gh_ex is Req-native, so Req.Test drives it with no live API. Add Plug to your test dependencies, then use GhEx.Testing.client/1 to install a named stub with retries disabled:

{:plug, "~> 1.16", only: :test}

A stubbed request

test "fetches a repo" do
  Req.Test.stub(MyApp.Stub, fn conn ->
    assert conn.request_path == "/repos/o/r"
    Req.Test.json(conn, %{"full_name" => "o/r"})
  end)

  client = GhEx.Testing.client(MyApp.Stub)

  assert {:ok, %{"full_name" => "o/r"}, _meta} = GhEx.REST.get(client, "/repos/o/r")
end

The stub receives a Plug.Conn, so you can assert on the method, path, query string, headers, and body. GhEx.Testing.json/2 adds a standard core rate-limit snapshot to the response so assertions can include meta.

Asserting the request

Req.Test.stub(MyApp.Stub, fn conn ->
  assert conn.method == "POST"
  {:ok, raw, conn} = Plug.Conn.read_body(conn)
  assert Jason.decode!(raw) == %{"title" => "Bug"}
  conn |> Plug.Conn.put_status(201) |> GhEx.Testing.json(%{"number" => 1})
end)

Because the client sets retry: false, a stubbed 500 or other transient failure returns immediately instead of following Req's production retry schedule.

Conditional and rate-limit responses

Use GhEx.Testing.not_modified/1 for an ETag poll's 304 path. The GhEx.Testing.rate_limited/2 variants cover retry-after, exhausted primary bucket, and body-only secondary rate limits:

Req.Test.stub(MyApp.Stub, fn conn ->
  GhEx.Testing.rate_limited(conn, :secondary)
end)

assert {:error, %GhEx.Error{} = error} =
         GhEx.REST.get(GhEx.Testing.client(MyApp.Stub), "/repos/o/r")

assert GhEx.Error.classify(error) == :rate_limited

Errors and pagination

Return a non-2xx status to exercise error handling, or a Link header to drive GhEx.REST.stream/3:

conn
|> Plug.Conn.put_resp_header("link", ~s(<#{next_url}>; rel="next"))
|> Req.Test.json([%{"n" => 1}])

App and installation auth

Minting an installation token runs inside the cache GenServer, a different process than the test. Allow it to use the stub with Req.Test.allow/3:

cache_pid = start_supervised!({GhEx.TokenCache.ETS, name: MyApp.Cache})
Req.Test.allow(MyApp.Stub, self(), cache_pid)

Then the stub handles both the access-tokens POST (as the app) and the API call (as the installation).