Disclosure: this piece was drafted with Claude Fable 5.1 on its launch day, inside the pipeline that writes every SharpHaw post. I read it, corrected it and edited it before it went out.
Anthropic shipped Claude Fable 5.1 on 1 September. The benchmarks are in every feed already. What is more useful to a developer who also runs a business is the page under the benchmarks: the release notes list what got better, and then, further down, what now behaves differently without you changing a line. I read both lists and sorted my own work into what I hand the model and what I keep. Here is the list, with the reasons taken from the notes rather than from vibes.
What changed without a code change
These are documented behaviour differences from Fable 5, not my guesses:
Parallel tool calling is more variable. In coding and computer-use loops where the next reads are only implied, it may issue one tool call per turn. Same answer quality, more round trips.
