How To Shrink The System Prompt For Codex — Development skill for Claude Code

How to shrink the built-in system-prompt in Codex

Codex can carry substantially more fixed context than you may expect.

The effective prompt seen by the model can include:

• Codex's bundled base instructions
• global AGENTS.md
• repository-level AGENTS.md
• directory-specific AGENTS.md
• developer_instructions
• skill instructions
• plugin/app instructions
• MCP tool definitions
• built-in tool schemas
• your actual conversation

For ordinary usage this may be fine. But if your goal is a **lean coding harness**, **lower token throughput**, **better prompt-cache behavior**, or a **minimal bootstrap environment**, reducing this fixed context can be worthwhile.

The most important trick is not to shorten Codex's built-in prompt manually.

Instead, **replace it**.

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1. Replace Codex's bundled base instructions

Current Codex versions support:

model_instructions_file = "/path/to/minimal-codex-prompt.md"

Add this to:

~/.codex/config.toml

The referenced file can contain a much smaller base prompt.

For example:

You are a coding agent operating in a repository.

Use the available tools to inspect and modify files as needed.

Rules:
- Follow the user's request precisely.
- Inspect relevant code before editing.
- Preserve existing behavior unless change is requested.
- Make minimal, coherent edits.
- Do not modify unrelated files.
- Run relevant tests when practical.
- Report changes and unresolved issues concisely.

This can reduce a fairly large bundled instruction set to only a few hundred tokens.

A minimal configuration might therefore begin with:

model_instructions_file = "/home/user/.codex/minimal-instructions.md"

On Windows:

model_instructions_file = "C:\\Users\\USERNAME\\.codex\\minimal-instructions.md"

[!IMPORTANT] Do not duplicate tool descriptions in the replacement prompt. Tool schemas are supplied separately by the harness. Repeating shell, editor, patch, or MCP tool instr