Claude Fable 5 vs Claude Opus 5:2026年に勝つAIモデルはどっち?

“I do medical imaging research and the bio classifier keeps flagging my MRI segmentation prompts, then it silently falls back to Opus 4.8 mid-session. At $50 per million output tokens I expect to at least know which model actually answered me.”

“Yes it's 2x the price of Opus and yes the turns are slow. But one overnight Fable run replaced what used to be a week of supervising shorter runs. On a per-task basis it's actually the cheapest model we use.”

“The 1M context is real, not marketing. I fed it our entire service mesh config plus six months of incident postmortems and it traced a flaky timeout to a retry policy nobody remembered writing. Opus 4.8 never connected those dots.”

“Gave it a monorepo migration that Opus 4.8 kept stalling on. It ran for about 40 minutes, came back with the whole thing done plus a test harness it wrote for itself. Felt like reviewing a senior engineer's PR, not babysitting a chatbot.”

“The sticker price is unchanged but my invoice is not: it writes essays where Opus 4.8 wrote answers. 68 seconds to first token killed it for our support chat, we went back to Sonnet 5.”

“Fed it a 40-tab financial model with cross-sheet formulas and asked for a scenario deck. It got the edge cases the analysts missed. For document-heavy enterprise work this is the best model I have used.”

“We moved off Fable 5 because the bio classifier kept flagging our genomics tooling. Opus 5 does the same work at half the price and I have not seen a single silent reroute since.”

“Migrated our agents from Opus 4.8 the day it dropped. Same bill, and tasks that used to stall at the planning stage now just finish. The 10-point SWE-bench jump is not marketing, our merge queue feels it.”