Authoring Agent System Prompts

Introduction and Context

In our last lesson, we learned how to use Codex to explore a new project and understand its structure. However, every time you start a new chat, the agent "forgets" your specific preferences unless you tell it again. To fix this, we use a file called AGENTS.md . Think of AGENTS.md as the permanent instruction manual for your repository. It acts as a System Prompt . Every time you launch Codex, it reads this file from your repository root to understand your rules, coding style, and safety boundaries. By keeping these instructions in a file, you ensure that Codex always behaves consistently, no matter who is using it or when the session starts. As a reminder from our previous work, you can always enter the interactive mode by typing this command in your terminal: Shell codex codex Once inside this interactive view, you are ready to start configuring your agent.

Initializing the Rulebook with /init

You don't have to create the AGENTS.md file by hand from scratch. Codex has a built-in shortcut to help you get started. Inside the Codex interactive session, you can use the /init command. Shell /init /init When you run this, Codex automatically generates a basic AGENTS.md file in the root directory of your project. In the CodeSignal IDE, you will see this new file appear in your file explorer on the left. The default file provides a skeleton structure. It often includes placeholders for the project's purpose and basic instructions. This saves you time and ensures the file is named and placed correctly so that Codex can find it later.

Defining Style and Formatting

The first major section you should add to your AGENTS.md is Coding Style . This ensures the AI doesn't write code that looks "out of place" compared to the rest of your project. Let's start building our file content. We will begin with a simple header and some style rules: Markdown # Agent Instructions ## Coding Style - Use 2 spaces for indentation. - Use double quotes for strings in TypeScript files. # Agent Instructions ## Coding Style - Use 2 spaces for indentation. - Use double quotes for strings in TypeScript files. # Agent Instructions : This is the main title of your rulebook. ## Coding Style : This sub-header tells Codex exactly where to look for formatting rules. The bullet points give clear, simple directions. By specifying 2 spaces and double quotes, you prevent the agent from using its own default settings, which might be different from your project's existing code. Now, let's add one more detail to make the code even cleaner: Markdown # Agent Instructions ## Coding Style - Use 2 spaces for indentation. - Use double quotes for strings in TypeScript files. - Always include a trailing comma in multi-line objects and arrays. # Agent Instructions ## Coding Style - Use 2 spaces for indentation. - Use double quotes for strings in TypeScript files. - Always include a trailing comma in multi-line objects and arrays. Adding the "trailing comma" rule helps keep your git diffs clean. When the agent knows these small details, you won't have to manually fix the formatting after the AI finishes its work.

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Markdown

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.

Markdown

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

Markdown

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- Run the fastest local test command after making changes and before presenting the final diff: `npm test -- --watchAll=false`
- If the test run fails, do not finalize the change. Investigate the failure, apply a fix, and re-run tests until they pass (or report the failure and ask for guidance if it can’t be resolved quickly).

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- Run the fastest local test command after making changes and before presenting the final diff: `npm test -- --watchAll=false`
- If the test run fails, do not finalize the change. Investigate the failure, apply a fix, and re-run tests until they pass (or report the failure and ask for guidance if it can’t be resolved quickly).

Markdown

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- After implementing a change, run the fastest local test command before presenting the final diff: `npm test -- --watchAll=false`

## Safety Boundaries
- Lockfiles: do not modify lockfiles (e.g., package-lock.json). If a lockfile change seems required, ask for confirmation first and explain why.
- Comments: keep existing explanatory comments. If a comment is inaccurate, update it rather than deleting it.

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- After implementing a change, run the fastest local test command before presenting the final diff: `npm test -- --watchAll=false`

## Safety Boundaries
- Lockfiles: do not modify lockfiles (e.g., package-lock.json). If a lockfile change seems required, ask for confirmation first and explain why.
- Comments: keep existing explanatory comments. If a comment is inaccurate, update it rather than deleting it.

Markdown

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- After implementing a change, run the fastest local test command before presenting the final diff: `npm test -- --watchAll=false`

## Safety Boundaries
- Lockfiles: do not modify lockfiles (e.g., package-lock.json). If a lockfile change seems required, ask for confirmation first and explain why.
- Comments: keep existing explanatory comments. If a comment is inaccurate, update it rather than deleting it.

## Commit Messages
- Use the Conventional Commits format.
- Examples: `feat: add login logic`, `fix: resolve crash in header`.

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- After implementing a change, run the fastest local test command before presenting the final diff: `npm test -- --watchAll=false`

## Safety Boundaries
- Lockfiles: do not modify lockfiles (e.g., package-lock.json). If a lockfile change seems required, ask for confirmation first and explain why.
- Comments: keep existing explanatory comments. If a comment is inaccurate, update it rather than deleting it.

## Commit Messages
- Use the Conventional Commits format.
- Examples: `feat: add login logic`, `fix: resolve crash in header`.

Markdown

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- Run the fastest local test command after making changes and before presenting the final diff: `npm test -- --watchAll=false`
- If the test run fails, do not finalize the change. Investigate the failure, apply a fix, and re-run tests until they pass (or report the failure and ask for guidance if it can’t be resolved quickly).

Markdown

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- After implementing a change, run the fastest local test command before presenting the final diff: `npm test -- --watchAll=false`

## Safety Boundaries
- Lockfiles: do not modify lockfiles (e.g., package-lock.json). If a lockfile change seems required, ask for confirmation first and explain why.
- Comments: keep existing explanatory comments. If a comment is inaccurate, update it rather than deleting it.

Markdown

# Agent Instructions

## Coding Style
- Use 2 spaces for indentation.
- Use double quotes for strings in TypeScript files.
- Always include a trailing comma in multi-line objects and arrays.

## Testing Requirements
- When changing behavior, you must add or adjust tests to match.
- After implementing a change, run the fastest local test command before presenting the final diff: `npm test -- --watchAll=false`

## Safety Boundaries
- Lockfiles: do not modify lockfiles (e.g., package-lock.json). If a lockfile change seems required, ask for confirmation first and explain why.
- Comments: keep existing explanatory comments. If a comment is inaccurate, update it rather than deleting it.

## Commit Messages
- Use the Conventional Commits format.
- Examples: `feat: add login logic`, `fix: resolve crash in header`.

Introduction and Context

Initializing the Rulebook with /init

Defining Style and Formatting

Join the 1M+ learners on CodeSignal

Enforcing Quality with Testing Rules

Setting Safety Boundaries and Commit Standards

Summary