Claude Fable 5.1 Agent Test: Sonnet Tied It at 1/8 the Cost

Posted on Oct 30 • Edited on Oct 30 • Originally published at ranthis.com • AI-assisted

Originally published on RanThis. Cover: AI-generated illustration.

Claude Fable 5.1 is not worth its price for a month-end close with tools. We gave Fable, Opus 5.5 and Sonnet 5.5 three exports and ten traps; all three closed the books correctly, and Fable cost the most and took the longest.

Anthropic sells Fable "for demanding reasoning and long-horizon agentic work", at $10/$50 per million tokens: 2.5 times the list price of Opus 5.5 and five times Sonnet 5.5. So we tested exactly that kind of job: paging through an invoice list, a bank statement and a card report, and matching every payment to an invoice. Finance teams do this at every month end, and AI agents are now pitched for it. We measured it on 28 Oct 2026 through the Anthropic API, from a Python script. The data is invented. The prompt, the answer key and the scoring code were committed before any model was called, and the raw logs of every run are kept with this article's source files.

* All ten answers were right, but the reply contained two JSON objects (see below), and the sealed scoring code reads only one.

None of the ten traps separated the models. Every model excluded the cancelled invoice, applied the credit note, matched the typo reference by customer and amount, saw that one double payment had been refunded and the other had not, and found the payout that had not reached the bank. Differences showed up in how they worked and in how they reported.

How they used the tools. Sonnet and Opus planned the reading: the first turn listed the files, and the next few turns asked for 8–10 pages at a time. Fable read sequentially in two of its four logged runs, one page per turn: 18 turns in round 1 and 81 turns in round 2, against 11–22 turns for the others on the same files. It reached the same answers, but an agent loop pays for every turn, and a 1,430-row job took Fable over six minutes against under three for Sonnet.

How they reported. The prompt asked for "only a JSON object". Fable and Opus often added notes anyway, and the notes are the useful part for a finance team. Opus in round 2 listed each judgement call with its effect on the totals: "If finance writes off the three small shortfalls, the count drops to 111 and the total to $527,322.32." Fable listed every adjustment under "Notes on how I got there, so you can check my judgement calls". Sonnet gave plain JSON in round 1. In round 2 it added a warning that it had "added up the figures by hand from the three exports, so a missed or misread line is possible". It had in fact used the calculator tool 21 times.

The one failed run. In Opus run 2 of round 1, the model sent a JSON object with nine keys, then wrote "Sorry, that's only nine keys. Here is the full object with all ten" and sent the full object. All ten numbers in the second object were right. The sealed scoring code expects one object and scored the run 0. If a script reads your agent's output, check that you get exactly one object back.

Anthropic lists Claude Fable 5.1 at $10 per million input tokens and $50 per million output tokens, with cache reads at $0.25 per million. In our test, one agent run closing a small month (about 300 rows) cost a median $0.70. A run five times the size cost $3.74, with prompt caching on.

Not on this job. Fable 5.1, Opus 5.5 and Sonnet 5.5 all found the same ten traps in every scored run. Fable cost 3.4 times as much as Opus in round 1 and 3.7 times in round 2, and it was the slowest, partly because it read one page per turn.

Anthropic's own guidance is to use Opus 5.5 for most workloads and to move to Fable 5.1 "when your evals on Claude Opus 5.5 at higher effort still fall short". Our results fit that advice. Run your own task on Sonnet or Opus first, and pay for Fable only if they fail it.

For a month-end close with tools, we would not use Claude Fable 5.1. Anthropic describes Fable as built for long-horizon agentic work, and this was such a job: 58 page reads, 10 traps, numbers to the cent. Sonnet 5.5 gave the same answers for about an eighth of the cost in round 1 and a seventh in round 2, in less than half the time. If you work in Claude, start agent jobs like this on Sonnet 5.5. Opus 5.5 explained its judgement calls most clearly in our runs, so it is the step up if you want that, and keep Fable for jobs where both of them fail. Whatever model you use, ask it to list the calls it made (written-off fees, overpayments) next to the numbers, because those notes are what a person checks before closing the month.

How this was made: an AI (Claude) helped write this article; the RanThis editor ran the test script, checked every number against the raw results and checked facts against the linked sources.