use-cases/codebase-onboarding.md Codex Docs, 2026-04-23 12:28 UTC → 2026-05-07 20:02 UTC

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

2 2name: Understand large codebases

3Codex use cases3tagline: Trace request flows, map unfamiliar modules, and find the right files fast.

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

5![](/assets/OpenAI-black-wordmark.svg)5 data flow, and point you to the next files worth reading before you edit.

6 6bestFor:

7![Codex](/assets/OAI_Codex-Lockup_Fallback_Black.svg)

8 

9Codex use case

10 

11# Understand large codebases

12 

13Trace request flows, map unfamiliar modules, and find the right files fast.

14 

15Difficulty **Easy**

16 

17Time horizon **5m**

18 

19Use Codex to map unfamiliar codebases, explain different modules and data flow, and point you to the next files worth reading before you edit.

20 

21## Best for

22 

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

24 - 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.

25 14 

26# Contents

27 

28[← All use cases](https://developers.openai.com/codex/use-cases)

29 

30Copy page [Export as PDF](https://developers.openai.com/codex/use-cases/codebase-onboarding/?export=pdf)

31 

32Use Codex to map unfamiliar codebases, explain different modules and data flow, and point you to the next files worth reading before you edit.

33 

34Easy

35 

365m

37 

38Related links

39 

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

41 

42## Best for

43 

44 - New engineers onboarding to a new repo or service

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

46 15 

47## Starter prompt

48 

49Explain how the request flows through <name of the system area> in the codebase.

50 Include:16 Include:

17 

51 - which modules own what18 - which modules own what

19 

52 - where data is validated20 - where data is validated

21 

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

54 End with the files I should read next.

55 23 

56[Open in the Codex app](codex://new?prompt=Explain+how+the+request+flows+through+%3Cname+of+the+system+area%3E+in+the+codebase.%0A%0AInclude%3A%0A-+which+modules+own+what%0A-+where+data+is+validated%0A-+the+top+gotchas+to+watch+for+before+making+changes%0A%0AEnd+with+the+files+I+should+read+next. "Open in the Codex app")

57 24 

58Explain how the request flows through <name of the system area> in the codebase.

59 Include:

60 - which modules own what

61 - where data is validated

62 - the top gotchas to watch for before making changes

63 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---

64 32 

65## Introduction33## Introduction

66 34 

70 38 

71If 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:

72 40 

73Explain this repo to me

74 

75If 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:

76 42 

771. 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.

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

90- 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?

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

92 

93## Related use cases

94 

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

96 

97### Iterate on difficult problems

98 

99Give Codex an evaluation system, such as scripts and reviewable artifacts, so it can keep...

100 

101Engineering Analysis](https://developers.openai.com/codex/use-cases/iterate-on-difficult-problems)[![](/images/codex/codex-wallpaper-1.webp)

102 

103### Create browser-based games

104 

105Use Codex to turn a game brief into first a well-defined plan, and then a real browser-based...

106 

107Engineering Code](https://developers.openai.com/codex/use-cases/browser-games)[![](/images/codex/codex-wallpaper-1.webp)

108 

109### Learn a new concept

110 

111Use Codex to study material such as research papers or courses, split the reading across...

112 

113Knowledge Work Data](https://developers.openai.com/codex/use-cases/learn-a-new-concept)