Tip: Fable 5.1 batches fewer tool calls and rewrites whole files, so add two lines to your agent-loop prompt

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Anthropic documents two behavior regressions that show up with no code change: Fable 5.1 may issue one tool call per turn where Fable 5 batched several (costing round trips and wall-clock time in bash-and-editor harnesses without reducing answer quality), and when editing text files it is more likely to rewrite the entire file than make a targeted edit. Both have prompt-level fixes in the Fable 5.1 prompting guide. A third gotcha: at `low` effort it answers from memory and calls search or retrieval tools less often, so raise effort on turns that need fresh information.