Claude Sonnet 5 vs GPT-5.5: Anthropic's Mid-Tier Dethrones OpenAI's Flagship

July 1, 2026. Anthropic drops Claude Sonnet 5 — and across the board, it beats GPT-5.5. Not just on one benchmark. On every single directly comparable benchmark. SWE-bench Pro: 63.2% vs 58.6%. Terminal-Bench 2.1: 80.4% vs 78.2%. HLE with tools: 57.4% vs 52.2%. And it does it at 40% cheaper input and 50% cheaper output. Anthropic's mid-tier model just leapfrogged OpenAI's flagship. Here's the complete comparison, sourced from Anthropic's Sonnet 5 System Card and OpenAI's GPT-5.5 announcement.

Full Benchmark Comparison

The table below shows all directly comparable benchmarks — where both models were evaluated on the same benchmark with the same metric. All Sonnet 5 scores are from Anthropic's System Card (Table 8.1.A). GPT-5.5 scores are from OpenAI's official announcement and independent verification on tbench.ai.

* GDPval uses different scales: Anthropic reports an Elo score (1618 for Sonnet 5), OpenAI reports a win-rate percentage (84.9% for GPT-5.5). Direct comparison is not meaningful. Sources: Anthropic Sonnet 5 System Card, OpenAI GPT-5.5 announcement, Morphllm benchmarks, BuildFastWithAI GPT-5.5 review, BenchLM GPQA leaderboard.

Coding: Sonnet 5's Uncontested Lead

SWE-bench Pro is the benchmark that matters most for professional software engineering. It tests real GitHub issue resolution across open-source repositories — the kind of work developers actually do. Sonnet 5 scores 63.2% vs GPT-5.5's 58.6%. That's a 4.6-point gap — not enormous, but consistent and meaningful.

What makes this remarkable is the pricing context. Sonnet 5 achieves this 4.6-point lead at 40% cheaper input cost and 50% cheaper output cost. For teams running AI-assisted code review, bug fixing, or autonomous PR resolution, that's a double win: better accuracy at lower cost.

On Terminal-Bench 2.1 — terminal-based agentic coding — Sonnet 5 leads 80.4% vs 78.2%. A narrower 2.2-point margin, but again: cheaper. For DevOps workflows involving package management, git operations, build systems, and Docker commands, Sonnet 5 is both more capable and more cost-effective.

The DeepSWE Wildcard

One important caveat: on DeepSWE — a third-party benchmark testing harder, longer-horizon coding tasks — the available data compares Opus 4.8, not Sonnet 5. GPT-5.5 scores 70% on DeepSWE vs Opus 4.8's 58%, suggesting GPT-5.5 may have an edge on the most complex, multi-hour coding tasks. Sonnet 5 hasn't been independently tested on DeepSWE yet.

Reasoning: The HLE Gap

Humanity's Last Exam is the benchmark designed to be the hardest test ever created — 3,000 expert-level questions across math, science, and humanities that models can't simply memorize. Two variants exist: without tools (raw reasoning) and with tools (browsers, code execution, shell access).

Without tools, Sonnet 5 leads 43.2% vs 41.4% — a slim 1.8-point margin. With tools, the gap widens to 5.2 points (57.4% vs 52.2%). This pattern mirrors what we saw with Opus 4.8: Anthropic models are particularly strong at tool-augmented reasoning.

Computer Use: Both Beat Humans, Sonnet Leads

OSWorld-Verified measures desktop automation. The human expert baseline is 72.4%. Both models clear it: Sonnet 5 at 81.2% (8.8 pts above humans), GPT-5.5 at 78.7% (6.3 pts above humans). For computer-use agents, Sonnet 5 delivers better automation at lower cost.

Where GPT-5.5 Fights Back

1. Abstract Reasoning (ARC-AGI-2)

GPT-5.5 scores 85.0%. Sonnet 5's score (~84.7%) is within the margin of error. A tie — but the only benchmark where GPT-5.5 isn't behind.

2. Ecosystem and Production Hardening

GPT-5.5 launched 68 days before Sonnet 5. It powers Codex CLI (4M weekly devs) with browser verification, persisted goals, and self-repair loops. Sonnet 5 is brand new and unproven in production.

3. Long-Context Retrieval

GPT-5.5 scores 74.0% on MRCR v2 at 512K-1M token contexts. Anthropic hasn't published comparable long-context benchmarks for Sonnet 5.

Specification Comparison

* Sonnet 5 introductory pricing of $2/$10 per MTok through August 31, 2026. Sources: Claude Platform Docs, Anthropic blog, OpenAI GPT-5.5 announcement.

The Tokenizer Caveat: Real Costs Matter

Sonnet 5 uses the updated tokenizer that Anthropic introduced with Opus 4.7. The same text produces roughly 1.3-1.4x more tokens compared to Sonnet 4.6 and GPT-5.5's tokenizer. Simon Willison's analysis: English text ~1.33-1.42x more tokens, Python code ~1.27-1.28x, Spanish ~1.33x, Simplified Chinese ~1.01x (essentially unchanged).

At effective rates of ~$3.90/$19.50 for English workloads, Sonnet 5 is still meaningfully cheaper than GPT-5.5 at $5/$30. With the introductory $2/$10 pricing, even with token inflation you're paying ~$2.60/$13.00 — roughly half the cost of GPT-5.5.

Should You Use Sonnet 5 or GPT-5.5?

The Ecosystem Reality Check

Benchmarks tell one story. Production tells another. GPT-5.5 has been in the wild for 68 days. It powers Codex CLI, which serves 4 million weekly developers. It's integrated into ChatGPT, Azure, and the OpenAI API with battle-tested reliability. The Codex ecosystem includes browser verification, persisted goals, and self-repair loops.

Sonnet 5 launched yesterday. The benchmarks are undeniable — it's the better model on paper. But Anthropic's developer ecosystem for Sonnet-tier models lags behind OpenAI's. Claude Code is excellent, but it doesn't have the scale, integrations, or production track record of Codex CLI.

Conclusion: The Best Model at the Best Price

Claude Sonnet 5 is the best mid-tier model ever released. It beats OpenAI's current flagship on every directly comparable benchmark. It does it at 40-50% lower cost. The only areas where GPT-5.5 holds ground are abstract reasoning (a tie), long-context retrieval (uncontested), and ecosystem maturity (temporary).

• Starting a new project today? Use Sonnet 5. Better benchmarks, lower price. Introductory $2/$10 through Aug 31 is the best deal in frontier AI.
• Deep in the OpenAI ecosystem? Wait a week for early adopter reports, then switch. The 4.6-point Pro lead at half the cost is too large to ignore.
• On GPT-5.4 or earlier? Upgrade to Sonnet 5 immediately. Better than GPT-5.5 at lower cost than GPT-5.4.

Anthropic's framing was that Sonnet 5 is "close to Opus 4.8 at lower prices." The data shows it's more than that: it's better than GPT-5.5 at lower prices. The mid-tier just became the new flagship.

Sources and Links

• Anthropic — Claude Sonnet 5 System Card — Table 8.1.A capability evaluation summary
• Anthropic — Introducing Claude Sonnet 5 — official launch announcement
• OpenAI — Introducing GPT-5.5 — official announcement with all benchmarks
• Claude Platform Docs — Models Overview
• BuildFastWithAI — GPT-5.5 Review (2026)
• Vellum — Everything You Need to Know About GPT-5.5
• Morphllm — Claude Benchmarks (2026)
• Morphllm — SWE-bench Pro Leaderboard
• BenchLM — GPQA Leaderboard
• Simon Willison — What's New in Claude Sonnet 5
• FailingFast — AI Coding Benchmarks
• O-mega.ai — GPT-5.5: The Complete Guide (2026)