GPT 5.6 Terra: historical model reference
400K tokens · Text / Vision / Code · Prompt cache
Okou no longer runs GPT 5.6 Terra. This page is kept as a reference for its specs, pricing and benchmarks. For the same kind of work, use GPT 6 Luna.
See GPT 6 LunaGPT 5.6 Terra is the balanced middle tier of OpenAI's GPT-5.6 preview generation — the everyday workhorse that handles the bulk of agentic coding and tool-use work without the flagship price. It keeps most of Sol's behavioural gains over GPT-5.5 while billing at 40% of the rate.
What is GPT 5.6 Terra?
July 2026 preview (middle tier of the GPT-5.6 family)
GPT-5.6 arrived in July 2026 as a three-tier family — Sol, Terra and Luna. Terra is the middle tier: OpenAI positions it as the balanced default that covers most agentic-coding and tool-use work, mirroring the role GPT-5.4 played in the previous generation. It shares the 400K-token context window and the reasoning_effort parameter with the rest of the family, so it drops into existing Codex agents unchanged.
As with the rest of the preview family, published numbers are early and will move, and OpenAI has flagged SWE-bench Verified contamination across frontier models. The durable signal is that Terra gives you most of the family's tool-call accuracy and first-attempt patch quality at the balanced price band — the reason it, not Sol, is meant to run everywhere.
What's notable about GPT 5.6 Terra
Headline architecture and capability features.
GPT 5.6 Terra keeps the 400K-token context window, billed at standard input pricing across the entire window. It supports the reasoning_effort parameter, prompt caching where cached input bills at one-tenth the input rate ($0.20 / 1M) plus a cache-write charge ($2.50 / 1M), and the Responses API surface the codex CLI uses by default. Tool-use, structured outputs and computer-use match the rest of the GPT-5.6 family. Inputs are multimodal across text, vision and code; there is no native image generation.
Specs at a glance
GPT 5.6 Terra benchmarks
Preview figures from OpenAI's GPT-5.6 materials, shown against the public GPT-5.4 numbers Terra succeeds. Treat all percentages as directional: the family is in preview and OpenAI has flagged SWE-bench Verified contamination across frontier models.
GPT 5.6 Terra pricing
Historical provider list prices, per 1M tokens. These reference figures are not current Okou charges.
How GPT 5.6 Terra behaves in practice
Observed behaviour from production agent runs.
Tool routing
Close to Sol on routine and moderately hard tool-routing. The gap opens only on the hardest edge cases — conditional selection and tool calls after long reasoning — where Sol's extra depth pays off.
First-attempt code edits
Strong patch quality on single- and few-file changes. Reach past Terra to Sol when a patch spans many files and must apply cleanly the first time; for most edits Terra lands them without a wasted CI run.
Computer use
Reliable on short-to-medium GUI sequences. For long multi-step computer-use runs where a mid-session derailment is expensive, Sol's higher OSWorld score is worth the premium.
Speed
Faster than Sol and a comfortable middle ground — around 90 tokens/sec at medium effort in early testing. Fast enough for interactive agents while keeping real reasoning depth.
Best agent tasks for GPT 5.6 Terra
The everyday coding agent
Use Terra as the default for the day-to-day work of a coding agent: reading code, writing functions, running tests, applying single- and few-file patches. It clears the bar on most tasks and keeps the credit bill at ×1.
The sub-agent under a Sol orchestrator
When Sol plans a ten-step job, Terra is the tier that executes most of those steps. You get near-flagship quality on the execution layer while paying the flagship rate only at the planner.
Interactive tool-use sessions
For an agent that a human is watching in real time, Terra's faster generation keeps the loop responsive while still handling conditional tool selection and structured outputs reliably.
Mixed workloads that don't justify the flagship
Support triage, doc drafting, code review comments, moderate refactors — the wide band of real work that needs solid reasoning but not the absolute frontier. Terra covers it at 40% of Sol's list price.
When to skip GPT 5.6 Terra
Skip Terra on the hardest multi-file refactors, long orchestration loops and graduate-level reasoning where Sol visibly does better, and on high-volume bulk classification or latency-critical replies where GPT 5.6 Luna is cheaper and faster.
GPT 5.6 Terra vs other models
GPT 5.6 Terra vs GPT 5.6 Sol
Sol is the flagship you escalate to; Terra is the default you run everywhere. Terra keeps most of Sol's tool-routing and coding quality at 40% of the list price and half the credit cost — promote only when a step visibly needs Sol's extra reasoning depth.
GPT 5.6 Terra vs GPT-5.5
Terra is the balanced GPT-5.6 tier, priced below the previous flagship GPT-5.5 while, in preview, matching or beating it on routine agentic work. Where GPT-5.5 was the escalation tier of its generation, Terra is meant to be the everyday default of the new one.
GPT 5.6 Terra vs Claude Sonnet 5
Peer balanced workhorses in different families. Sonnet 5 brings the 1M-token context window and Anthropic's ecosystem; Terra brings the Codex framework and OpenAI's computer-use profile. Pick by which framework your existing agents target and which context length you need.
Frequently asked questions
What is GPT 5.6 Terra's context window?
400,000 tokens, with up to 128K tokens of output per response. The full window bills at standard rates.
Does GPT 5.6 Terra support prompt caching?
Yes. Cached input bills at $0.20 per 1M tokens with a cache-write charge of $2.50 per 1M. Worth enabling whenever your system prompt or tool schema is stable across calls.
Alternatives
Availability of GPT 5.6 Terra on Okou
GPT 5.6 Terra is no longer offered on Okou. See GPT 6 Luna for a currently supported alternative. This page retains historical model information, not current Okou pricing.
