One is Anthropic's current flagship; the other is OpenAI's 2025 reasoning workhorse still sitting in the model picker. We ran the same three tasks on both — here is the full comparison.
by Anthropic
Winnerby OpenAI
Claude Opus 5 wins our head-to-head overall — it beat ChatGPT o3 on the debugging and long-document tasks we ran, handles a 1M-token context versus o3's 200K, and scores 68.1% on long-context retrieval (GraphWalks BFS at 1M) where no 200K-class model competes. ChatGPT o3 still owns competition math (96.7% on AIME 2024, per OpenAI's published results) and is far cheaper at the API layer ($2 in / $8 out per 1M tokens versus Opus-tier pricing). If you live in a codebase or feed models long documents, take Opus 5. If your work is math-heavy or budget-sensitive, o3 remains a legitimate pick despite being the older model.
On September 10, 2026, we ran the identical three-task set on Claude Opus 5 (claude.ai, extended thinking on) and ChatGPT o3 (model picker, reasoning effort high), in fresh chats with no prior context. No tools, no file upload — pure model output.
| Metric | Claude Opus 5 | ChatGPT o3 |
|---|---|---|
| Math task correct (9 pairs) | Yes ✓ | Yes ✓ |
| Reasoning chain easy to verify | Yes ✓ | Longer, brute-force flavored |
| Found both planted bugs, first pass | Yes ✓ | 1 of 2, needed a nudge |
| Added regression test unprompted | Yes ✓ | No |
| Spec edge cases found (of 11) | 11/11 ✓ | 9/11 |
| Hallucinated a citation | No ✓ | Yes (1 section) |
| Winner | 🏆 Opus 5 (2 decisive wins) | Math co-winner |
Side-by-side breakdown across key categories
| Feature | Claude Opus 5 | ChatGPT o3 | Winner |
|---|---|---|---|
| Context window | 1M tokens | 200K tokens | Claude Opus 5 |
| Long-context retrieval (GraphWalks BFS 1M) | 68.1% | N/A (200K limit) | Claude Opus 5 |
| AIME 2024 (competition math, official) | Not published | 96.7% (OpenAI) | ChatGPT o3 |
| SWE-bench Verified (agentic coding) | 74.9% | 69.1% (OpenAI, official) | Claude Opus 5 |
| Codeforces Elo (competitive programming) | ~2200 (vendor-reported) | 1876 (OpenAI, official) | Claude Opus 5 |
| Our debugging task (2 bugs, first pass) | Both found | One found | Claude Opus 5 |
| API price (per 1M tokens, in/out) | $15 / $75 | $2 / $8 | ChatGPT o3 |
| Agent/tool orchestration (Claude Code, MCP) | Native, first-party | Via ChatGPT tools & API | Claude Opus 5 |
| Tier | Claude Opus 5 | ChatGPT o3 |
|---|---|---|
| Free | Not available (Sonnet 5 instead) | Limited o3 messages |
| Plus / Pro $20/mo | Opus 5 via usage credits; Sonnet 5 as the daily driver; Projects | o3 selectable in the model picker alongside GPT-5.6 / GPT-6 |
| Max / Pro $200/mo | Generous Opus 5 caps, extended thinking, priority | GPT-6 Astra access (o3 included) |
| API (per 1M tokens) | $15 in / $75 out | $2 in / $8 out |
The pricing gap is the sharpest in this comparison: o3 costs roughly one-seventh of Opus 5 per input token and one-ninth per output token, per OpenAI's and Anthropic's published API price lists. In the consumer apps the picture inverts — a ChatGPT Plus subscription includes o3 outright, while Opus 5 on Claude Pro draws down limited usage credits, so heavy Opus users effectively need the $200 Max plan. For API-driven math pipelines, o3's price is the deciding factor; for hands-on coding and long documents in the browser, Opus 5's quality justifies the credits.
This is a flagship versus a value pick that refuses to die. Opus 5 wins on breadth — coding, agents, long context. o3 wins on pure math and price. Here is how we would split the decision.
Opus 5 found both planted bugs on the first pass, retrieved 11/11 spec edge cases with correct citations, and works at 1M tokens where o3 tops out at 200K. For coding, agents, and long documents, it is the stronger model — and the scores back it up (SWE-bench Verified 74.9% vs 69.1%).
o3's published 96.7% on AIME 2024 remains the number to beat, and it matched Opus 5 on our math task with a correct (if longer) derivation. If your workload is math-dense, o3 is the rational choice.
At $2 in / $8 out per 1M tokens, o3 delivers 90%-class reasoning at roughly a tenth of Opus 5's output price. For high-volume batch jobs that fit in 200K tokens, the math is not close.
1M-token context, first-party Claude Code and MCP orchestration, and the highest agentic coding score of the two. If the model has to act — read repos, edit files, follow multi-page specs — Opus 5 is the tool.
Claude Opus 5 won our hands-on on debugging, retrieval, and scale. ChatGPT o3 is the math-and-budget pick — and it is included with a Plus subscription. Both have free entry points.
Still deciding? Check out these related comparisons and best-of guides.