claude-prompting-best-practices changedbuild-with-claude/prompt-engineering/claude-prompting-best-practices
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2121| Model | Guide | What’s different |
2222| -------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
23| Claude Fable 5.2 and Claude Mythos 5.2 | [Prompting Claude Fable 5.2](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2) | Differences from earlier models: effort calibration, finishing long tasks, approvals, code ownership, user-facing progress updates, subagent delegation, caution that builds up, instructions written for earlier models, final reports, and safeguard refusals. |
23| Claude Fable 5.2 and Claude Mythos 5.2 | [Prompting Claude Fable 5.2](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2) | Differences from earlier models: effort calibration, finishing long tasks, approvals, code ownership, user-facing progress updates, subagent delegation, caution that builds up, prompts and harnesses designed for earlier models, final reports, and safeguard refusals. |
2424| Claude Fable 5.1 and Claude Mythos 5.1 | [Prompting Claude Fable 5.1](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1) | Differences from Claude Fable 5: effort levels, finishing long tasks, user-facing progress updates, passing thinking blocks back unchanged, tool-call batching in agent loops, search triggering at low effort, formatting, and writing density. |
2525| Claude Fable 5 and Claude Mythos 5 | [Prompting Claude Fable 5](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5) | Differences from Claude Opus 4.8: effort levels, instruction following, long-run progress claims, memory systems, and the `reasoning_extraction` refusal category. |
2626| Claude Sonnet 5.5 | [Prompting Claude Sonnet 5.5](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-sonnet-5-5) | Differences from Claude Sonnet 5: effort calibration, initiative and scope, running without up-front thinking, JSON output on reasoning tasks, user-facing progress updates, tool use in chat, mid-turn user messages, verification on coding tasks, tool-call handling, complex visual inputs, and safeguard refusals. |
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114011406. **Tune anti-laziness prompting:** If your prompts previously encouraged the model to be more thorough or use tools more aggressively, reduce that guidance. Claude 4.6 models are more proactive and might overtrigger on instructions that were needed for previous models.
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11427. **Pass thinking blocks back unchanged and keep history append-only:** Append each assistant turn exactly as the API returned it, thinking blocks included. On Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, editing earlier messages, rebuilding `system` or `tools`, or summarizing older turns in place between requests [invalidates every later thinking block](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation). Move those changes to mid-conversation system messages and server-side context management. See [Keep the conversation history append-only](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#keep-the-conversation-history-append-only).
11427. **Pass thinking blocks back unchanged and keep history append-only:** Append each assistant turn exactly as the API returned it, thinking blocks included. On Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, editing earlier messages, rebuilding `system` or `tools`, or summarizing older turns in place between requests [invalidates every later thinking block already in the history](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation). Move those changes to mid-conversation system messages and server-side context management. See [Keep the conversation history append-only](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#keep-the-conversation-history-append-only).
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11441144For detailed migration steps, see the [Migration guide](https://platform.claude.com/docs/en/about-claude/models/migration-guide).
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11521152<CardGroup cols={2}>
11531153 <Card title="Prompting Claude Fable 5.2" icon="terminal" href="https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2">
1154 Behavioral differences and prompting patterns for Claude Fable 5.2, covering effort, finishing long tasks, approvals, code ownership, progress updates, subagents, caution that builds up, instructions written for earlier models, final reports, and safeguard refusals.
1154 Behavioral differences and prompting patterns for Claude Fable 5.2, covering effort, finishing long tasks, approvals, code ownership, progress updates, subagents, caution that builds up, prompts and harnesses designed for earlier models, final reports, and safeguard refusals.
11551155 </Card>
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11571157 <Card title="Prompting Claude Fable 5.1" icon="terminal" href="https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1">
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