go-openapi

---
name: go-openapi
description: >
license: MIT
---

# Go OpenAPI

The spec is the source of truth. Types, routes, and clients are generated from
it — never hand-written alongside it, because two hand-maintained copies of a
contract diverge within one sprint.

## Operating Modes

- **Adopt** — the project has hand-written handlers and no spec, or a spec
  nobody generates from. Introduce generation without a rewrite.
- **Extend** — the pipeline exists. Change the spec, regenerate, implement.
- **Review** — check that handlers, spec, and published client agree, and
  that the change is not silently breaking.

## 1. Choose the Generator Once

| Tool | Use it when |
|---|---|
| `oapi-codegen` | Default. Types + server interface + client, works with `net/http`, chi, echo, gin |
| `ogen` | You want a fully generated, strictly validating server and can accept its opinions |
| `swaggo/swag` | Only for a legacy code-first project you are not converting — annotations generate the spec, so the spec cannot be reviewed before the code exists |

Do not mix. A project with both annotations and a checked-in spec has two
sources of truth again.

## 2. Wire Generation Into the Build

Pin the generator as a module tool (Go 1.24+), so every developer and CI run
uses the same version:

```bash
go get -tool github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen
```

```yaml
# oapi-codegen.yaml
package: api
output: internal/api/openapi.gen.go
generate:
  models: true
  std-http-server: true   # Go 1.22+ ServeMux; use chi-server / echo-server if that is the router
  strict-server: true     # typed request/response structs instead of raw http.ResponseWriter
  embedded-spec: true     # lets the service serve its own spec
output-options:
  skip-prune: false
```

```go
//go:generate go tool oapi-codegen -config oapi-codegen.yaml ../../api/openapi.yaml
```

Commit generated files. Reviewers need to see the diff of a contract change,
and a build must not depend on a generator being installed.

CI must fail when they are stale:

```bash
go generate ./... && git diff --exit-code
```

## 3. Implement the Generated Interface

Strict mode gives typed requests and responses, so the compiler enforces the
contract.

```go
// Generated: type StrictServerInterface interface { GetUser(ctx, GetUserRequestObject) (GetUserResponseObject, error) }

type Server struct{ users UserStore }

var _ api.StrictServerInterface = (*Server)(nil) // compile-time compliance

func (s *Server) GetUser(ctx context.Context, req api.GetUserRequestObject) (api.GetUserResponseObject, error) {
    u, err := s.users.Find(ctx, req.Id)
    if errors.Is(err, ErrNotFound) {
        return api.GetUser404JSONResponse{Title: "user not found", Status: 404}, nil
    }
    if err != nil {
        return nil, fmt.Errorf("find user %s: %w", req.Id, err) // 500 via the error handler
    }
    return api.GetUser200JSONResponse{Id: u.ID, Email: u.Email}, nil
}
```

Rules:

- Assert `var _ api.StrictServerInterface = (*Server)(nil)`. Adding an
  endpoint to the spec then becomes a compile error, not a 404 in staging.
- Return a typed response for every documented status. Return a Go `error`
  only for the undocumented failure path.
- Never edit `*.gen.go`. Every change starts in the YAML.

## 4. Validate Requests Against the Spec

Generated types check shape, not constraints. `minLength`, `pattern`,
`enum`, and `required` on query parameters are enforced only if you add the
validation middleware.

```go
spec, err := api.GetSwagger()
if err != nil {
    return fmt.Errorf("load spec: %w", err)
}
spec.Servers = nil // otherwise the server URL must match exactly

mux := http.NewServeMux()
handler := nethttpmiddleware.OapiRequestValidator(spec)(mux)
```

This rejects malformed input at the boundary with a 400 before any handler
runs. Keep domain validation in the domain — the middleware enforces the
contract, not the business rules.

## 5. Errors: RFC 9457 Problem Details

Define one error schema and reference it from every failure response.

```yaml
components:
  schemas:
    Problem:
      type: object
      required: [type, title, status]
      properties:
        type:   { type: string, format: uri, default: "about:blank" }
        title:  { type: string }
        status: { type: integer }
        detail: { type: string }
        instance: { type: string }
```

Serve it as `application/problem+json`. Never return a bare string, and never
put an internal error message in `detail` — log the wrapped error, return a
stable, safe title.

## 6. Versioning and Breaking Changes

Detect breaking changes mechanically; reviewers miss them.

```bash
go install github.com/oasdiff/oasdiff@latest
oasdiff breaking api/openapi.yaml.base api/openapi.yaml --fail-on ERR
```

Run it in CI against the spec on the main branch. Breaking, in practice:
removing an endpoint or field, narrowing a type, adding a required request
field or a required response field the client must understand, changing a
status code, removing an enum value from a response.

Additive changes are safe. Version the path (`/v2/...`) only when a break is
unavoidable, and keep the previous version serving until clients have moved.

## 7. Contract Testing

The spec is only a contract if something fails when the implementation
disagrees.

```go
func TestGetUser_MatchesSpec(t *testing.T) {
    spec, err := api.GetSwagger()
    require.NoError(t, err)
    spec.Servers = nil

    srv := httptest.NewServer(newTestHandler(t, spec))
    t.Cleanup(srv.Close)

    // Generated client — if the spec changed, this stops compiling
    c, err := api.NewClientWithResponses(srv.URL)
    require.NoError(t, err)

    resp, err := c.GetUserWithResponse(t.Context(), "u-1")
    require.NoError(t, err)
    require.Equal(t, http.StatusOK, resp.StatusCode())
    require.Equal(t, "u-1", resp.JSON200.Id)
}
```

Use the generated client in tests, not a hand-rolled `http.NewRequest`. It
turns contract drift into a compile failure.

## 8. Keep the Spec Reviewable

```bash
vacuum lint -d api/openapi.yaml     # or: spectral lint, redocly lint
```

- One file per API, under `api/`, checked in, reviewed like code.
- Every operation has an `operationId` — it becomes the Go method name.
- Every schema has a `description`; it becomes the godoc on the generated type.
- Split large specs with `$ref` to `components/`, not by generating fragments.

## Verification Checklist

1. Exactly one source of truth: a checked-in spec, no annotation generator alongside it
2. The generator is pinned via the `go.mod` tool directive
3. Generated files are committed and CI fails on `go generate` + `git diff --exit-code`
4. `var _ api.StrictServerInterface = (*Server)(nil)` present
5. No hand edits in `*.gen.go`
6. Request validation middleware installed and covered by a 400 test
7. Errors use a single `Problem` schema, served as `application/problem+json`
8. `oasdiff breaking` runs in CI against the base spec
9. At least one test drives the generated client against the real handler
10. The spec lints clean and every operation has an `operationId`