What is the difference between GOPATH mode and Go modules?

0-2 years experience

1. 1You need to add a new third‑party library to a small Go project that currently uses GOPATH. How would you fetch and import it, and what would change if the project used Go modules instead?
2. 2If you run `go build` inside a directory that contains a `go.mod` file, what does the Go toolchain do differently compared to a directory without one?
3. 3Your CI script sets `GOPATH` and runs `go get ./...`. After adding a `go.mod` file the build fails. Why does this happen?

2-5 years experience

1. 1Your team is adding a new microservice that lives in a GOPATH‑based repo, but management wants to adopt Go modules for better versioning. What migration steps would you take and what pitfalls would you watch for?
2. 2During a release a dependency version conflict appears only when building with modules enabled. How would you diagnose and resolve it?
3. 3A build that succeeded on a developer’s machine using GOPATH started failing on the CI server after the CI switched to module mode. What could be causing the discrepancy?

5-8 years experience

1. 1You are designing a shared library that will be consumed by dozens of services across the company. How would you structure its repository and choose between GOPATH and Go modules to ensure reproducible builds and easy upgrades?
2. 2Our monorepo contains many legacy packages that still rely on GOPATH. Introducing modules caused duplicate import paths and version skew. How would you architect a migration strategy that minimizes disruption and supports incremental adoption?
3. 3Discuss the performance and caching implications of using a module proxy versus GOPATH vendoring in a large CI pipeline that runs thousands of builds per day.

8+ years experience

1. 1The organization plans to deprecate GOPATH entirely and enforce module usage across all teams. What organization‑wide policies, tooling, and migration road‑map would you propose to handle legacy code, vendor directories, and external dependencies?
2. 2Consider multiple teams needing to share internal packages with different version requirements. How would you design a versioning and publishing strategy using Go modules that balances stability and flexibility, and what governance model would you put in place?
3. 3If you had to support both Go 1.13 (module‑aware) and older Go versions that only understand GOPATH, how would you architect the build system and repository layout to serve both without duplication?