System Prompt Modes

When customizing system prompts in Msty Studio (ie, with Projects and Personas), you can control how your custom instructions are combined with the default system prompt using three modes:

• Prepend
• Replace
• Append

Each mode affects how the model interprets behavior and tone. Below are examples and explanations for each mode, along with their advantages and disadvantages. The examples use the following prompts:

Your custom prompt: "You are a pirate who speaks like a pirate."
Default system prompt: "You are a helpful assistant."

Prepend

Definition: Your custom prompt is added before the default system prompt.

Example:

Final Prompt:
"You are a pirate who speaks like a pirate. You are a helpful assistant."

Advantages:

• Gives your custom instructions priority.
• Useful for setting tone, style, or specific roles early on.
• Keeps the base safety and helpfulness rules.

Disadvantages:

• May conflict with the system prompt if the tones differ.
• Custom behavior may not fully override the assistant’s defaults.

Best For: When you want to strongly guide behavior or persona, but still retain the assistant’s helpful baseline.

Replace

Definition: Your custom prompt completely overrides the system prompt.

Example:

Final Prompt:
"You are a pirate who speaks like a pirate."

Advantages:

• Gives you full control of the assistant’s behavior.
• Ideal for custom personas, creative characters, or specialized workflows.

Disadvantages:

• Removes all default behavior (e.g., safety, neutrality, helpfulness).
• Requires careful crafting to avoid issues or undesired responses.

Best For: When you want to define the assistant’s role or tone entirely from scratch.

Append

Definition: Your custom prompt is added after the default system prompt.

Example:

Final Prompt:
"You are a helpful assistant. You are a pirate who speaks like a pirate."

Advantages:

• Preserves the default assistant behavior.
• Great for subtle additions, like tone adjustments or specific response styles.
• Less likely to conflict with system rules.

Disadvantages:

• Lower priority — your custom instructions may be overridden or ignored.
• Not ideal for dramatic behavior shifts.

Best For: When you want to enhance or refine the assistant’s responses, without changing its core behavior.

Comparison Table