GPT-5.6 vs Grok 4.5 vs Claude Fable 5: Early Comparison

Three frontier models are now competing for the same expensive corner of the AI stack: coding, agents, research, and complex professional work. The launches arrived close together. Their jobs are already separating.

Claude Fable 5 makes the strongest documented case for difficult work that may run for hours or days. Grok 4.5 is the value challenger, with aggressive pricing and a broad technical and office-work pitch. GPT-5.6 gives OpenAI users the widest range inside one family, from Luna through Terra to Sol. This is an evidence-based comparison built from official launch material, pricing pages, and provider documentation. It is not a controlled Choosely benchmark, and the vendors have naturally selected evidence that makes their own models look rather capable.

GPT-5.6, Grok 4.5 and Claude Fable 5 at a glance

The prices above are standard API token prices rather than consumer subscription fees (OpenAI pricing update, xAI pricing, Anthropic pricing). OpenAI says GPT-5.6 is generally available across ChatGPT, Codex, and the API (OpenAI availability). xAI says Grok 4.5 is available in Grok Build, Cursor on all plans, and the SpaceXAI console and API (xAI availability). Anthropic says Fable 5 is available to Claude users, on the Claude Platform, and through AWS, Google Cloud, and Microsoft Foundry (Anthropic availability).

What is GPT-5.6?

GPT-5.6 arrives as a three-tier family.

• Sol: OpenAI's flagship model.
• Terra: a balanced model for everyday work.
• Luna: a fast and lower-cost tier.

OpenAI says Terra is competitive with GPT-5.5 at half the cost, while Luna brings strong capability at the family's lowest price (OpenAI preview). The more interesting change is how OpenAI packages reasoning and agentic work.

OpenAI says GPT-5.6 introduces a new max reasoning effort for Sol and an ultra mode that uses subagents to accelerate complex work (OpenAI capabilities). That pushes GPT-5.6 further beyond the idea of one chatbot response. At least in OpenAI's own framing, this is a model family meant to coordinate harder, longer, multi-step work.

Where is GPT-5.6 available?

OpenAI says the GPT-5.6 family is generally available across ChatGPT, Codex, and the API. Access differs by plan: ChatGPT users receive model choices based on their tier, ChatGPT Work and Codex expose configurable reasoning effort, and API developers can use Sol, Terra, and Luna directly (OpenAI availability). That broader rollout changes the practical question raised in Choosely's earlier provider-risk article.

Now that access is broader, which GPT-5.6 tier earns a defined job in a real stack?

Where OpenAI says GPT-5.6 is strongest

OpenAI's early material focuses on a few areas in particular.

• Command-line coding and tool coordination.
• Long-horizon biology and genomics work.
• Defensive cybersecurity and vulnerability work.
• Multi-agent execution through ultra mode.
• More efficient reasoning compared with GPT-5.5.

OpenAI says GPT-5.6 Sol sets a new state of the art on Terminal-Bench 2.1 and improves on GPT-5.5 on GeneBench v1 while using fewer tokens (OpenAI capabilities). The practical caveat is simple.

GPT-5.6 can now be tested against real workflows, but much of the launch evidence still comes from OpenAI's own evaluations. Its strongest-model claim should remain provisional until broader independent testing accumulates.

GPT-5.6 pricing

OpenAI reduced Terra pricing by 20% and Luna pricing by 80% on July 30. Sol pricing did not change (OpenAI pricing update). OpenAI also says GPT-5.6 introduces more predictable prompt caching. Cache writes are billed at 1.25 times the uncached input rate, while cache reads keep the 90% cached-input discount (OpenAI pricing).

The tiering gives developers a cleaner routing strategy. Instead of using one expensive frontier model for everything, a product could use Luna for routine operations, Terra for normal professional work, and Sol only when a task genuinely requires deeper reasoning.

What is Grok 4.5?

Grok 4.5 is xAI's newest frontier model, built primarily for coding, agentic tasks, and knowledge work (xAI launch). xAI says Grok 4.5 was trained on datasets spanning coding, science, engineering, and mathematics, and that it was trained alongside Cursor (xAI launch). Its launch remains heavily focused on technical execution, but calling it only a coding model would undersell how xAI is positioning it.

xAI is also pitching the model for office work. Its launch post says Grok 4.5 is now the default model in Grok Build and highlights Excel, PowerPoint, and Word workflows, including native add-ins for those Microsoft apps (xAI office work). Grok 4.5 is currently available through the following channels.

• Grok Build.
• Cursor on all plans.
• The SpaceXAI console and API.
• Word, PowerPoint, and Excel add-ins.

xAI says Grok 4.5 is available now through Grok Build, Cursor, and the SpaceXAI console and API (xAI availability).

Grok 4.5's strongest argument: price

Grok 4.5 costs the following at standard API rates. $2 per million input tokens and $6 per million output tokens (xAI pricing).

xAI's reasoning docs also show that grok-4.5 supports reasoning.effort settings of low, medium, and high, with high as the default, and that it can use function calling, web search, X search, and code execution through the platform (xAI reasoning docs). At listed API rates, Grok 4.5's output is the following.

• Five times cheaper than GPT-5.6 Sol.
• Five times more expensive on output than GPT-5.6 Luna at its reduced July 30 price.
• More than eight times cheaper than Claude Fable 5.

Price alone does not make Grok the better model, but it gives xAI a serious argument for high-volume coding and agentic workloads where every run has to earn its cost. xAI also says Grok 4.5 is served at 80 tokens per second and used 15,954 output tokens on average on its published SWE-Bench Pro comparison, versus 67,020 for Claude Opus 4.8 at max effort (xAI launch). Those are still vendor-run figures, so they should be treated as early directional evidence rather than a final market truth.

xAI is also offering limited-time free Grok 4.5 usage in Grok Build and Cursor, which lowers the barrier to practical testing (xAI availability).

What xAI's own benchmarks show

Interestingly, xAI's benchmark table does not show Grok 4.5 winning every engineering test. Its published figures place Claude Fable 5 ahead of Grok 4.5 on DeepSWE 1.0, DeepSWE 1.1, Terminal-Bench 2.1, and SWE-Bench Pro, while Grok leads on SWE Marathon (xAI launch).

That makes the more credible Grok argument this.

Strong frontier-level capability, fast delivery, and aggressive pricing, rather than universal benchmark dominance.

For developers running large volumes of coding or agentic tasks, being close to the frontier at a substantially lower cost may matter more than topping every leaderboard.

What is Claude Fable 5?

Claude Fable 5 is Anthropic's most capable generally available model and is designed for ambitious work that may take hours or days rather than one short prompt (Anthropic Fable page). Anthropic positions it around several kinds of work.

• Large software migrations.
• Multi-stage engineering projects.
• Deep research.
• Complex professional analysis.
• Enterprise knowledge work.
• Long-running autonomous agents.
• Document-heavy work involving charts, tables, and diagrams.

Anthropic says Fable 5 can work for days inside an agent harness, plan across stages, delegate to sub-agents, and check its own work (Anthropic Fable page). Its role is fairly clear.

Fable 5 is far too expensive for every small rewrite, classification task, or summary. Its case begins with substantial projects where quality and sustained reasoning matter more than token cost.

Claude Fable 5 pricing

Claude Fable 5 costs the following at standard API rates. $10 per million input tokens and $50 per million output tokens (Anthropic pricing).

Anthropic also says prompt caching keeps its existing 90% input-token discount, but Fable remains considerably more expensive than Grok 4.5 and every announced GPT-5.6 tier (Anthropic pricing). For lightweight everyday work, that premium may be hard to justify.

For a complex engineering migration, professional analysis, or research project that might otherwise consume many expert hours, the calculation is different. The more useful metric becomes the cost of the completed outcome rather than the price of each individual token. That cost lens also lines up with Choosely's earlier article on usage-based AI pricing.

The unusual Fable 5 rollout

Anthropic launched Claude Fable 5 on 9 June 2026 (Anthropic launch). On 12 June 2026, Anthropic suspended Fable 5 and Mythos 5 after US government export controls required access restrictions it could not reliably enforce in real time (Anthropic suspension and redeployment).

Anthropic says those controls were lifted on 30 June 2026, with Fable 5 returning globally from 1 July 2026 (Anthropic redeployment). Anthropic also says Pro, Max, Team, and select Enterprise plans included Fable 5 for up to 50% of weekly usage through 7 July 2026, after which it moved to usage credits (Anthropic redeployment).

Fable 5's safeguards and fallback model

Claude Fable 5 includes strengthened safeguards across cybersecurity and biology (Anthropic Fable page, Anthropic launch). Anthropic says flagged requests in those areas may be routed to Claude Opus 4.8, and that users are not charged Fable 5 prices for rerouted requests (Anthropic Fable page).

Anthropic also says more than 95% of Fable sessions involve no fallback at all, while acknowledging that its stricter safeguards can still trigger on benign requests and will need refinement over time (Anthropic launch). That matters for cybersecurity teams and developers working near sensitive technical boundaries.

Fable 5 may provide extremely strong underlying capability, but some legitimate tasks can experience more friction because Anthropic has deliberately chosen a conservative safety threshold.

Which model is best for coding?

There is no universal coding winner yet, which is inconvenient for launch-day scorecards and useful for everyone else.

Claude Fable 5

Fable appears best suited to the following kinds of work.

• Large repositories.
• Multi-day implementations.
• Complex migrations.
• Projects requiring planning, testing, and repeated self-correction.
• High-value engineering where model cost is secondary.

Anthropic's material gives it the strongest documented case for long-horizon autonomous coding available today, but it is also by far the most expensive option.

Grok 4.5

Grok appears best suited to the following kinds of work.

• Cost-sensitive coding agents.
• Frequent or high-volume development work.
• Terminal and tool-based workflows.
• Developers already working in Cursor or Grok Build.
• Applications where speed and unit economics matter.

It may offer the strongest early price-to-capability ratio, even where it does not lead every benchmark.

GPT-5.6 Sol

Sol could become the strongest option for particularly difficult engineering tasks, especially through max reasoning and ultra multi-agent execution. GPT-5.6 is now broadly available, but its position still needs independent testing against real repositories and completed production work.

Which model is best for agents?

All three companies are converging on the same destination: models that do not merely answer questions, but perform extended work. Their approaches appear different.

OpenAI is introducing explicit subagent orchestration through GPT-5.6 ultra mode (OpenAI capabilities). xAI is emphasizing fast, economical technical execution through Grok Build, tool use, and code execution (xAI launch, xAI reasoning docs).

Anthropic is positioning Fable 5 as a persistent autonomous worker capable of planning, delegating, and operating for days (Anthropic Fable page). The likely dividing line is this.

• Claude Fable 5 for the most ambitious, long-running individual projects.
• Grok 4.5 for economical and repeated agent workloads.
• GPT-5.6 for users who want one model family spanning low-cost work through intensive multi-agent execution.

That assessment remains provisional. Reliability, latency, integration quality, usage limits, and the cost of completed work will matter as much as benchmark scores.

The practical choice by use case

These are early Choosely positioning recommendations, not the result of a controlled head-to-head benchmark. They should be revised as GPT-5.6 becomes broadly available and more independent testing emerges.

Do you need all three?

Probably not. Most teams should choose one primary model, give it a defined job, and add a second only when it solves a real weakness. Paying for every frontier launch is not a strategy. It is a collection.

OpenAI-first teams can route routine work to Luna or Terra and reserve Sol for the difficult jobs. Cost-sensitive technical teams should test Grok 4.5 hard. Teams handling large migrations, deep research, or prolonged professional work have the clearest reason to keep Fable 5 available. The winning stack is the one with fewer overlapping subscriptions and clearer model roles.

The Choosely verdict

Claude Fable 5 gets our nod for the hardest long-running work, provided the project is valuable enough to justify the premium. Grok 4.5 has the strongest early value case for repeated coding, agent, and office workloads. GPT-5.6 offers the broadest single-provider range and the cleanest internal routing story. There is still no universal winner. Choose the role first, then make the model earn its place on completed work rather than benchmark theater.

What could change this view

• Independent results on real repositories and long-running agent tasks.
• The number of retries and corrections each model needs before the work is usable.
• Latency and total workflow cost at the reasoning settings people actually use.
• Reliability of subagent coordination, tool use, and safeguard behavior during legitimate technical work.
• Changes to availability, plan limits, or API pricing.

Sources

• OpenAI: GPT-5.6
• OpenAI: July 30 GPT-5.6 pricing update
• xAI: Introducing Grok 4.5
• xAI Docs: Reasoning
• Anthropic: Claude Fable 5
• Anthropic: Claude Fable 5 and Claude Mythos 5
• Anthropic: Redeploying Claude Fable 5

Originally published: 10 July 2026. Launch and availability details substantially updated: 17 July 2026. Pricing updated: 31 July 2026.

GPT-5.6 is generally available across ChatGPT, Codex, and the OpenAI API. Grok 4.5 is available through Grok Build, Cursor, and the SpaceXAI console and API. Claude Fable 5 is available through Anthropic's products, API, and supported platforms. Choosely will keep this comparison current as independent evidence and availability change.