GPT-5.6
OpenAI’s GPT-5.6 family for reasoning, coding, and agents. GPT-6 Astra (2026-09-03) is the new peak; ChatGPT Plus gets it in Work and Codex, not the Chat picker.
OpenAI’s GPT-5.6 family (Sol flagship, Terra balanced, Luna cost-efficient) for complex reasoning, coding, multimodal understanding, and agentic tool use. The gpt-5.6 API alias routes to Sol. GPT-6.1 Sol (gpt-6.1-sol, 2026-09-29) is the current lower-cost Sol; GPT-6 Sol and Luna (2026-09-22) are earlier cost tiers. This entry remains GPT-5.6, which is still available.
Why GPT-5.6 matters
GPT-5.6 remains the default OpenAI brain many teams ship: mature tool calling, multimodal understanding, and Sol / Terra / Luna cost routing. GPT-6 Astra is the new capability ceiling (API gpt-6-astra): Plus ChatGPT includes it in Work and Codex; Chat “GPT-6 Pro” is Pro/Business/Enterprise. It costs more at Standard list rates, is generally available in GitHub Copilot (2026-09-04), is in private preview on Snowflake Cortex AI (2026-09-09), and is not available in Cursor.
Vision · Tool calling · Thinking · MCP · Coding
Last reviewed: 15 October 2026
When to choose GPT-5.6
Decision guidance for architects—not a feature list.
Best for
- Complex multi-step reasoning
- Software engineering agents
- Multimodal analysis
- MCP / tool-heavy workflows
Avoid if
- You require open weights or on-prem-only inference
- Cost and vendor lock-in are hard blockers
Strengths
DataAIHub qualitative scores from 1 to 5, for architects. Not official benchmark results or a public ranking.
- Tool calling / agents★★★★★
- Ecosystem integrations★★★★★
- Coding★★★★★
- Open weights / self-host★☆☆☆☆
- Cost efficiency★★★☆☆
Ecosystem
Built by
- OpenAI
GPT-5 is OpenAI’s flagship foundation-model family.
Competes with
- Claude Opus
Primary rival for enterprise assistants, coding agents, and careful instruction following.
- Gemini 3.1 Pro
Competes on multimodal frontier capability and cloud platform distribution.
Works with
Recommended for
- AI Agents
Mature tool calling and structured output for agent workloads.
- Large language models
Reference proprietary frontier model for LLM fundamentals.
How GPT-5.6 evolved
Key moments in chronological order.
- Model
OpenAI releases gpt-6.1-sol in the API and in ChatGPT Work and Codex for Plus, Pro, Business, Enterprise, and Edu. Standard API price is $2 input / $0.10 cached input / $2.50 cache writes / $10 output per 1M tokens. Context is 1,050,000 tokens and max output is 128,000. Knowledge cutoff is 2026-04-30. It is not in Chat. Tool calling uses the Responses API. The model page does not say gpt-6-sol was removed.
- API
OpenAI documents Ultrafast as service_tier ultrafast on gpt-6-astra, up to 8× Standard, for all API users at low default TPM (500K tiers 1–3, 1M tier 4, 5M tier 5). US and global processing only. The guide does not print a dollar rate. GPT-6.1 Sol Ultrafast is announced for the coming days and is not in that guide.
- Deprecation
Legacy Completions IDs listed under Past deprecations
OpenAI lists gpt-3.5-turbo-instruct, babbage-002, davinci-002, and gpt-3.5-turbo-1106 under Past deprecations with shutdown date 2026-09-28 and replacement gpt-5.6-terra. That section still says access will be shut down, unlike the March 2026 section, which says access was shut down.
- Model
OpenAI releases gpt-6-sol and gpt-6-luna in the API and in ChatGPT Work and Codex for Plus, Pro, Business, Enterprise, and Edu. Standard API prices are $2/$10 (Sol) and $0.10/$0.50 (Luna) per 1M tokens. Both have a 1,050,000-token context. They are not in Chat. Free and Go can use Luna in the desktop app. GPT-5.6 remains available. The announcement names Sol and Luna only.
- Model
OpenAI launches GPT-6 Astra (gpt-6-astra) as the new capability peak. This GPT-5.6 family (Sol/Terra/Luna) remains the volume and cost-routing path. ChatGPT Plus gets Astra in Work and Codex; Chat “GPT-6 Pro” is Pro/Business/Enterprise (Enterprise off by default). Standard API $10/$50 per 1M tokens. Not supplied to Cursor.
- Platform
GPT-5.6 Sol promotional API pricing
Sol API drops to $4 input / $20 output per 1M tokens (−20% / −33%); promotional pricing through at least Nov 21, 2026. Applies to API and eligible Codex/Work credits; ChatGPT subscription prices unchanged.
- Platform
Ultrafast mode preview for Sol
Limited API preview of Ultrafast for GPT-5.6 Sol (Cerebras-backed): up to ~750 output tokens/sec and up to ~14× Standard. Pricing and GA not published.
- Product
ChatGPT Sol update + Luna for Free/Go
Plus/Pro get an updated GPT-5.6 Sol with a reasoning slider; Free/Go move to Luna defaults with unlimited text chats rolling out. Distinct from July Codex/Work API builds.
- Platform
Terra/Luna API price reductions
Luna −80% and Terra −20% API pricing; Sol unchanged. Fast mode replaces Priority Processing in the API.
- Model
GPT-5.6 family (Sol / Terra / Luna)
Capability tiers make routing explicit: Sol for peak, Terra balanced, Luna cost-efficient.
- API
gpt-5.6 API routing
API aliases route to Sol by default with Terra/Luna for cost control.
- Product
MCP and agent tooling deepen
Tool use, structured output, and MCP-style workflows become first-class for agents.
- Model
Multimodal + long context
Text, image, and audio understanding with million-token-class context for research agents.
- Research
Reasoning-model era (o1 lineage)
OpenAI popularizes deliberate reasoning traces that later feed GPT-5 thinking modes.
Overview
Capabilities
- Vision: Yes
- Audio input: No
- Tool calling: Yes
- Thinking: Yes
- MCP: Yes
- Coding: Yes
- Structured output: Yes
Technical specifications
- Provider
- OpenAI
- License
- Proprietary
- Context window
- 1.1M
- Parameters
- Undisclosed
- Architecture
- Transformer (proprietary)
- Release
- 2026-07
- Modalities
- Text, Image
- Vision
- Yes
- Audio input
- No
- Tool calling
- Yes
- Thinking
- Yes
- MCP
- Yes
- Open weights
- No
- API
- Yes
- Pricing (input)
- Sol ~$4 / Terra ~$2 / Luna ~$0.20 per 1M tokens
- Pricing (output)
- Sol ~$20 / Terra ~$12 / Luna ~$1.20 per 1M tokens
Supported modalities
Text · Image
Context window
1.1M (1,050,000 tokens)
Pricing
Aug 21 2026: Sol promotional API pricing −20% input / −33% output ($4/$20) through at least Nov 21, 2026. Jul 30 2026: Luna −80%, Terra −20%. Sol Fast mode ~2× Sol price. Ultrafast (Aug 13 preview) has no published price yet—verify OpenAI pricing.
Availability
API: gpt-5.6-sol / terra / luna. ChatGPT (Aug 6 2026): Plus/Pro use updated Sol with a reasoning slider; Free/Go default to Luna (unlimited text chats rolling out). Ultrafast Sol (Aug 13): limited API preview via Cerebras (~14× Standard / up to ~750 tok/s); pricing and GA not published. OpenAI notified SpaceX it intends to stop supplying OpenAI models to Cursor (proposed shutoff 2026-11-12); ChatGPT, API, and Codex are unchanged. GPT-6 Astra (gpt-6-astra) launched 2026-09-03 as the OpenAI peak — Plus ChatGPT gets Astra in Work and Codex, not the Chat picker. GPT-6.1 Sol (gpt-6.1-sol, 2026-09-29) is the current lower-cost Sol. GPT-6 Sol (gpt-6-sol) and Luna (gpt-6-luna) launched 2026-09-22 as earlier cost tiers. This slug remains the GPT-5.6 family, including Sol promotional pricing through at least Nov 21, 2026. OpenAI lists gpt-3.5-turbo-instruct, babbage-002, davinci-002, and gpt-3.5-turbo-1106 under Past deprecations with shutdown date 2026-09-28 and replacement gpt-5.6-terra. That section still says access will be shut down, unlike the March 2026 section, which says access was shut down.
Use cases
- Complex multi-step reasoning and research
- Software engineering and code agents
- Multimodal analysis (docs, charts, images)
- Tool-using assistants and MCP workflows
Strengths
- Clear Sol / Terra / Luna routing for capability vs cost
- Mature tool-calling and structured output ecosystem
- Broad third-party integrations
Limitations
- Closed weights; limited inspectability
- Sol cost and rate limits at high volume
- Vendor lock-in for proprietary features
Related guides
Related benchmarks
Related research
- OpenAI model system cards / technical reports
Evaluation
- OpenAI API deprecations
Evaluation
Related GitHub
Related tools
Related rankings
Related comparisons
Companies
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