Article:Anthropic
Prompting Guide for Claude Opus 5
Anthropic outlines behavioral differences and prompt patterns for Claude Opus 5, focusing on verbosity management, subagent delegation, scoping, and thinking controls.
Prompting Guide for Claude Fable 5.1 and Mythos 5.1
ArticleResource
Anthropic's official documentation outlines behavioral differences and prompting patterns for Claude Fable 5.1 and Claude Mythos 5.1, focusing on effort levels, tool call batching, progress updates, and conversation history management.
Key points
- Existing Claude Fable 5 prompts remain compatible, but developers should adapt to key behavioral shifts.
- Effort levels (low, medium, high, xhigh, max) act as the primary control for intelligence, latency, and cost; medium matches Fable 5 performance at lower cost.
- The model outputs fewer user-facing updates between tool calls by default; updates can be requested via prompt or thinking display settings (display: 'updates').
- Independent tool calls in agent loops should be batched using turn-scoped system messages to avoid unnecessary round trips.
- Conversation history must remain append-only; editing earlier turns restarts prompt caching and invalidates subsequent thinking blocks.
- Safety classifiers may trigger stop_reason: 'refusal' on benign coding requests, requiring prompt adjustments to avoid false positives.
Why it matters
Engineers building agent loops and API integrations can significantly reduce latency and operational costs while maintaining output quality by leveraging the new effort controls and batching patterns.
Analysis
The documentation underscores Anthropic's focus on cost-performance tuning via explicit effort parameters and cache efficiency. It highlights practical agent architecture challenges, such as handling tool call narration and preserving cache hits with immutable conversation histories.
Mentioned links
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Summary generated by AI (Gemini 3.8 Flash, 27 September 2026). It may contain errors – the original source is authoritative.
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