4. Prompt, Plan, and Goal: Let Codex Think Before Coding

Codex can do many things—so you need to articulate your tasks clearly.

The official best practices offer an excellent framework: a well-defined task should ideally consist of four parts—Goal, Context, Constraints, and Done When. In Chinese, these correspond to: Objective, Background, Boundaries, and Acceptance Criteria.

When writing prompts for Codex, I start by decomposing any vague request into these four components:

• What we aim to achieve,
• What must remain untouched,
• How we plan to proceed, and
• How we’ll know it’s complete.

This habit drastically reduces back-and-forth clarification.

If a task spans more than three files, I typically ask Codex to first generate a plan. If the task unfolds over multiple rounds of interaction, I use /goal to maintain continuity and alignment. Don’t treat Plan as mere lip service—or Goal as just another to-do item. Their real value lies in preserving stable context and unambiguous acceptance criteria.

A Weak Prompt vs. A Strong Prompt

Weak prompt:

Before invoking Codex, explicitly state the objective, background context, scope of changes, and verification commands. Planning isn’t ceremonial—it determines whether subsequent modifications can be objectively validated.

Help me optimize the homepage.

The problem? Optimize what? Performance? SEO? Styling? Conversion rate? Mobile responsiveness? Structured data? Without clear boundaries, Codex can only guess.

A stronger version:

Goal: Optimize the first viewport of the homepage so users can access today’s AI news and hands-on articles faster.
Context: Please first read app/page.tsx, the navigation component, and the homepage styles.
Constraints: Do not modify login, registration, API, or database logic; do not introduce new dependencies.
Done when: Buttons do not overflow on mobile or desktop; `npm run build` passes; and a summary of the diff is provided.

This isn’t “prompt engineering mysticism”—it’s engineering-grade task specification.

For Complex Tasks: Start with /plan

When a task is complex, ambiguous, or spans multiple files, begin in plan mode. As documented in the Codex manual, Plan mode enables Codex to first gather context, ask clarifying questions, and draft a step-by-step plan—before implementing anything.

After reading “Prompt, Plan, and Goal: Let Codex Think First, Then Act”, take one minute to reflect:

• Are the core concepts clearly distinguished?
• Can the practice steps be reliably reproduced?
• Can you rephrase the key conclusions in your own words?

You can trigger plan mode in CLI, IDE, or App using:

/plan

Or simply say:

Please don’t modify any files yet. First, read the relevant code, then outline a step-by-step plan—including risks and verification methods—and wait for my approval before proceeding.

I usually require that every plan includes:

• Which files need to be read,
• What will be modified at each step,
• Which files must not be touched,
• How correctness will be verified, and
• Where rollback risks lie.

Use /goal When the Objective Is Clear

Goal mode suits longer-running tasks. As explained in the official prompting documentation, a goal represents a persistent objective—Codex uses it to guide its next actions and assess completion.

For example:

/goal
Fix the mobile layout of this project’s login page. Done when: no horizontal scrolling at 320px width, form buttons are fully visible (not obscured), and the build passes.

If the goal remains fuzzy, start with /plan: let Codex help crystallize your vague idea into a concrete, actionable objective—then set /goal.

Avoid Concurrent Modifications to the Same Files

Codex supports multi-threading and parallel tasks—but the official manual cautions against having two threads simultaneously modify the same set of files. Doing so invites merge conflicts and makes it hard to trace which thread introduced a given issue.

Parallelism works well when threads handle independent concerns, e.g.: ✅ One thread writes documentation, ✅ Another adds tests, ✅ A third researches background information.

It does not work well when: ❌ Three threads concurrently edit the same homepage.

Every Task Needs Clear “Done When” Criteria

Acceptance criteria can be simple:

After reading “Prompt, Plan, and Goal: Let Codex Think First, Then Act”, try walking through a small, concrete example end-to-end—then assess which steps you can now execute independently.

By this point, you should be able to distill “Prompt, Plan, and Goal: Let Codex Think First, Then Act” into a concise reflection table: clarify the main thread first, then test it on a small task.

• Build passes.
• Specified tests pass.
• Page renders without overflow on mobile.
• Git diff contains only intended files.
• Changes are strictly limited to the agreed-upon scope.

Without explicit acceptance criteria, Codex may reply, “I’ve optimized it!”—but what you need isn’t a declaration of completion. You need verifiable, inspectable outcomes.

References:

• Codex Best Practices
• Codex Prompting Guide
• Codex App Commands