Comparison of Anthropic's Prompting Guidelines for Claude Models
Claude models respond similarly to most instructions, but there are a few key differences to be aware of. Keep these in mind when using the same prompt, skill, or agent with different models.
| Category | Fable 5 / Mythos 5 | Opus 5 | Sonnet 5 | Opus 4.8 | Summary |
|---|---|---|---|---|---|
| Effort | high default. xhigh only for capability-critical work; low and medium are genuinely fine — they often beat xhigh on older models. |
high default. Use low/medium liberally as your primary cost lever; step to xhigh only for demanding agentic work. |
high default. Raise to xhigh for the hardest coding and agentic tasks. |
Start at xhigh for coding and agentic work; never below high for anything intelligence-sensitive. |
The arrow reverses. Carrying 4.8's always-xhigh habit into Opus 5 or Fable 5 burns tokens for no gain; carrying Opus 5's low is fine back to 4.8 under-thinks. |
| Thinking | Adaptive only. No budgets, summarized output only. | On by default. Can be disabled only at high effort or below. |
On by default — changed from 4.6. Turn off with thinking:{type:"disabled"}. |
Off unless you explicitly set {type:"adaptive"}. |
A prompt written for 4.8 assumes no thinking. The same request on Sonnet 5 or Opus 5 now spends max_tokens on thinking and can truncate mid-answer. |
| Verbosity | One short brevity rule is enough — it replaces enumerating every pattern you dislike. | Prompt for length explicitly. Lowering effort cuts thinking, not the visible answer. | Length self-calibrates to task complexity. Tune only if your product needs a fixed style. | Length self-calibrates. Tune only if your product needs a fixed style. | On Opus 5, dropping effort to get shorter replies is the wrong knob. On 4.8 and Sonnet 5 it partly works, which is why the habit survives. |
| Verify-your-work | Add it. Make self-verification explicit and run fresh-context verifier subagents on an interval. | Delete it. Explicit verification triggers over-verification. Drop "double-check your answer" too. | not flagged | not flagged | A flat contradiction. The long-run prompt that makes Fable 5 reliable makes Opus 5 pay twice for the same work. |
| Subagents | Delegates readily — so use them more: async, long-lived, running in parallel. | Delegates readily — so cap it. Never delegate verification of its own work; keep spawn counts low. | not flagged | Spawns too few. Prompt it to fan out across files and items. | Three different directions. 4.8 needs encouragement, Opus 5 needs a leash, Fable 5 wants more delegation than either. |
| Prompt detail | Strip it back. Skills tuned for older models are too prescriptive and actively degrade output. | Remove legacy verification scaffolding; otherwise 4.8 prompts run as-is. | Sonnet 4.6 prompts work out of the box. | Prescriptive prompting is how you steer it. | Fable 5 is the one model where your most carefully tuned legacy prompt is the liability. More specific stops being better. |
| Instruction style | Give intent and the reason behind the ask. Short instructions steer broadly. | Constrain scope explicitly on narrow tasks — it expands scope on its own. | Literal. Won't generalize an instruction from one item to the next. State the scope. | Literal. Same as Sonnet 5. | Sonnet 5 and 4.8 need every case spelled out. Fable 5 penalizes that same enumeration. |
| Progress scaffolding | Replace it with a send_to_user tool plus elicitation language, and add a re-grounding rule for final summaries. | Remove it, then tune narration down — Opus 5 over-narrates. | Remove it. Updates are already good. | Remove it. Updates are already good. | Only Fable 5 wants new machinery here. Only Opus 5 needs the volume turned down after you delete the old scaffolding. |
| Reasoning rules | Never ask it to echo or explain its reasoning — that trips the reasoning_extraction refusal and elevated fallbacks. |
Never tell it "don't think" or "don't reason" — that increases <thinking> tag leakage when thinking is off. |
— | — | Opposite failure modes, same root cause. Both resolve to: write no rules about reasoning at all. |
| Sampling parameters | Not accepted. | Not accepted. | 400 error on temperature, top_p, top_k — new for Sonnet-class models. |
Not accepted. | Anything using temperature for stylistic or design variety has to move to asking the model for several directions up front. |
| Frontend defaults | — | — | Settles into a house style; generic negatives just move it to a different fixed palette. | A documented house style: cream #F4F1EA, serif display, terracotta accent. |
"Don't use cream" fails on both. Give a concrete spec, or ask for four directions and pick one. |
| Code review recall | Higher bug-finding recall, but cyber and bio safety classifiers can decline findings outright. | Ask for everything; filter in a second pass. | Ask for everything; filter in a second pass. | Ask for everything; filter in a second pass. | The rare axis where all four agree — and it still breaks you. "Only high-severity" is now obeyed literally, and measured recall collapses. |
| Watch for | Runs for minutes to hours, so fix client timeouts. Ground progress claims in tool results. State boundaries on unrequested actions. Give it a memory file. Don't show it a context countdown. | Files it writes run long — calibrate document length. With thinking off, tool calls can leak into the response as plain text. | Tool use is up versus 4.6 but drops with thinking off, so nudge explicitly. The new tokenizer produces roughly 30% more tokens. | Prefers reasoning over tool calls. Raise effort to get more tool use. | Same knob, opposite ends: Sonnet 5 and 4.8 need agentic behavior pushed up, Fable 5 and Opus 5 need it reined in. |