I made a small script to dump the first codex request. It points codex at a fake local server and captures the JSON.

chmod +x dump-codex-context.sh
./dump-codex-context.sh

The first request contains the user prompt as well as:

  • top-level instructions
  • AGENTS.md / project instructions, when present
  • environment context
  • skills instructions
  • plugin instructions
  • full tool declarations, including parameter docs

The default setup has:

top-level instructions: 21,335 chars
developer input block:   7,092 chars
environment context:       431 chars
tools:                  15 JSON schemas (81,575 chars)

Codex Internals

In codex-rs/core/src/session/mod.rs, codex builds the initial context roughly like this:

initial context =
  base model instructions
  + permissions instructions
  + collaboration-mode instructions
  + apps instructions
  + skills instructions
  + plugin instructions
  + environment context
  + tool schemas

In codex-rs/core/src/config/mod.rs, most of those context blocks default to on:

include_permissions_instructions = true
include_apps_instructions = true
include_collaboration_mode_instructions = true
include_skill_instructions = true
include_environment_context = true

So unless you turn them off, codex adds them.

model_instructions_file is the other important knob. It replaces the base model instructions:

model_instructions_file = "/Users/amin/.codex/system.md"

That means I can replace the giant generic prompt with one that is written for how I actually want the agent to work.

My Minimal Config

model_instructions_file = "/Users/amin/.codex/system.md"

include_permissions_instructions = true
include_apps_instructions = false
include_collaboration_mode_instructions = false
include_environment_context = true

[skills]
include_instructions = true

[skills.bundled]
enabled = false

[features]
plugins = false
browser_use = false
computer_use = false
enable_mcp_apps = false
in_app_browser = false
memories = false

And this is the kind of ~/.codex/system.md I want:

You are Codex, a coding agent.

Optimize for correctness, small diffs, and repo-local conventions.
Read the code before editing; use fast cli tools such as rg.
Do not invent abstractions unless they remove real complexity.
Verify changes with the narrowest useful test. Preserve user changes.

That is enough, the model already knows how to write code!