use-cases/codebase-onboarding.md +57 −0 added
1---
2name: Understand large codebases
3tagline: Trace request flows, map unfamiliar modules, and find the right files fast.
4summary: Use Codex to map unfamiliar codebases, explain different modules and
5 data flow, and point you to the next files worth reading before you edit.
6bestFor:
7 - New engineers onboarding to a new repo or service
8 - 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.
14
15
16 Include:
17
18 - which modules own what
19
20 - where data is validated
21
22 - the top gotchas to watch for before making changes
23
24
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---
32
33## Introduction
34
35When you are new to a repo or dropped into an unfamiliar feature, Codex can help you get oriented before you start changing code. The goal is not just to get a high-level summary, but to map the request flow, understand which modules own what, and identify the next files worth reading.
36
37## How to use
38
39If you're new to a project, you can simply start by asking Codex to explain the whole codebase:
40
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:
42
431. Give Codex the relevant files, directories, or feature area you are trying to understand.
442. Ask it to trace the request flow and explain which modules own the business logic, transport, persistence, or UI.
453. Ask where validation, side effects, or state transitions happen before you edit anything.
464. End by asking which files you should read next and what the risky spots are.
47
48A useful onboarding answer should leave you with a concrete map, not just a list of filenames. By the end, Codex should have explained the main flow, highlighted the risky parts, and pointed you to the next files or checks that matter before you start editing.
49
50## Questions to ask next
51
52Once Codex gives you a first pass, keep going until the explanation is specific enough that you would trust yourself to make the first edit. Good follow-up questions usually force it to call out assumptions, hidden dependencies, and the checks that matter after a change.
53
54- Which module owns the actual business logic versus the transport or UI layer?
55- Where does validation happen, and what assumptions are enforced there?
56- What related files or background jobs are easy to miss if I change this flow?
57- Which tests or checks should I run after editing this area?