use-cases/codebase-onboarding.md Codex Docs, 2026-04-08 00:40 UTC → 2026-05-07 17:08 UTC

1# Understand large codebases | Codex use cases1---

2 2name: Understand large codebases

3[← All use cases](https://developers.openai.com/codex/use-cases)3tagline: Trace request flows, map unfamiliar modules, and find the right files fast.

4 4summary: Use Codex to map unfamiliar codebases, explain different modules and

5Use Codex to map unfamiliar codebases, explain different modules and data flow, and point you to the next files worth reading before you edit.5 data flow, and point you to the next files worth reading before you edit.

6 6bestFor:

7Easy

8 

95m

10 

11Related links

12 

13[Codex app](https://developers.openai.com/codex/app)

14 

15## Best for

16 

17 - New engineers onboarding to a new repo or service7 - New engineers onboarding to a new repo or service

18 - Anyone trying to understand how a feature works before changing it8 - Anyone trying to understand how a feature works before changing it

9starterPrompt:

10 title: Explain the System Area I Need

11 body: >-

12 Explain how the request flows through <name of the system area> in the

13 codebase.

19 14 

20## Starter prompt

21 15 

22Explain how the request flows through <name of the system area> in the codebase.

23 Include:16 Include:

17 

24 - which modules own what18 - which modules own what

19 

25 - where data is validated20 - where data is validated

21 

26 - the top gotchas to watch for before making changes22 - the top gotchas to watch for before making changes

23 

24 

27 End with the files I should read next.25 End with the files I should read next.

26 suggestedModel: gpt-5.3-codex-spark

27 suggestedEffort: medium

28relatedLinks:

29 - label: Codex app

30 url: /codex/app

31---

28 32 

29## Introduction33## Introduction

30 34 

34 38 

35If you're new to a project, you can simply start by asking Codex to explain the whole codebase:39If you're new to a project, you can simply start by asking Codex to explain the whole codebase:

36 40 

37Explain this repo to me

38 

39If you need to contribute a new feature to an existing codebase, you can ask codex to explain a specific system area. The better you scope the request, the more concrete the explanation will be:41If you need to contribute a new feature to an existing codebase, you can ask codex to explain a specific system area. The better you scope the request, the more concrete the explanation will be:

40 42 

411. Give Codex the relevant files, directories, or feature area you are trying to understand.431. Give Codex the relevant files, directories, or feature area you are trying to understand.

53- Where does validation happen, and what assumptions are enforced there?55- Where does validation happen, and what assumptions are enforced there?

54- What related files or background jobs are easy to miss if I change this flow?56- What related files or background jobs are easy to miss if I change this flow?

55- Which tests or checks should I run after editing this area?57- Which tests or checks should I run after editing this area?

56 

57## Related use cases

58 

59[![](/images/codex/codex-wallpaper-3.webp)

60 

61### Iterate on difficult problems

62 

63Give Codex an evaluation system, such as scripts and reviewable artifacts, so it can keep...

64 

65Engineering Analysis](https://developers.openai.com/codex/use-cases/iterate-on-difficult-problems)[![](/images/codex/codex-wallpaper-1.webp)

66 

67### Create browser-based games

68 

69Use Codex to turn a game brief into first a well-defined plan, and then a real browser-based...

70 

71Engineering Code](https://developers.openai.com/codex/use-cases/browser-games)[![](/images/codex/codex-wallpaper-2.webp)

72 

73### Analyze datasets and ship reports

74 

75Use Codex to clean data, join sources, explore hypotheses, model results, and package the...

76 

77Data Analysis](https://developers.openai.com/codex/use-cases/datasets-and-reports)