Prompt Craft

Prompt Craft

Write production-grade prompts for Claude's latest models. Based on Anthropic's official prompting best practices (April 2026, Claude 4.6).

Golden Rule

Show your prompt to a colleague with minimal context. If they'd be confused, Claude will be too.

Prompt Structure Template

<system>
  [Role definition — one sentence]
  [Key constraints and rules]
  [Output format specification]
</system>

<context>
  [Background the model needs — documents, data, history]
</context>

<examples>
  <example>
    <input>[Sample input]</input>
    <output>[Desired output]</output>
  </example>
</examples>

<instructions>
  [The actual task — clear, sequential steps]
</instructions>

Document placement: Long documents go at the TOP, queries at the BOTTOM. This improves quality by up to 30%.

Core Techniques

1. Be Explicit, Not Implicit

Claude 4.6 is literal. It follows precise instructions rather than inferring intent.

Say what to do, not what not to do. Provide the why behind instructions — Claude generalizes from motivation.

2. XML Tags for Structure

Wrap distinct content types in descriptive tags. This eliminates ambiguity.

• <instructions>, <context>, <input>, <output> — top-level sections
• <documents> → <document index="1"> → <source>, <document_content> — multi-doc inputs
• <example> inside <examples> — few-shot examples
• <rules>, <constraints> — hard requirements
• Custom tags work too: <smoothly_flowing_prose_paragraphs> steers format

3. Few-Shot Examples (3-5)

Examples are the most reliable way to steer format, tone, and structure.

Requirements:

• Relevant: Mirror real use cases
• Diverse: Cover edge cases, avoid unintended patterns
• Wrapped: Use <example> tags to distinguish from instructions

4. Role Prompting

One sentence in the system prompt focuses behavior:

You are a senior security engineer reviewing code for vulnerabilities.

5. Chain-of-Thought

For complex reasoning, ask Claude to work step-by-step. But prefer adaptive thinking (thinking: {type: "adaptive"}) over manual CoT — it outperforms in internal evals.

Guide thinking with:

"Choose an approach and commit to it. Avoid revisiting decisions
unless new information directly contradicts your reasoning."

Claude 4.6 Specifics

Prefills Are Gone

No more prefilling the assistant turn. Migrate to:

• Format control → explicit formatting instructions
• No preamble → "Begin your response with..."
• Continuations → provide context in user message

Concise by Default

Claude 4.6 is more direct, less verbose. If you want detail:

"After completing a task with tool use, provide a quick summary of the work done."

Opus Overthinks

Opus 4.6 does extensive upfront exploration. To control:

• Remove "If in doubt, use [tool]" → now causes overtriggering
• Replace "Default to using [tool]" with "Use [tool] when it would enhance understanding"
• Lower effort setting as a fallback

Parallel Tool Calling

Claude excels at parallel execution. Boost to ~100% with:

"If no dependencies between calls, make all independent calls in parallel."

Tool Design (ACI)

Invest as much effort in Agent-Computer Interface as you would in Human-Computer Interface.

Tool Description Principles

1. Claude's perspective: Is it obvious how to use this tool from the description alone?
2. Great docstrings: Write like you're onboarding a junior dev
3. Natural formats: Keep close to what the model sees in training data (not JSON-escaped code)
4. Room to think: Give enough output tokens before the model paints itself into a corner
5. Poka-yoke: Make mistakes structurally impossible (absolute paths > relative paths)
6. Test and iterate: Run many inputs, observe actual mistakes, fix the tool design

Tool Schema Tips

Prompt Optimization Workflow

1. Define success criteria before writing the prompt
2. Write a first draft — clear, explicit, structured
3. Test with real inputs — observe actual failures
4. Add examples targeting the failure modes
5. Iterate — each fix should address a specific observed problem
6. Measure — use evals, not vibes

Output Format Control

Ranked by effectiveness:

1. XML format indicators: <smoothly_flowing_prose_paragraphs>
2. Explicit format instructions: "Respond in JSON with keys: name, score, reasoning"
3. Match prompt style to desired output style (remove markdown from prompt → less markdown in output)
4. Few-shot examples showing exact format

Anti-Patterns