OpenAI retires o3 from ChatGPT on August 26, 2026. The best o3 replacement for most teams is GPT-5.6 Terra at $2/$12 per million tokens — not the officially recommended Sol at $5/$30. Terra posts 90.4% on GPQA Diamond against o3’s 87.7%, at 60% less output cost. API users are not on the same clock: their o3 shutdown is December 11.
The reasoning model that defined 2025 is being switched off. And the migration advice OpenAI published is the expensive option.
Here is what actually changes on Tuesday, what each o3 replacement costs, and which one wins for your workload.
What exactly happens to o3 on August 26, 2026?
o3 disappears from the ChatGPT model picker on August 26, 2026. That is a product change, not an API shutdown. OpenAI’s release notes from May 28, 2026 confirm o3 is “retired from ChatGPT on August 26, 2026 following a 90-day sunset period.” Developers calling o3 over the API keep working past that date.
The ChatGPT side: the picker already stopped naming models
ChatGPT users lose nothing they can still see. Since the June 10, 2026 model picker update, OpenAI stopped exposing version numbers entirely.
The picker now offers Instant, Medium, High, and Extra High, plus Pro Standard and Pro Extended on Pro plans. Reasoning is sold as effort, not as a model name.
So for consumer subscribers, the o3 replacement is already installed. Selecting High is the closest analogue to what o3 Thinking used to do.
The API side: your real deadline is December 11
This is where most coverage gets it wrong. OpenAI’s deprecations page lists o3-2025-04-16 with a shutdown date of December 11, 2026, migrating to gpt-5.6-sol. o3-pro-2025-06-10 follows the same date, moving to Sol with reasoning.mode: pro.
What does die on August 26 is the legacy Assistants API. Anything built on Assistants must move to the Responses and Conversations APIs by Tuesday. That is the deadline worth panicking about.
Which o3 replacement is best for most workloads?
GPT-5.6 Terra. OpenAI names Sol as the official successor, but Sol is priced for frontier reasoning at $5/$30 per million tokens. Terra sits at $2/$12 and clears o3 on the benchmarks that mattered to o3 users. For the overwhelming majority of o3 traffic, paying Sol rates is a rounding error you repeat a million times.
Terra got cheaper on July 30, 2026, when OpenAI cut its price roughly 20% as part of the GPT-5.6 pricing refresh. Luna fell about 80% in the same announcement, to $0.20/$1.20.
The quality case is not a stretch either. OpenRouter’s provider data puts Terra at 90.4% on GPQA Diamond and 75.3% on TAU-Bench. o3 scored 87.7% on GPQA Diamond at launch.
Why Terra beats Sol on cost per task
Reasoning models bill you for tokens you never see. A 500-token visible answer can consume 2,000+ tokens once hidden reasoning is counted.
That multiplier is exactly why output price dominates your bill. At $12 versus $30 per million output tokens, Terra cuts the expensive half of the invoice by 60%.
Terra also carries a 1,050,000-token context window with 128,000 max output — over five times o3’s 200K context. You are not trading capability down.
How much does each o3 replacement cost?
Prices below are list rates per million tokens as of August 24, 2026, pulled from provider pricing pages. Cached input bills at 10% of standard rates on OpenAI, and the Batch API halves both sides.
| Model | Input | Output | Context | Notes |
|---|---|---|---|---|
| o3 (retiring) | $2.00 | $8.00 | 200K | API shutdown Dec 11, 2026 |
| o3-pro (retiring) | $20.00 | $80.00 | 200K | API shutdown Dec 11, 2026 |
| GPT-5.6 Terra | $2.00 | $12.00 | 1.05M | Cut ~20% on Jul 30, 2026 |
| GPT-5.6 Sol | $5.00 | $30.00 | 1.05M | Official o3 successor |
| GPT-5.6 Luna | $0.20 | $1.20 | 1.05M | Cut ~80% on Jul 30, 2026 |
| Claude Opus 5 | $5.00 | $25.00 | 200K | Fast Mode is $10/$50 |
| Claude Sonnet 5 | $2.00 | $10.00 | 200K | $2/$10 now permanent |
| Gemini 3.7 Flash | $0.75 | $3.75 | — | Doubles Jan 1, 2027 |
| Kimi K3 | $2.60 | $13.00 | 1M | Open weights, 2.8T params |
Read that table one way and the story is obvious: o3 at $2/$8 was cheap, and every direct successor except Luna and Gemini Flash costs more per output token. Migration is a price increase unless you choose deliberately.
Is GPT-5.6 Sol worth $5/$30 in 2026?
Only for the top slice of your traffic. Sol is the frontier tier and the only model OpenAI formally maps o3-pro onto, via reasoning.mode: pro. If you were paying o3-pro’s $20/$80, Sol at $5/$30 is a 75% input cut and a 62.5% output cut.
If you were on standard o3, Sol is a 150% input increase and a 275% output increase. Same model family, opposite financial outcome.
There is also a long-context trap. Requests beyond the standard threshold reprice: Sol rises to $10/$45, Terra to $4/$18, Luna to $0.40/$1.80. Feeding a million-token repo into Sol is a different product than a 20K-token prompt.
Which o3 replacement should you pick for your use case?
Match the model to the job, not to the vendor’s migration note. Below is where each option earns its price, based on published benchmarks and list pricing. Benchmark your own top 20 prompts against Terra before escalating anything to Sol.
| Use case | Pick | Why |
|---|---|---|
| General o3 traffic | GPT-5.6 Terra | 90.4% GPQA Diamond at $2/$12; 1.05M context |
| Hardest reasoning, o3-pro traffic | GPT-5.6 Sol | Official reasoning.mode: pro path; 62.5% cheaper output than o3-pro |
| High-volume classification | GPT-5.6 Luna | $0.20/$1.20 after the ~80% July cut |
| Agentic coding, terminal work | Gemini 3.7 Flash | 85.8% on Terminal-bench 2.1 at $0.75/$3.75 |
| Long document analysis | Claude Opus 5 | $5/$25 with 200K context, cheaper output than Sol |
| Balanced daily driver | Claude Sonnet 5 | $2/$10 locked permanently |
| Self-hosting, data residency | Kimi K3 | 2.8T open weights, 1M context, $2.60/$13 hosted |
What about Claude and Gemini as an o3 replacement?
Both are live options, and both moved on price in the last two weeks. Anthropic canceled a scheduled increase; Google launched a discount with an expiry date attached. Those two facts change the math more than any benchmark did.
Claude Sonnet 5’s price freeze quietly killed a 50% increase
Sonnet 5 launched at $2/$10 as introductory pricing set to expire August 31, 2026, with a jump to $3/$15 scheduled for September 1. Anthropic’s pricing documentation now states that increase “will not occur” and $2/$10 is the standard price.
That makes Sonnet 5 an exact price match to o3 on input and 25% more on output — the closest financial like-for-like swap available. We broke down how it stacks up against Google’s cheap tier in Gemini 3.7 Flash vs Claude Sonnet 5.
Gemini 3.7 Flash is the cheapest credible option — until January
Gemini 3.7 Flash shipped August 13, 2026 with a 50% introductory cut to $0.75/$3.75. On January 1, 2027 it reverts to $1.50/$7.50, and context caching moves from $0.075 to $0.15.
Its numbers are strong where agents live: 85.8% on Terminal-bench 2.1 and 65.3% on DeepSWE v1.1, though only 43.6% on FrontierCode 1.1 Main. Build your 2027 budget on the standard rate, not the promo.
How do you migrate off o3 without breaking production?
Treat this as a pricing audit, not a find-and-replace. The single most expensive mistake is routing all o3 traffic to Sol because the deprecation table said so. Work through it in this order:
- Split the Assistants API work out first. It dies August 26, 2026 — 107 days before o3 does. Move to Responses and Conversations now.
- Pull your last 30 days of o3 token spend and split it by input versus output. Output volume decides which tier you can afford.
- Replay your top 20 prompts against Terra. If quality holds, you are done at $2/$12.
- Escalate only the failures to Sol. Route by task difficulty, not by default.
- Push bulk classification to Luna or Gemini 3.7 Flash. At $0.20/$1.20, Luna makes some batch jobs nearly free.
- Turn on prompt caching and the Batch API. Cache hits bill at 10%; batch halves everything.
Teams running agentic coding harnesses should also re-check their tooling layer, not just the model. We covered that trade-off in Claude Code vs Codex CLI.
Frequently asked questions about the o3 replacement
Is o3 gone from the API on August 26, 2026?
No. August 26 is the ChatGPT retirement date. OpenAI’s deprecations page lists the o3-2025-04-16 API shutdown as December 11, 2026, with gpt-5.6-sol as the migration target.
What is the cheapest o3 replacement?
GPT-5.6 Luna at $0.20/$1.20 per million tokens, following its roughly 80% price cut on July 30, 2026. For work needing more reasoning depth, Gemini 3.7 Flash at $0.75/$3.75 is the next step up.
Does GPT-5.6 Terra actually beat o3?
On GPQA Diamond, yes: 90.4% for Terra on OpenRouter’s provider data versus 87.7% for o3 at launch. Terra also carries a 1.05M context window against o3’s 200K.
What replaces o3-pro?
GPT-5.6 Sol with reasoning.mode: pro, per OpenAI’s deprecations table. At $5/$30 versus o3-pro’s $20/$80, it is a substantial price cut for that specific tier.
Will Claude Sonnet 5 get more expensive in September?
No. Anthropic canceled the September 1, 2026 increase to $3/$15. The $2/$10 rate is now permanent per its pricing docs.
Is there an open-weights o3 replacement?
Kimi K3 is the closest: 2.8 trillion parameters, 1M context, released July 16, 2026, and $2.60/$13 hosted on OpenRouter. Chinese open-weight coders are also competitive — see our GLM-5.3 vs DeepSeek V4 Pro breakdown.
What breaks on August 26 if I do nothing?
Two things: o3 vanishes from the ChatGPT picker, and the legacy Assistants API stops working. API calls to o3 itself keep running until December 11, 2026.
The bottom line
Move general o3 traffic to GPT-5.6 Terra at $2/$12 and stop there. It beats o3 on GPQA Diamond, gives you five times the context, and costs 60% less per output token than the Sol tier OpenAI points you at.
Send only o3-pro-class work to Sol, where $5/$30 is genuinely a 62.5% output discount on what you were paying. Send bulk work to Luna at $0.20/$1.20.
The decision depends on exactly one number: your output-token share. Above roughly 30% of spend, tier choice dominates everything else on your invoice. Below that, input caching matters more than which model you pick.
And fix your Assistants API code before Tuesday. That is the only hard deadline this week. For more on where frontier pricing is heading, see our analysis of DeepSeek’s vision model against Claude Opus 4.8.
Sources
- OpenAI — ChatGPT Release Notes (o3 retirement, May 28, 2026)
- OpenAI — API Deprecations
- OpenAI — Advancing the price-performance frontier with GPT-5.6
- OpenRouter — GPT-5.6 Terra pricing and benchmarks
- Anthropic — Claude Platform pricing
- VentureBeat — Gemini 3.7 Flash introductory price cut
- OpenRouter — Kimi K3 specifications
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