golang-dependency-management

**Persona:** You are a Go dependency steward. You treat every new dependency as a long-term maintenance commitment — you ask whether the standard library already solves the problem before reaching for an external package.

**Dependencies:**

- govulncheck: `go install golang.org/x/vuln/cmd/govulncheck@latest`

# Go Dependency Management

## AI Agent Rule: Ask Before Adding Dependencies

**Before running `go get` to add any new dependency, AI agents MUST ask the user for confirmation.** AI agents can suggest packages that are unmaintained, low-quality, or unnecessary when the standard library already provides equivalent functionality. Using `go get -u` to upgrade an existing dependency is safe.

Before proposing a dependency, evaluate:

- Does the standard library already cover the use case?
- Is the license compatible?
- Are there well-known alternatives?
- What it does and why it's needed?

The `samber/cc-skills-golang@golang-popular-libraries` skill contains a curated list of vetted, production-ready libraries. Prefer recommending packages from that list. When no vetted option exists, favor well-known packages from the Go team (`golang.org/x/...`) or established organizations over obscure alternatives.

## Key Rules

- `go.sum` MUST be committed — it records cryptographic checksums of every dependency version, letting `go mod verify` detect supply-chain tampering. Without it, a compromised proxy could silently substitute malicious code
- `govulncheck ./...` or `go tool govulncheck ./...` before every release — catches known CVEs in your dependency tree before they reach production
- Maintenance status, license compatibility, and stdlib alternatives are important considerations before adding a dependency — every dependency increases attack surface, maintenance burden, and binary size
- `go mod tidy` before every commit that changes dependencies — removes unused modules and adds missing ones, keeping go.mod honest

## go.mod & go.sum

### Essential Commands

| Command           | Purpose                                      |
| ----------------- | -------------------------------------------- |
| `go mod tidy`     | Add missing deps, remove unused ones         |
| `go mod download` | Download modules to local cache              |
| `go mod verify`   | Verify cached modules match go.sum checksums |
| `go mod vendor`   | Copy deps into `vendor/` directory           |
| `go mod edit`     | Edit go.mod programmatically (scripts, CI)   |
| `go mod graph`    | Print the module requirement graph           |
| `go mod why`      | Explain why a module or package is needed    |

### Vendoring

Use `go mod vendor` when you need hermetic builds (no network access), reproducibility guarantees beyond checksums, or when deploying to environments without module proxy access. CI pipelines and Docker builds sometimes benefit from vendoring. Run `go mod vendor` after any dependency change and commit the `vendor/` directory.

## Installing & Upgrading Dependencies

### Adding a Dependency

```bash
go get github.com/google/uuid          # Latest version
go get github.com/google/uuid@v1.6.0   # Specific version
go get github.com/google/uuid@latest   # Explicitly latest
go get github.com/google/uuid@<commit> # Specific commit (pseudo-version)
```

### Upgrading

```bash
go get -u ./...            # Upgrade ALL direct+indirect deps to latest minor/patch
go get -u=patch ./...      # Upgrade to latest patch only (safer)
go get github.com/pkg@v1.5 # Upgrade specific package
```

**Prefer `go get -u=patch`** for routine updates. Patch and minor updates are usually lower risk than major upgrades, but still require review. For dependency updates, run:

```bash
go get -u=patch ./...
go mod tidy
go test ./...
go vet ./...
govulncheck ./...   # or: go tool govulncheck ./...
```

Release notes and changelogs for libraries affecting persistence, serialization, networking, authentication, authorization, cryptography, or public APIs may contain important information about breaking changes.

### Removing a Dependency

```bash
go get github.com/google/uuid@none  # Mark for removal
go mod tidy                          # Clean up go.mod and go.sum
```

### Installing CLI Tools

For Go 1.24+ modules, pin executable tools in `go.mod` with `tool` directives. Do not create a new `tools.go` blank-import file unless the module must support Go <1.24.

```bash
# Add tools to the current module.
go get -tool github.com/golangci/golangci-lint/v2/cmd/golangci-lint@latest
go get -tool golang.org/x/vuln/cmd/govulncheck@latest
go get -tool golang.org/x/perf/cmd/benchstat@latest

# Run pinned tools reproducibly.
go tool golangci-lint run ./...
go tool govulncheck ./...
go tool benchstat old.txt new.txt

# Install all module-pinned tools into GOBIN/PATH when needed.
go install tool

# Update pinned tools deliberately, then review go.mod/go.sum.
go get -u tool
go mod tidy
```

`go.mod` shape for a module targeting Go 1.26 or newer. This is an example target, not a cap; keep the project's actual `go` directive and do not change it just to add tools.

```go.mod
module example.com/project

go 1.26

tool (
    github.com/golangci/golangci-lint/v2/cmd/golangci-lint
    golang.org/x/vuln/cmd/govulncheck
    golang.org/x/perf/cmd/benchstat
)
```

For Go <1.24 only, use the legacy `tools.go` blank-import workaround:

```go
//go:build tools

package tools

import (
    _ "github.com/golangci/golangci-lint/v2/cmd/golangci-lint"
    _ "golang.org/x/vuln/cmd/govulncheck"
)
```

Rule: Go 1.24+ = `tool` directives. Go <1.24 = `tools.go` fallback.

### Go 1.26+ module target note

When using a Go 1.26 or newer toolchain, `go mod init` may create a module with an older default `go` directive. If the project intentionally targets Go 1.26+ APIs, update the directive deliberately:

```bash
go mod edit -go=1.26
go mod tidy
```

For future Go versions, use the project's intended target version. Do not use APIs newer than the module's `go` directive until the projec