---
title: "Modifying system prompts Code Agent SDK"
source: https://docs.autohand.ai/agent-sdk/customize/system-prompts
---

# Modify the system prompt

The CLI ships with a default system prompt. You have two ways to influence it: append your own guidance on top, or replace it entirely. Append is the right default for most integrations.

## Append vs replace

-   **Append**: keep the CLI defaults and add your own coding standards, repo conventions, or persona. Recommended.
-   **Replace**: take full control of the system prompt. Only do this if you have a strong reason; you lose the CLI's built-in tool guidance, safety rules, and formatting.

Both methods accept either inline text or a file path. The file path mirrors the CLI's \`--sys-prompt\` / \`--append-system-prompt\` flags.

## Append to the default prompt

Use this for repo-specific rules, brand voice, or workflow guardrails.

The TypeScript and Python SDKs also accept a file path. Anything that looks like a path is read from disk; everything else is treated as inline text.

## Replace the default prompt

Use this only when you need to fully define the agent's behaviour. You are responsible for tool guidance, safety rules, and output formatting.

## Per-agent instructions

The high-level \`Agent.create()\` API takes an \`instructions\` field that becomes part of the appended prompt. Use this for short-lived agents that need a focused persona.

## Best practices

-   Default to appending. Replacing is rarely the right answer.
-   Keep prompts in files under version control. Put them next to the code they govern.
-   Be specific about success criteria (“tests pass”, “file compiles”) so the agent knows when it can stop.
-   State what the agent should not do. “Do not edit migration files” is more useful than a long list of allowed paths.
-   If you maintain multiple agents, share a base prompt and append per-agent overrides on top.