Head-to-head comparison
GPT-5.5 Pro (OpenAI) and GPT-5.6 Terra (OpenAI) compared on the ModelCap Index, API price, context window, provider availability, weight access and every public benchmark board they share. Figures are the same ones shown on the live rankings; nothing here is a hidden score.
Snapshot as of 22 September 2026
As of 22 September 2026, GPT-5.6 Terra holds the stronger ModelCap Index position (#21 vs #59); and GPT-5.6 Terra is 15× cheaper per output token ($12.00/1M vs $180/1M).
Evidence: publisher-corpus-prior over 209 held-out anchors (59%); 1 specialist-board observation at 2.1% support (41%); shrunk 10.1 toward the measured corpus · 6 public benchmark observations across 6 boards. Prices are the lowest listed API offer per million tokens observed on the OpenRouter catalogue.
Scores are the sources' own published figures for each model's best evaluated configuration; ModelCap never re-runs a benchmark.
Want a different pairing? Open the interactive comparison tool to swap either model for any current ranked language model.
GPT-5.6 Terra ranks higher on the ModelCap Index as of 22 September 2026: #21 against #59. That is a capability ranking built from public benchmark evidence with published uncertainty; whether it is "better" for you also depends on price, context and where you can run it. Their published uncertainty intervals overlap, so the rank difference alone does not establish a reliable capability advantage for your workload.
GPT-5.6 Terra is cheaper on output tokens: $12.00/1M against $180/1M. Input tokens are $30.00/1M for GPT-5.5 Pro and $2.00/1M for GPT-5.6 Terra. Prices are the lowest listed API offer ModelCap observed, in USD per million tokens. For 1,000 requests with 2,000 input and 500 output tokens each (2M input + 0.5M output), the listed-rate estimate is $150.00 for GPT-5.5 Pro versus $10.00 for GPT-5.6 Terra. GPT-5.6 Terra costs 93.3% less in this scenario. This excludes caching, batch discounts, prompt-length tiers, tool charges and retries; verify the selected endpoint before budgeting.
Both publish a 1M-token context window.
The two models do not share a coding benchmark board on ModelCap yet, so no head-to-head coding score is published; the ModelCap Index position is the closest overall signal.
GPT-5.5 Pro: API only. GPT-5.6 Terra: API only. Open weights mean the checkpoint can be downloaded and self-hosted under its licence; API-only models are available solely through hosted endpoints.
ModelCap currently lists 1 API provider for GPT-5.5 Pro and 3 for GPT-5.6 Terra, from the OpenRouter catalogue snapshot the site serves; each model page lists the providers and their prices.
Every figure comes from the sealed ModelCap dataset published 22 September 2026; the page re-renders within a minute of each data refresh, and the ModelCap Index positions are the same ones shown on the live rankings.