{"schema":"postcutoff/event@1","as_of":"2026-11-13T13:49:00+01:00","url":"https://postcutoff.com/e/2025-09-29-claude-sonnet-4-5/","md":"https://postcutoff.com/e/2025-09-29-claude-sonnet-4-5/index.md","disclosure":{"written_by":"AI agents (Claude Opus 5.5 in Claude Code)","editor":"human editor","policy":"https://postcutoff.com/about/"},"license":null,"id":"2025-09-29-claude-sonnet-4-5","date":"2025-09-29","date_precision":"day","short_title":"Anthropic releases Claude Sonnet 4.5","deck":null,"takeaway":"Claude Sonnet 4.5 became the state-of-the-art model on SWE-bench Verified and OSWorld, able to maintain focus on complex tasks for over 30 hours; Anthropic also launched the Claude Agent SDK and Claude Code 2.0.","category":"model-release","category_label":"Model releases","importance":4,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":[{"n":1,"title":"Anthropic Engineering: Effective context engineering for AI agents (Sept 29, 2025)","url":"https://www.anthropic.com/engineering/effective-context-engineering-for-ai-agents","type":"official","group":"primary","domain":"anthropic.com"},{"n":2,"title":"Introducing Claude Sonnet 4.5 (Anthropic)","url":"https://www.anthropic.com/news/claude-sonnet-4-5","type":"official","group":"primary","domain":"anthropic.com"},{"n":3,"title":"Introducing Claude Haiku 4.5 (Anthropic)","url":"https://www.anthropic.com/news/claude-haiku-4-5","type":"official","group":"primary","domain":"anthropic.com"}],"official":3,"filed":"2026-09-29","updated":"2026-10-11","orgs":["Anthropic"],"title":"Anthropic releases Claude Sonnet 4.5","summary":"Claude Sonnet 4.5 became the state-of-the-art model on SWE-bench Verified and OSWorld, able to maintain focus on complex tasks for over 30 hours; Anthropic also launched the Claude Agent SDK and Claude Code 2.0. Claude Haiku 4.5 followed on 15 October 2025.","key_facts":["Released 29 September 2025","SWE-bench Verified: 77.2%, per Anthropic","OSWorld: 61.4%, per Anthropic","Observed working autonomously for more than 30 hours on complex tasks","Same price as Sonnet 4: $3 / $15 per million tokens"],"key_numbers":[],"tags":["llm","claude","coding","agents","computer-use"],"science":null,"body_md":"## What happened\n\nAnthropic released its best coding and computer-use model at the time, alongside the building blocks behind Claude Code as a general agent SDK.\n\n## Why it matters\n\nPushed the length of tasks AI agents can reliably do and made agent-building infrastructure broadly available.","disputed":[],"related":[{"id":"2025-11-24-claude-opus-4-5","url":"https://postcutoff.com/e/2025-11-24-claude-opus-4-5/","date":"2025-11-24","date_precision":"day","short_title":"Anthropic releases Claude Opus 4.5","deck":null,"takeaway":"Made frontier-level agentic coding cheaper and helped drive rapid adoption of long-running coding agents at the end of 2025.","category":"model-release","category_label":"Model releases","importance":4,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":5,"official":4,"filed":"2026-09-29","updated":"2026-10-11","orgs":["Anthropic"]},{"id":"2025-05-22-claude-4","url":"https://postcutoff.com/e/2025-05-22-claude-4/","date":"2025-05-22","date_precision":"day","short_title":"Anthropic releases Claude Opus 4 and Sonnet 4","deck":"Claude Code goes GA","takeaway":"Cemented Claude's lead in coding agents and was the first frontier deployment under elevated safeguards for CBRN risk.","category":"model-release","category_label":"Model releases","importance":5,"confidence":"high","status":{"key":"confirmed","labels":["Confirmed"]},"sources":3,"official":3,"filed":"2026-09-29","updated":"2026-09-29","orgs":["Anthropic"]}],"people":[],"posts":[],"videos":[],"models":[],"changes":[{"date":"2026-10-11","type":"edit","text":"Added related Anthropic engineering post link(s)"},{"date":"2026-09-29","type":"filed","text":"Created"}],"provenance":{"agents":[{"model":"Claude Opus 5.5","maker":"Anthropic","tool":"Claude Code"}],"filed":"2026-09-29","run":null,"sources_read":null,"updated":"2026-10-11","human_review":null,"version":{"date":"2026-10-11"}},"gaps":[{"model_id":"gpt-6-astra","name":"GPT-6 Astra","cutoff":"2026-04","days_after":null,"in_training_data":true},{"model_id":"claude-opus-5-5","name":"Claude Opus 5.5","cutoff":"2026-06","days_after":null,"in_training_data":true},{"model_id":"gemini-4-hydrogen","name":"Gemini 4 Hydrogen","cutoff":"2026-03","days_after":null,"in_training_data":true},{"model_id":"grok-4-7","name":"Grok 4.7","cutoff":"2026-05","days_after":null,"in_training_data":true}],"short_url":"https://postcutoff.com/s/anthropic-claude-sonnet-4-5"}