Claude Opus 5 changelog

Arena text leaderboard: Gemini 4 Argon first, Opus 5.5 second, GPT-6 Astra 73rd

Arena's October 20, 2026 text leaderboard ranks Gemini 4 Argon (High) first at 1532 (4,900 votes), Claude Opus 5.5 (High) second at 1516, Claude Opus 5 (Max) fourth at 1509, Claude Sonnet 5.5 (xHigh) 28th at 1473, GPT-6.1 Sol (Max) 66th at 1443 and GPT-6 Astra (Max) 73rd at 1441.

Arena WebDev leaderboard: Opus 5.5 first, GPT-6 Astra second

Arena's October 19, 2026 WebDev leaderboard ranks Claude Opus 5.5 (Max) first at 1807 (2,925 votes), GPT-6 Astra (Max) second at 1774, Claude Sonnet 5.5 (xHigh) third at 1768, GPT-6.1 Sol (Max) fourth at 1753, Claude Opus 5 (Max) eighth at 1689 and Gemini 4 Argon (High) tenth at 1678.

Vals AI Terminal-Bench 4.0 with one harness: Opus 5.5 65.2%, Astra 59.6%, Argon 57.6%

Running every model in the same Mini-SWE-agent harness, Vals AI reports Claude Opus 5.5 at 65.15%, Sonnet 5.5 64.14%, GPT-6 Astra 59.60%, Gemini 4 Argon 57.58%, GPT-6.1 Sol 55.05% and Opus 5 53.53%. GPT-6.1 Sol cost $1.72 per test against $13.20 for Opus 5.5.

Copilot to remove Claude Opus 4.8, Claude Opus 5 and GPT-5.6 Terra on November 16

Copilot will deprecate Claude Opus 4.8 and Claude Opus 5 (replace with Claude Opus 5.5) and GPT-5.6 Terra (replace with GPT-6.1 Sol or GPT-6 Luna) on November 16, 2026. Opus 4.8 stays for individual annual-plan subscribers.

Takes effect Nov 16, 2026 · in 15 days

Who is affected:

Copilot users and integrations pinned to those models.

Action needed:

Move to the suggested alternatives before November 16, 2026.

Opus 5 in Claude Code scores 53.9% on Terminal-Bench 4.0 at xhigh effort

Claude Code with Opus 5 scored 53.94% at xhigh, 51.82% at max (posted August 27), 50.3% at high, 44.85% at medium and 34.85% at low, each over 330 trials.

Who is affected:

Teams still on Opus 5 deciding whether to move to Opus 5.5.

What to do:

Compare against Opus 5.5 (64.85%) before upgrading.

Vals AI retires SWE-bench Verified with Opus 5 first at 97.0%

Vals AI archived its SWE-bench Verified leaderboard because performance has saturated. As of September 1, 2026, Claude Opus 5 with Mini-SWE-agent ranks first at 97.00%, ahead of DeepSeek V4 Pro 0813 (96.40%) and GPT-5.6 Sol (96.20%). Newer models such as Opus 5.5 and GPT-6 are not run.

Who is affected:

Anyone citing SWE-bench Verified to compare current models.

What to do:

Use Terminal-Bench 4.0 or SWE-Bench Pro results for newer models.

Claude Code default model for seat-based Enterprise switches to Opus 5

Claude Code 2.1.251 changed the default model for seat-based Enterprise subscriptions to Opus 5, matching other premium plans.

Who is affected:

Claude Code users on seat-based Enterprise plans.

What to do:

Pick a model with /model if you want a different default.

Report: Claude Code 2.1.237 sent the 'high' effort setting as the old 'low' value

A MarsBit report on KuCoin says a developer found Claude Code 2.1.237 mapped the 'high' reasoning level to the value 10, previously used for 'low', as part of an undisclosed A/B test, and that an Anthropic engineer confirmed the issue but said it did not affect model performance.

Who is affected:

Claude Code users on version 2.1.237 who selected high effort.

What to do:

Update Claude Code and confirm the effort level with /effort.

Hacker News thread: Why does Opus 5 feel worse to work with?

A widely shared post said Opus 5 benchmarks higher than Opus 4.7 and 4.8 but needs more supervision because it makes assumptions and reinterprets plans without asking. The thread drew 993 points and 874 comments.

Who is affected:

Developers using Claude Opus 5 for interactive coding.

What to do:

Compare Opus 5.5 on your own tasks before staying on Opus 5.

Claude Opus 5 available in Perplexity Search and Computer

Claude Opus 5 went live across Perplexity Search and Computer. Perplexity says it beat every model it tested on its WANDR benchmark except Fable 5, at 57% lower cost per task.

Claude Code makes Opus 5 the default Opus model

Claude Code 2.1.219 added Claude Opus 5 as the default Opus model and removed Opus 4.7 from fast mode. Anthropic also made Opus 5 the default model on Claude Max.

Who is affected:

Claude Code users on the opus alias or the Max plan.

What to do:

Pin claude-opus-4-8 with /model if you need the previous model.

Claude Opus 5 released

Claude Opus 5 launched at $5 / $25 per million tokens, the same as Opus 4.8, with a 1M token context window and 128k max output. It became the default model on Claude Max.

Who is affected:

Developers and subscribers using Claude Opus 4.8.

What to do:

Test claude-opus-5. Disabling thinking now fails at xhigh or max effort.

Claude Opus 5 rejects disabled thinking at xhigh and max effort

On Claude Opus 5, sending thinking disabled together with effort xhigh or max returns a 400 error.

Who is affected:

Code moving from Claude Opus 4.8 to Opus 5 that turns thinking off.

What to do:

Use effort high or below when thinking is disabled.

Claude Opus 5 available in GitHub Copilot

Claude Opus 5 is available on Copilot Pro+, Max, Business and Enterprise at provider API list price. GitHub notes its cyber safeguards may block some security-related requests.

Who is affected:

GitHub Copilot users on Pro+ and above.

What to do:

Copilot Business and Enterprise admins: enable the Claude Opus 5 policy if you want it.