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This Week in Claude · 25 September 2026

This Week in Claude: 25 September 2026

Claude's top model got cheaper to run, and Claude gained more doors into your books. Re-cost your workflows, and open one door, not three.

Week of 18 to 25 September 2026. Scan it in 30 seconds, read the takeaways in three minutes, or go the whole way in ten.

Nhial Majok
Nhial MajokFounder · AI & Automation Lead · Melbourne

The scan

Sorted by what to do about it.

  • Act now

    Anthropic shipped Claude Opus 5.5 on 22 September at US$4 in and US$20 out per million tokens, which it says costs 40% less to run than Opus 5. Re-cost every live Claude workflow.

  • Act now

    Claude Sonnet 4.5 retires no sooner than 29 September and Haiku 4.5 no sooner than 15 October. Anything pinned to a 4.5 model needs an owner.

  • Watch

    A bookkeeping firm released a free connector that gives Claude read and write access to Xero, Amazon opened Seller Central to Claude through a plugin, and Anthropic launched the Claude Marketplace with more than 2,000 connectors and plugins. More doors into the books. Open one.

  • Watch

    Claude Code 2.1.280 switched the default model on Pro and Team Standard from Sonnet to Opus. That changes usage without anyone touching a setting.

  • Noise, for now

    Anthropic's IPO reportedly slipped to November at about US$2 trillion, and Claude found a new enzyme system. Interesting. Nothing to do on Monday.

Numbers that moved

  • 40% cheaper to run Opus 5.5
  • US$4 and US$20 per million tokens
  • 2,000-plus connectors and plugins in the Claude Marketplace
  • 15 October, the earliest Haiku 4.5 retirement.

Takeaways for your business

What it means on Monday.

  1. 01Claude's top model got cheaper to run. Re-cost everything.

    What happened
    On Tuesday Anthropic released Claude Opus 5.5 at US$4 in and US$20 out per million tokens, with cache reads at US$0.20, and says it costs 40% less to run than Opus 5 on typical work.
    Why it matters
    Any quote you got for Claude work in August is now wrong in your favour. Budget-tier models keep getting cheaper, and this week the top tier did too.
    Next 30 days
    List every AI workflow you run, note which model it's pinned to, and re-cost it on the new prices. If a vendor bills you a flat monthly fee for AI features, ask them what their model costs did this week.
    Say it on a call
    "The clever part of your AI setup is the workflow and the data. The model underneath got cheaper on Tuesday, and it'll happen again."
    For operators

    Opus 5.5 has adaptive thinking always on (the thinking: disabled parameter returns a 400), tool_choice: any and tool_choice: tool also return 400, and computer use needs the computer_toolset_20260801 toolset. Sonnet 5.5 and Haiku 5.5 are "coming in the weeks ahead", so hold the budget-tier migration until they land. Claude Code 2.1.280 switched the default model on Pro and Team Standard from Sonnet to Opus, which will change client usage patterns without anyone touching a setting.

  2. 02Claude is getting more doors into your accounting stack. Open one of them.

    What happened
    On 22 September Meridian (part of Pilot) released a free MCP connector that lets Claude and Cowork create invoices, bills and journals and run reports across multiple Xero organisations, with per-organisation read-only or read-write settings. Amazon opened Seller Central to Claude via a plugin on 23 September, and Anthropic launched the Claude Marketplace with more than 2,000 connectors and plugins.
    Why it matters
    Last week Claude arrived inside Xero and MYOB through Anthropic's own integration. This week a third party built a second bridge with write access, for free. That's convenient and it's also more surface to govern.
    Next 30 days
    Pick one door. Xero's native Claude for Small Business integration is the safer default because the vendor owns both ends. Run it read-only for two weeks before any write. Do not stack a third-party connector on top for the same job.
    Say it on a call
    "You'll be offered more than one way to plug Claude into Xero this month. We're using one, read-only first, and we're keeping the others in reserve."
    For operators

    Before granting a third-party connector write access, read its data-handling terms as carefully as the vendor's. Meridian is a bookkeeping business, so ask where the tokens flow and what's retained.

What I’m doing about it

Re-costing the two client workflows I quoted in August on Tuesday's prices.

Watchlist

What we’re tracking.

  • Anthropic IPO. No change in substance. Timing reported to have moved from mid-October to November so Q3 numbers can be shown. Up to US$100 billion raise at about US$2 trillion. Still no public filing.
  • Claude 5.5 rollout and 4.5 retirements. New. Sonnet 5.5 and Haiku 5.5 due "in the coming weeks". Claude Sonnet 4.5 retires no sooner than 29 September, Haiku 4.5 no sooner than 15 October. Anything pinned to a 4.5 model needs an owner.

Dropped from the list: the Claude SMB trainer program. Still US-only, no ANZ program announced. It comes back when that changes.

Corrections from last week: none.

The full report

Anthropic and Claude

So what: the flagship got cheaper to run, and the ways to connect Claude to business software multiplied.

Claude Opus 5.5 (confirmed, primary source). Launched 22 September, model ID claude-opus-5-5. US$4 in, US$20 out per million tokens, cache reads US$0.20, cache writes US$5, fast mode at US$8 and US$40. Anthropic says it "costs 40% less to run than Opus 5 on typical workloads". Terminal-Bench 4.0 at 66.4%, OSWorld 2.0 at 81.8%, GDPval-AA v2.1 at 1846 Elo. Available on the Claude API, Bedrock, Google Cloud and Microsoft Foundry. Sonnet 5.5 and Haiku 5.5 to follow in the coming weeks. Retirement of Opus 5.5 listed as not sooner than 22 September 2027.

The Claude Marketplace and partners (confirmed, primary source). The Claude Marketplace launched 23 September with 2,000-plus connectors and plugins, agents from partners like Cursor and Snowflake, and service partners including Accenture and Deloitte, usable against committed Anthropic spend for enterprise customers. Anthropic and Accenture (18 September) each committed at least US$1 billion over five years to an embedded evaluation partnership with employee-level access to Anthropic, the first concrete implementation of Amodei's "pace the frontier" essay. Amazon's Seller Assistant plugin lets sellers manage inventory, pricing and listings through Claude, US beta, human approval per action.

Research (confirmed, primary source). Claude discovered a new enzyme system (23 September) using about 950 agents over 21 hours and 210 million tokens, validated in Anthropic's own lab; sceptics say it's a long way from a CRISPR rival.

The IPO (reported, WSJ via Yahoo Finance). Anthropic's IPO reportedly moved to November.

For operators: Claude and agentic tooling notes

So what: a heavy week of API changes. Read the release notes before Monday.

All confirmed from Anthropic release notes and the Claude Code changelog. Inline tools beta (inline-tools-2026-09-15) lets a tool definition arrive mid-conversation without invalidating prompt cache, and MCP toolsets can ride the same block. Cache diagnostics went GA on 23 September. From 24 September, refusals that arrive before any output are billed when the category is bio, frontier_llm or reasoning_extraction; other pre-output categories stay unbilled. The Compliance API now returns Claude in Chrome transcripts for Enterprise. Claude Code 2.1.277 reads AGENTS.md when there's no CLAUDE.md, 2.1.278 defaults auto mode to the free server-side classifier, 2.1.280 makes Opus 5.5 the default Opus model and switches the Pro and Team Standard default from Sonnet to Opus, 2.1.281 adds Bedrock assume_role and guardrail support, 2.1.282 adds maxProseWidth and fixes resumed sessions dropping prior reasoning. Retirements to diarise: Claude Sonnet 4.5 not before 29 September, Claude Haiku 4.5 not before 15 October.

Sources (13)

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