Claude Prompting Best Practices

Opus 4.5 Behavioral Characteristics

Opus 4.5 has specific behavioral characteristics that affect prompt design:

• Highly responsive to system prompts - Emphatic language causes overtriggering
• Stricter instruction adherence - Won't infer implied requests; needs explicit direction
• Sensitive to "think" word - When extended thinking is disabled, triggers unintended behavior
• Tendency to over-engineer - May create extra files, unnecessary abstractions, or unrequested features
• May skip code exploration - Can propose solutions without reading relevant files

Core Principles

Use Positive Framing

Describe what to do rather than what to avoid. Negative prompting can backfire.

Soften Emphatic Language

Opus 4.5 overtriggers on emphatic language.

Exception: Emphatic language is appropriate for code exploration directives where skipping leads to incorrect solutions based on unread code:

• "Read and understand relevant files before proposing code edits"

Replace "Think" Variants

When extended thinking is disabled, Opus 4.5 is sensitive to "think."

Be Explicit and Specific

Opus 4.5 requires explicit direction. Vague instructions underperform.

# Vague (less effective)

- Prioritize simplicity and well-structured code
- Use good architecture patterns

# Explicit (more effective)

- Implement exactly what was requested, nothing more
- Place domain logic in models, external API calls in service objects
- Use Capybara's click_on, fill_in methods with visible text labels

Provide Context and Motivation

Explain why behavior matters to help Claude understand goals:

# Without context

- Let code fail loudly on invalid assumptions

# With context (more effective)

- Let code fail loudly on invalid assumptions - this reveals programming
  errors early rather than masking bugs with defensive checks

Skill Structure

File Naming and Layout

skill-name/
├── SKILL.md              # Required - uppercase, contains frontmatter + content
└── references/           # Optional - supporting reference material
    └── reference.md      # Claude reads these only when needed

YAML Frontmatter

---
name: skill-name # lowercase, hyphens, max 64 chars
description: What the skill does. Use when [trigger conditions]. Covers [scope].
---

Description requirements:

• Maximum 1024 characters
• Include BOTH what it does AND when to use it (trigger terms)
• Be specific, not generic

# Effective description
description: Guide for writing Ruby and Rails code in this codebase. Use when
implementing features, fixing bugs, refactoring, or reviewing code. Covers
code style, architecture patterns, testing, and Rails conventions.

Skill Naming

• Lowercase letters, numbers, and hyphens only
• Maximum 64 characters
• Specific and descriptive
• Match domain terminology users would search for

Scope: One Capability Per Skill

A skill should address one focused problem domain. If you need "and/or" or long lists of unrelated capabilities, consider splitting.

Examples of appropriately focused skills:

• "pdf-form-filling"
• "rails-hotwire"
• "customer-support-debugging"

Content Best Practices

Use Concrete Examples

Provide working code examples like StripeService.create_subscription(team, plan) rather than abstract descriptions like "Use service objects for external API calls."

Structure with Tables for Reference

Tables make lookup information scannable. Use them for tool references, command options, and comparison matrices.

Use Step-by-Step Workflows

Explicit numbered steps give clear actions. List the specific sequence of operations to perform.

Move Reference Content to Separate Files

Keep SKILL.md focused on workflows. Move large reference tables and lookup data to references/ subdirectory.

Battle-Tested Patterns

These snippets from Anthropic's Opus 4.5 migration demonstrate effective prompt patterns:

Over-Engineering Prevention: "Avoid over-engineering. Only make changes directly requested or clearly necessary. Don't add features, refactor code, or make improvements beyond what was asked. Don't create helpers, utilities, or abstractions for one-time operations."

Code Exploration: "Read and understand relevant files before proposing code edits. If the user references a specific file or path, open and inspect it before suggesting changes."

Frontend Design Quality: "Create distinctive, production-grade interfaces. Use purposeful typography choices (avoid overused fonts like Arial, Inter, Roboto). Commit to cohesive color schemes with dominant colors and sharp accents. Employ strategic motion and animations. Create atmospheric backgrounds rather than flat colors."

Context Management

Keep Content Focused

Every token in a skill enters the context window when activated, affecting both cost and performance. Optimize for signal by removing redundant examples, avoiding repetition of standard conventions Claude already knows, and moving lookup tables to reference files.

Match Prompt Style to Output Style

Formatting in prompts influences response formatting. Use prose paragraphs unless bullet lists are necessary, as this reduces markdown artifacts in responses.

Anti-Patterns to Avoid