The 40 percent savings on Anthropic's new default model is mostly real only if you change how you use it.
Anthropic released Claude Opus 5.5 on September 22, 2026, and made it the default model everywhere, including Claude Code on every paid plan, the same tool behind his own Work OS and blog tooling. Anthropic says it matches Claude Fable 5.1, the earlier top model, on most tasks. It writes answers more than 30 percent faster than Opus 5 and is pitched as costing about 40 percent less to run. That last number only holds at default settings for typical work, a condition most teams will miss. It also now ranks first on the Artificial Analysis Intelligence Index, a standard test many teams use to compare models.
The list price only fell 20 percent, to $4 per million input tokens and $20 per million output tokens (tokens are chunks of text, roughly three quarters of a word each). The rest of the promised saving comes from the model using fewer tokens per task, and that only happens if you change how you prompt it. One early tester used Opus 5.5 to audit and fix a 200,000-line codebase in under three hours, a job that took Opus 5 more than 20 hours. In Anthropic’s own test of earnings research reports, 16 of 18 Opus 5.5 reports had no made-up figures or quotes. Fable 5.1 and Opus 5 did not produce a single clean report between them. One company, Quantium, cut a coding task from 38 back-and-forth prompts over 4 days down to 11 prompts finished in 3 hours.
Three habits eat the whole saving. First, running the model at its highest effort setting: at max effort it used about 119,000 output tokens per task, 1.6 times what Opus 5 used. The cost per task ends up the same as Opus 5, and the 40 percent saving disappears. The default effort level, called medium, already matches or beats Opus 5 running at its high setting on Anthropic’s own coding and knowledge-work tests, so there is rarely a reason to go higher. Second, carrying over old prompt habits: deleting lines like “think carefully” made Opus 5.5 start replying sooner with no clear drop in quality. Anthropic has also blocked requests to see the model’s step-by-step reasoning, known as its chain of thought, and since September 24 the API bills you even for those blocked requests. Third, trusting a long unattended run to finish on its own: on long tasks Opus 5.5 sometimes stops partway through and writes a progress report instead of finishing. A program that lets the model take several steps on its own with no human in between can mistake that progress report for a finished task.
There is also a real quality gap to manage. A code checker called Sonar found 44 percent more concurrency problems, meaning bugs that show up when several things run at once, per line in Opus 5.5’s code than in Opus 5’s. A security testing firm, Endor Labs, found only 33.5 percent of Opus 5.5’s bug fixes passed security checks once fixes it had simply memorized were excluded. The fix is a short set of habits: start every task at medium effort, hand over the whole job in one message with a clear finish line, tell the model exactly when to stop and ask instead of guessing, and put extra review on the work it is weakest at, meaning concurrency and security fixes. Going from the lowest effort setting to medium buys a real jump in quality for about 80 cents a task. Going from the second-highest setting to the highest buys almost nothing and costs about 70 percent more.
The guide behind this turns it into a copyable system: a one-message brief template for handing off long tasks without hand-holding, rules for when Claude Fable 5.1 is worth two and a half times the price, when a cheaper model called GPT-6 Sol is the better choice, and why a promise not to retain your data can decide the matter for regulated teams, plus cost controls for long Claude Code sessions and guidance for recovering from the most common failures. It is aimed at people doing work that touches revenue, code, or money, the exact bracket his own SMB search and Work OS tooling sit in.