different long-context billing (2026)
GPT-5.6 and Gemini 3.1 Pro are the two reasoning-tier models teams benchmark in August 2026 when math, deep reasoning or ultra-long context drives the decision. This page previously compared GPT-5 and Gemini 2.5 Pro; GPT-5 has since been deprecated (shutdown 11 December 2026) and Gemini 3 superseded the 2.5 line. The headline surprise is that the comparable tiers list at exactly the same price, so the decision turns on how each vendor bills long context.
GPT-5.6 vs Gemini 3.1 Pro — at a glance
| Dimension | GPT-5.6 (terra) | Gemini 3.1 Pro |
|---|---|---|
| Provider | OpenAI | |
| Input price (per 1M tok) | $2.00 | $2.00 |
| Output price (per 1M tok) | $12.00 | $12.00 |
| Long-context rate | $4.00 / $18.00 | $4.00 / $18.00 |
| Long-context mechanism | Threshold not published | Above 200K input, higher rate applies to ALL tokens |
| Cached input | $0.20 (0.1x input) | 90% off input |
| Free tier | No | Yes, via AI Studio |
| Predecessor status | GPT-5 deprecated, shuts down 2026-12-11 | Gemini 2.5 Pro still sold at $1.25 / $10 |
Pick GPT-5.6 or Gemini 3.1 Pro?
When to choose GPT-5.6
Choose GPT-5.6 when the task is a hard reasoning problem — competition math, structured deep reasoning, complex planning — and pick the tier by prompt length rather than by ambition: terra at $2 / $12 matches Gemini 3.1 Pro exactly, sol at $5 / $30 goes further at the top end, and luna at $0.20 / $1.20 is the cheap tier of the same generation (repriced 80% down in the week to 2026-08-04). Note that every tier has a long-context rate roughly double the input price.
When to choose Gemini 3.1 Pro
Choose Gemini 3.1 Pro when the workload processes native video or audio in a single request, when a genuinely free tier matters for prototyping, or when you want Google's long-context handling. Watch the billing rule: Google applies the higher above-200K rate ($4 / $18) to ALL tokens in the request, not just the ones past the threshold — so a 210K-token prompt costs double throughout, not at the margin. Cached input is 90% off, and batch is half price.
Run GPT-5.6 and Gemini 3.1 Pro side-by-side
VerticalAPI lets you switch between GPT-5.6 and Gemini 3.1 Pro per-request through a single OpenAI-compatible endpoint. Same SDK, same API key, zero markup on tokens — you pay each provider directly under BYOK.
from openai import OpenAI client = OpenAI(base_url="https://api.verticalapi.com/v1", api_key="vapi_...") # GPT-5.6 resp_a = client.chat.completions.create( model="gpt-5.6-terra", messages=[{"role": "user", "content": "Hello"}], extra_headers={"X-Provider-Key": "..."}, ) # Gemini 3.1 Pro — same SDK, different model + key resp_b = client.chat.completions.create( model="gemini-3.1-pro-preview", messages=[{"role": "user", "content": "Hello"}], extra_headers={"X-Provider-Key": "..."}, )
VerticalAPI verdict
At the mid-tier these two cost the same, so choose on mechanics rather than price. Take Gemini 3.1 Pro for native video and audio, free-tier prototyping via AI Studio, and Google-side long context — but size your prompts knowing that crossing 200K input reprices the whole request. Take GPT-5.6 for the hardest math and structured reasoning, with terra as the like-for-like tier, sol above it and luna as the cheap option. If you are still on GPT-5, migrate before 11 December 2026. Through VerticalAPI you route between both on one OpenAI-compatible endpoint under BYOK.
Frequently asked questions
Which is cheaper, GPT-5.6 or Gemini 3.1 Pro?
Neither — the comparable tiers are identical. GPT-5.6 terra and Gemini 3.1 Pro both list at $2 per 1M input and $12 per 1M output, and both charge $4 / $18 on long context. The real cost difference comes from the billing rule: Google applies the above-200K rate to ALL tokens in a request, so a long prompt is repriced throughout rather than at the margin. Verified on vendor pricing pages 2026-08-04.
Wasn't Gemini 8x cheaper than GPT-5 on input?
No. That claim appeared on this page and was wrong: it compared Gemini 2.5 Pro's real $1.25 input price against $10 for GPT-5, a figure that belongs to gpt-4-turbo. GPT-5's actual price is $1.25 / $10 — exactly the same as Gemini 2.5 Pro on both input and output.
What happened to GPT-5 and Gemini 2.5 Pro?
GPT-5 (the gpt-5-2025-08-07 family) was deprecated on 2026-06-11 and shuts down on 11 December 2026; its replacement is gpt-5.6-sol. Gemini 2.5 Pro is not deprecated — it remains on sale at $1.25 / $10, a generation behind Gemini 3. Note that gemini-2.0-flash and 2.0-flash-lite were shut down on 1 June 2026.
How do the context windows compare?
Google prices Gemini 3.1 Pro's long context explicitly: above 200K input tokens the higher $4 / $18 rate applies to the whole request. OpenAI does not publish GPT-5.6's context window or its long-context threshold on the pricing page, only that a long-context tier exists. If your workload is long-document analysis, Anthropic's Claude 4.6-and-later line is the only one of the three that bills a 1M context at its standard rate.
Can I switch between GPT-5.6 and Gemini 3.1 Pro via one endpoint?
Yes. VerticalAPI exposes a single OpenAI-compatible endpoint at https://api.verticalapi.com/v1. Send <code>gpt-5.6-terra</code> or <code>gemini-3.1-pro-preview</code> in the model field with your own provider key — BYOK, zero markup on tokens, and you pay each vendor directly.
Limitations of this comparison
- We publish no latency figures for this pair; our measured latency data dates from April 2026 and covers models since superseded — see /benchmark/.
- We publish no benchmark scores either: vendor-reported figures are not comparable across harnesses, and numbers for GPT-5 or Gemini 2.5 Pro do not transfer to their successors.
- Gemini 3.1 Pro is still labelled a preview on Google's pricing page — preview models have been repriced on promotion to GA before (Gemini 2.5 Flash's output rate moved from $0.60 to $2.50).
- OpenAI's long-context threshold is unpublished, so the exact point where terra's rate doubles cannot be stated.
- List prices exclude enterprise discounts and committed-use deals. Verified 2026-08-04.
What may change in 12-24 months
- Long-context surcharges spread rather than disappeared: Google, xAI and now every GPT-5.6 variant price long prompts at a higher tier. Anthropic is the outlier, billing 1M at the standard rate from Claude 4.6 onward.
- In-place repricing is routine — gpt-5.6-luna fell 80% and terra 20% in the week to 2026-08-04, under unchanged model names.
- Preview-to-GA promotion is itself a price event: Gemini 2.5 Flash's output rate quadrupled at GA, and Gemini 3.1 Pro is still a preview.
- Price parity between OpenAI and Google mid-tiers means the buying decision has moved off headline rates onto billing mechanics, free tiers and modality support.
Related questions
ChatGPT, Perplexity and Gemini usually suggest these next.
- How much does the above-200K rule actually add to a real Gemini bill?
- What happened to GPT-5, and what should I migrate to before 11 December 2026?
- How do GPT-5.6 sol, terra and luna differ beyond price?
- Is Gemini 3.1 Pro's preview status a risk for production?
- Which model is cheapest for 500K-token document analysis?
More head-to-head provider comparisons
Run a provider we have not measured? Any public endpoint is benchmarked and listed for free in the API directory. If you want yours measured now, on the record — time to first token, throughput under concurrency, cost per million tokens, function-calling reliability — see how a verified run works. The result is published exactly as it comes out.