System messages for behavior control

1. System messages for behavior control

Hello, welcome to this video on system messages for behavior control.

2. What are system messages?

Picture a support bot that must stay polite, concise, and end every reply with “Is there anything else I can help you with today?”

3. What are system messages?

A single system message locks that behavior for the whole chat, with no need to repeat instructions in every prompt. System messages give Claude a lasting character: tone, constraints, output format, and even a specialized role.

4. Writing effective system prompts

The key to effective system prompts is specificity. A vague message like 'be helpful' doesn't give Claude much guidance. A strong system message defines the exact role, specifies the writing style, sets clear constraints, and establishes boundaries. In code, the weak prompt produces generic text; the strong prompt, mentioning the technical writer, use of active voice, under 100 words, and actionable advice, yields more specific output.

5. Common system message patterns

System messages typically follow common patterns. Role-based messages establish who Claude should be. Constraint-based messages set hard limits on format or length. Tone-based messages define the personality. Task-specific messages give specialized work instructions. The code examples show each of the patterns: defining Claude as a customer service rep, setting exact bullet point constraints, establishing an enthusiastic tone, or giving specific debugging instructions. Each serves different needs and can be combined for precise control.

6. Testing and iterating system messages

System messages rarely work perfectly on the first try, so testing and iteration are crucial. We need to test with diverse user inputs to discover edge cases where our system message might not work as expected. We should monitor for undesired outputs or behavior drift, where Claude gradually moves away from our intended behavior. The code example shows how a vague 'be concise' message led to overly short responses, so we refined it to specify 2-3 sentences with relevant context. This iterative process helps us build robust system messages that work consistently.

7. Advanced system message techniques

Let's say we're building an automated code review system that needs to prioritize security issues over performance, give specific feedback formats, and handle cases where code is already perfect. We can combine multiple instructions with clear priorities - notice how the system message establishes security as priority one, performance as two, and style as three. We can use conditional logic for different scenarios - 'If no issues: Code looks good' versus suggesting improvements. We can include examples right in the system message to show the exact format we want. And we can layer this with few-shot prompting for even more control. The code example shows this sophisticated approach - it creates a consistent, prioritized code review system that handles multiple scenarios and always follows our specified format.

8. Let's practice!

Let's dive in and practice crafting powerful system messages!

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