· Sep 29, 2025
Anthropic introduces Claude Sonnet 4.5 and the Claude Agent SDK
Markdown · JSON · Original source
published this announcement on September 29, 2025. It presents Sonnet 4.5 as a stronger model for coding, computer use, reasoning and math, and says the model can maintain focus for more than 30 hours on complex, multi-step tasks. Anthropic also announces product changes, including checkpoints in Claude Code, tools for longer-running agents in its , and code execution and file creation in Claude apps. It is making the Claude Agent SDK—the infrastructure it says powers Claude Code—available to developers building agents for tasks beyond coding. Anthropic backs its capability claims with evaluation results and examples from early customers, while noting that the results have specific testing methods. It also says the model has improved on several alignment measures, and describes safeguards for its release under AI Safety Level 3 protections. The announcement notes that safety classifiers can sometimes flag ordinary content, although Anthropic says it has reduced these false positives. The results and customer accounts offer evidence of performance in selected tests and uses; they do not establish how reliably the model will perform across other tasks or settings. The announcement does not provide enough detail to assess the independent verification of its customer reports or how the model’s performance and safeguards will hold up in broader use.
Headline and summary by Escape Meridian, prepared by Luna from the attributed source. A source check is not independent confirmation of the source’s claims.
Original source
· Article
Source details
Original source date: Sep 29, 2025 (2025-09-29T00:00:00.000Z).
Date basis: Date stated on the publisher’s page.
Added to Escape Meridian: 2026-11-11T23:53:24.731Z. First observed: 2026-10-20T07:55:25.843Z. Retrieved: 2026-10-24T17:29:42.208Z.
Referenced by
Recorded citations and public file attachments. A reference alone does not establish agreement or influence.
No public references recorded here yet.
Limits
Text as retrieved on October 24, 2026; originally published September 29, 2025. ’s performance and safety claims are presented in its own announcement; the cited customer accounts are attributed testimonials. The announcement does not establish how independently those accounts were verified. Evaluation scores depend on the stated methods and test setups; results alone do not establish performance across all real-world tasks.
Open question
How will Sonnet 4.5 perform across real-world tasks beyond the evaluations and early customer uses described in ’s announcement?
Source-stated
Anthropic describes Claude Sonnet 4.5 as its strongest model for coding and complex agents, and says it performs well on computer use, reasoning and math.
Claude Sonnet 4.5 is the best coding model in the world. It's the strongest model for building complex agents. It’s the best model at using computers.
Source-stated
Anthropic says it observed the model maintaining focus for more than 30 hours on complex, multi-step tasks.
Practically speaking, we’ve observed it maintaining focus for more than 30 hours on complex, multi-step tasks.
Source-stated
Anthropic says it is making the Claude Agent SDK available so developers can build agents for tasks beyond coding.
The Claude Agent SDK is the same infrastructure that powers Claude Code, but it shows impressive benefits for a very wide variety of tasks, not just coding. As of today, you can use it to build your own agents.
Source-stated
Anthropic says safety classifiers may flag normal content, and reports reducing false positives by a factor of ten since it originally described them.
These classifiers might sometimes inadvertently flag normal content. We’ve made it easy for users to continue any interrupted conversations with Sonnet 4, a model that poses a lower CBRN risk. We've already made significant progress in reducing these false positives, reducing them by a factor of ten
Escape Meridian interpretation
Escape Meridian reads Anthropic’s evaluations and customer accounts as evidence about selected tests and uses, not as a demonstration of performance across other tasks or settings.
For complete technical details and evaluation results, see our system card, model page, and documentation.
Source and contributor context
Anthropic · Publisher
The publisher is identified here. No individual author has been inferred.
Source media
Media retrieval status: unchecked.
Terms and references
Reference context selected by Escape Meridian; separate from source evidence. Entity mentions do not establish authorship, involvement or endorsement.
API
An application programming interface: a defined way for software to request data or actions from another system.
Anthropic
Anthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Claude
Claude is Anthropic’s family of AI models and assistants for tasks such as writing, analysis and coding.
Editorial provenance
Version 1. Publication ID: e84be8fd-708a-6f8b-4d90-01c9e2e9cd3a.
Prepared by openai/gpt-6-luna; grounding review by anthropic/claude-opus-5.5.
Source check: uncertain by anthropic/claude-opus-5.5 on 2026-11-11T23:49:18.638Z. This checks fidelity to the source, not whether its claims are independently true.
Editorial selection: accept; reasons: bridge_admitted_by_model, second_opinion_categorical.
Observation: f00fa636415414bebc4b19a6d0d1395f9657ba55bb251524c5ea5b09f7c00fab. Source text hash: 2313a4dca4d1308e444848de2a7a8c3685478ec436df9362f0e7eff2f17fea0b.
Revision and source-check history
Version 1 · 2026-11-11T23:53:24.731Z
Anthropic introduces Claude Sonnet 4.5 and the Claude Agent SDK
published this announcement on September 29, 2025. It presents Sonnet 4.5 as a stronger model for coding, computer use, reasoning and math, and says the model can maintain focus for more than 30 hours on complex, multi-step tasks. Anthropic also announces product changes, including checkpoints in Claude Code, tools for longer-running agents in its , and code execution and file creation in Claude apps. It is making the Claude Agent SDK—the infrastructure it says powers Claude Code—available to developers building agents for tasks beyond coding. Anthropic backs its capability claims with evaluation results and examples from early customers, while noting that the results have specific testing methods. It also says the model has improved on several alignment measures, and describes safeguards for its release under AI Safety Level 3 protections. The announcement notes that safety classifiers can sometimes flag ordinary content, although Anthropic says it has reduced these false positives. The results and customer accounts offer evidence of performance in selected tests and uses; they do not establish how reliably the model will perform across other tasks or settings. The announcement does not provide enough detail to assess the independent verification of its customer reports or how the model’s performance and safeguards will hold up in broader use.
Original source
· Article
Source details
Original source date: Sep 29, 2025 (2025-09-29T00:00:00.000Z).
Date basis: Date stated on the publisher’s page.
Added to Escape Meridian: 2026-11-11T23:53:24.731Z. First observed: 2026-10-20T07:55:25.843Z. Retrieved: 2026-10-24T17:29:42.208Z.
Limits
Text as retrieved on October 24, 2026; originally published September 29, 2025. ’s performance and safety claims are presented in its own announcement; the cited customer accounts are attributed testimonials. The announcement does not establish how independently those accounts were verified. Evaluation scores depend on the stated methods and test setups; results alone do not establish performance across all real-world tasks.
Open question
How will Sonnet 4.5 perform across real-world tasks beyond the evaluations and early customer uses described in ’s announcement?
Source-stated
Anthropic describes Claude Sonnet 4.5 as its strongest model for coding and complex agents, and says it performs well on computer use, reasoning and math.
Claude Sonnet 4.5 is the best coding model in the world. It's the strongest model for building complex agents. It’s the best model at using computers.
Source-stated
Anthropic says it observed the model maintaining focus for more than 30 hours on complex, multi-step tasks.
Practically speaking, we’ve observed it maintaining focus for more than 30 hours on complex, multi-step tasks.
Source-stated
Anthropic says it is making the Claude Agent SDK available so developers can build agents for tasks beyond coding.
The Claude Agent SDK is the same infrastructure that powers Claude Code, but it shows impressive benefits for a very wide variety of tasks, not just coding. As of today, you can use it to build your own agents.
Source-stated
Anthropic says safety classifiers may flag normal content, and reports reducing false positives by a factor of ten since it originally described them.
These classifiers might sometimes inadvertently flag normal content. We’ve made it easy for users to continue any interrupted conversations with Sonnet 4, a model that poses a lower CBRN risk. We've already made significant progress in reducing these false positives, reducing them by a factor of ten
Escape Meridian interpretation
Escape Meridian reads Anthropic’s evaluations and customer accounts as evidence about selected tests and uses, not as a demonstration of performance across other tasks or settings.
For complete technical details and evaluation results, see our system card, model page, and documentation.
Source and contributor context
Anthropic · Publisher
The publisher is identified here. No individual author has been inferred.
Source media
Media retrieval status: unchecked.
Terms and references
Reference context selected by Escape Meridian; separate from source evidence. Entity mentions do not establish authorship, involvement or endorsement.
API
An application programming interface: a defined way for software to request data or actions from another system.
Anthropic
Anthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Claude
Claude is Anthropic’s family of AI models and assistants for tasks such as writing, analysis and coding.
Editorial provenance
Version 1. Publication ID: e84be8fd-708a-6f8b-4d90-01c9e2e9cd3a.
Prepared by openai/gpt-6-luna; grounding review by anthropic/claude-opus-5.5.
Source check: uncertain by anthropic/claude-opus-5.5 on 2026-11-11T23:49:18.638Z. This checks fidelity to the source, not whether its claims are independently true.
Editorial selection: accept; reasons: bridge_admitted_by_model, second_opinion_categorical.
Observation: f00fa636415414bebc4b19a6d0d1395f9657ba55bb251524c5ea5b09f7c00fab. Source text hash: 2313a4dca4d1308e444848de2a7a8c3685478ec436df9362f0e7eff2f17fea0b.
uncertain · 2026-11-11T23:49:18.638Z · anthropic/claude-opus-5.5
Publication: e84be8fd-708a-6f8b-4d90-01c9e2e9cd3a
attribution — pass: The reading consistently attributes the capability, alignment, SDK and classifier claims to Anthropic, the publisher of this single launch announcement. It treats the customer quotes as attributed testimonials and labels the one interpretive claim as Escape Meridian's reading.
Claude Sonnet 4.5 is available everywhere today.
summary — uncertain: The summary is faithful, but central terms are left unexplained for a non-specialist. 'Agents', 'alignment', 'AI Safety Level 3', 'Claude Code' and 'computer use' get no plain explanation, and 'SDK' is only glossed as infrastructure. Only API is defined beside the reading. Smallest repair: briefly explain agents (AI systems that carry out multi-step tasks), alignment, and ASL-3 safeguards, or add term-library entries for them. The reading also softens 'best coding model in the world' to 'stronger model'; restoring Anthropic's attributed superlative would preserve its force.
Claude Sonnet 4.5 is being released under our AI Safety Level 3 (ASL-3) protections
date — pass: September 29, 2025 matches the UTC day of evidence.originalPublished, and kind is publisher-page, so no archive disclosure is needed. The page was edited after publication (dateModified 2026-10-15), but the passages the reading relies on are plainly the original launch text, which is consistent with Sonnet 4 leading four months earlier. The reading uses no later material and frames no forecasts as outcomes.
Just four months ago, Sonnet 4 held the lead at 42.2%.
support — pass: Each claim is source-stated and supported by an exact excerpt: the 30-hour focus claim, the Agent SDK for tasks beyond coding, and the tenfold reduction in false positives. The interpretation about test scope is explicitly labeled and appropriately cautious. Although the page was edited later, the cited passages are core announcement text. The question follows from the evidence and is not leading.
reducing them by a factor of ten since we originally described them
media — pass: No media was selected and no verified candidate media was supplied, so there is no identity or relevance issue to review.
Challenge or correct this reading
Send a correction or an opposing source with this reading’s URL. Disagreement and uncertainty can remain unresolved.
Anthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore AnthropicAnthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore AnthropicClaude is Anthropic’s family of AI models and assistants for tasks such as writing, analysis and coding.
Explore ClaudeAn application programming interface: a defined way for software to request data or actions from another system.
Definition referenceAnthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore AnthropicAnthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore AnthropicClaude is Anthropic’s family of AI models and assistants for tasks such as writing, analysis and coding.
Explore ClaudeAnthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore AnthropicAnthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore AnthropicClaude is Anthropic’s family of AI models and assistants for tasks such as writing, analysis and coding.
Explore ClaudeAn application programming interface: a defined way for software to request data or actions from another system.
Definition referenceAnthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore AnthropicAnthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore AnthropicClaude is Anthropic’s family of AI models and assistants for tasks such as writing, analysis and coding.
Explore ClaudeAnthropic develops Claude and publishes research about AI capability, safety and how people use its systems.
Explore Anthropic