Claude Opus 4.6 vs OpenAI o3 Pro: Which AI Wins at Complex Reasoning in 2026?
Two of the most powerful reasoning models on the market — here is the complete benchmark breakdown with a clear verdict for every reasoning domain.
TL;DR — Quick verdict
- Best for science & legal reasoning: Claude Opus 4.6 (GPQA: 91.3%, BigLaw: 90.2%)
- Best for competition math: OpenAI o3 Pro (AIME 2025: 88.9%)
- Best for coding & engineering: Claude Opus 4.6 (SWE-bench: 80.8%)
- Best for abstract reasoning: Claude Opus 4.6 (ARC-AGI-2: 68.8%)
- Best price/performance: Claude Opus 4.6 ($5/$25 vs o3 Pro $20/$80 per M tokens)
What is the difference between Claude Opus 4.6 and OpenAI o3 Pro?
Claude Opus 4.6 is Anthropic's flagship model, released in February 2026. It is designed as a general-purpose frontier model with exceptional performance across science, law, coding, and multi-step agentic tasks. It supports a 1 million token context window and is the model powering Claude Code — currently the most capable AI coding agent available.
OpenAI o3 Pro is the high-compute version of OpenAI's o3 reasoning model — part of the o-series dedicated reasoning tier. Unlike GPT-5.4 (which aims for speed and breadth), o3 Pro is designed specifically for tasks that benefit from extended thinking time: hard math, structured scientific reasoning, and problems that require working through long chains of logic before answering.
Model overview
| Claude Opus 4.6 | OpenAI o3 Pro | |
|---|---|---|
| Company | Anthropic | OpenAI |
| Released | February 2026 | Early 2026 |
| Model type | General frontier | Dedicated reasoning |
| Context window | 1M tokens | 200K tokens |
| Input price (per M tokens) | $5.00 | $20.00 |
| Output price (per M tokens) | $25.00 | $80.00 |
| Speed (typical response) | Fast–Medium | Slow (extended thinking) |
| Best for | Science, law, coding, agents | Competition math, hard logic |
Benchmark comparison: complex reasoning
| Benchmark | Claude Opus 4.6 | OpenAI o3 Pro | Domain |
|---|---|---|---|
| GPQA Diamond | 91.3% ✓ | ~83% | Graduate-level science |
| AIME 2025 (math) | ~74% | 88.9% ✓ | Competition mathematics |
| AIME 2024 (math) | ~71% | 91.6% ✓ | Competition mathematics |
| SWE-bench Verified | 80.8% ✓ | 69.1% | Software engineering |
| ARC-AGI-2 | 68.8% ✓ | ~55% | Abstract reasoning |
| BigLaw Bench | 90.2% ✓ | N/A | Legal reasoning |
| Multi-step reasoning | 78.7% ✓ | ~76% | General complex tasks |
| Humanity's Last Exam | ~22% | 20.3% | Hardest mixed tasks |
Domain-by-domain verdict
Science and research
Claude Opus 4.6Claude Opus 4.6 scores 91.3% on GPQA Diamond — graduate-level questions across biology, chemistry, and physics. This is 8 percentage points above o3's ~83%. For researchers, scientists, and analysts working with complex scientific literature or data, Claude is the stronger choice. It handles multi-step scientific reasoning, hypothesis generation, and literature synthesis with notably fewer errors.
Competition and advanced mathematics
OpenAI o3 Proo3 Pro's extended thinking architecture was purpose-built for structured mathematical reasoning. Its 88.9% on AIME 2025 and 91.6% on AIME 2024 are the best available scores for any model on competition math benchmarks. Claude Opus 4.6 scores roughly 71–74% on the same tests. If your work centers on formal mathematics, theorem proving, or quantitative research at competition difficulty, o3 Pro is the better model.
Software engineering and coding
Claude Opus 4.6Claude Opus 4.6 scores 80.8% on SWE-bench Verified — the highest of any frontier model, significantly ahead of o3's 69.1%. Claude Code, built on Opus 4.6, is the most capable AI coding agent in 2026 for real-world engineering tasks: multi-file refactoring, debugging complex codebases, writing tests, and autonomous development workflows. For software teams, Claude is the clear winner.
Legal reasoning
Claude Opus 4.6On BigLaw Bench — a benchmark for legal document analysis, contract review, and legal reasoning — Claude Opus 4.6 scores 90.2%. No published o3 Pro score exists for this benchmark. In practice, Claude's long-context window (1M tokens vs o3's 200K) gives it a decisive advantage for legal tasks that involve reviewing entire contracts, case histories, or regulatory documents.
Abstract and novel reasoning
Claude Opus 4.6ARC-AGI-2 tests the ability to solve novel puzzles that cannot be memorized — true abstract reasoning. Claude Opus 4.6 scores 68.8%, compared to o3's estimated ~55%. This matters for complex planning, novel problem-solving, and tasks where a model must reason about situations it has not been explicitly trained on.
Cost-efficiency for reasoning tasks
Claude Opus 4.6o3 Pro costs $20/$80 per million tokens — four times more expensive than Claude Opus 4.6 ($5/$25). For most professional reasoning workloads, Claude delivers equal or superior performance at 25% of the cost. Only in pure mathematical domains does o3 Pro justify its significant price premium. For budget-conscious teams needing strong reasoning across multiple domains, Claude is the efficient choice.
When to choose o3 Pro over Claude
o3 Pro is the right choice in a narrow but important set of situations:
- Your primary task is formal or competition-level mathematics — theorem proving, quantitative finance, or academic math research
- You need extended multi-step deductive reasoning on fully structured problems (puzzles, proofs, logic chains)
- Speed is not a requirement and you need maximum effort applied to a hard problem
- Budget is not a constraint and the task is high-stakes enough to warrant $80 per million output tokens
For everything else — science, law, coding, business analysis, writing, agentic workflows — Claude Opus 4.6 delivers better or equal performance at dramatically lower cost.
Using both models via a single platform
The engineers and research teams getting the most out of AI reasoning in 2026 are not locked into a single model. They route mathematical tasks to o3 Pro and everything else to Claude Opus 4.6 — getting the best of both without managing two separate subscriptions.
Platforms like Happycapy give you access to Claude Opus 4.6, o3 Pro, GPT-5.4, and Gemini 3.1 Pro through a single workspace. Switch models mid-conversation, compare responses side-by-side, or let the platform route by task type — without paying for multiple API keys.
Claude Opus 4.6 + o3 Pro. One platform.
Happycapy gives you access to every frontier reasoning model — switch between Claude, o3 Pro, and GPT-5.4 depending on your task, without managing multiple subscriptions.
Try Happycapy Free →Frequently asked questions
Which is better for complex reasoning — Claude Opus 4.6 or OpenAI o3?
It depends on the domain. Claude Opus 4.6 is superior for graduate-level science (GPQA Diamond: 91.3%), legal reasoning (BigLaw Bench: 90.2%), software engineering (SWE-bench: 80.8%), and abstract reasoning (ARC-AGI-2: 68.8%). OpenAI o3 is stronger for competition-level mathematics (AIME 2025: 88.9%) and structured mathematical problem-solving. For most professional reasoning tasks outside pure math, Claude Opus 4.6 is the stronger choice.
What is OpenAI o3 Pro?
OpenAI o3 Pro is the high-compute version of OpenAI's o3 reasoning model. It belongs to the o-series — OpenAI's dedicated reasoning tier designed for complex tasks that benefit from extended 'thinking' time before responding. o3 Pro runs the same underlying model as o3 but with more compute allocated per response, making it significantly slower and more expensive but more accurate on hard reasoning tasks like competition math and scientific problem-solving.
How much does o3 Pro cost compared to Claude Opus 4.6?
OpenAI o3 Pro is priced at approximately $20 per million input tokens and $80 per million output tokens — making it one of the most expensive models available. Claude Opus 4.6 costs $5 per million input tokens and $25 per million output tokens. For high-volume complex reasoning workloads, Claude Opus 4.6 delivers competitive or superior performance at roughly 25–30% of the cost of o3 Pro.
Which model is better for coding and software engineering?
Claude Opus 4.6 is significantly better for software engineering tasks. It scores 80.8% on SWE-bench Verified — the leading score among frontier models — compared to o3's 69.1%. Claude Code, Anthropic's coding agent built on Opus 4.6, is widely considered the most capable AI coding tool in 2026 for multi-file refactoring, debugging complex codebases, and writing production-quality code.
Related reading
Sources: Artificial Analysis AI benchmark tracker, Anthropic model card (Claude Opus 4.6), OpenAI o3 system card, SWE-bench leaderboard (swebench.com), ARC Prize benchmark results (arcprize.org), BigLaw Bench evaluation report.