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+{-# LANGUAGE LambdaCase #-}
+
+module Minilang.IntegrationSpec where
+
+import Control.Exception (bracket)
+import Data.Aeson (eitherDecodeFileStrict)
+import Data.Proxy (Proxy (..))
+import Data.Text (Text)
+import Minilang.API
+import Minilang.Server (Server (..), startServer, stopServer)
+import Network.HTTP.Client (defaultManagerSettings, newManager)
+import Servant.API ((:<|>) (..))
+import Servant.Client hiding (mkClientEnv)
+import qualified Servant.Client as SC
+import Test.Hspec
+
+-- ---------------------------------------------------------------------------
+-- Servant client functions derived from the API type
+-- ---------------------------------------------------------------------------
+
+registerClient :: User -> ClientM Token
+unregisterClient :: Maybe Token -> ClientM Score
+leaderboardClient :: ClientM [Score]
+scoreClient :: Maybe Token -> ClientM Score
+challengeClient :: Maybe Token -> ClientM Challenge
+answerClient :: Maybe Token -> Text -> ClientM ()
+
+registerClient
+  :<|> unregisterClient
+  :<|> leaderboardClient
+  :<|> scoreClient
+  :<|> challengeClient
+  :<|> answerClient
+  :<|> _ =
+    client (Proxy :: Proxy LambdaAPI)
+
+-- ---------------------------------------------------------------------------
+-- Helpers
+-- ---------------------------------------------------------------------------
+
+-- | Build a 'ClientEnv' pointing at localhost on the given port.
+mkEnv :: Int -> IO ClientEnv
+mkEnv port = do
+  mgr <- newManager defaultManagerSettings
+  pure $ SC.mkClientEnv mgr (BaseUrl Http "127.0.0.1" port "")
+
+-- | Run a servant client action, failing the test on a Left.
+runC :: ClientEnv -> ClientM a -> IO a
+runC env action =
+  runClientM action env >>= \case
+    Left err -> fail $ "Servant client error: " <> show err
+    Right v -> pure v
+
+-- ---------------------------------------------------------------------------
+-- Spec
+-- ---------------------------------------------------------------------------
+
+spec :: Spec
+spec =
+  around withServer $
+    describe "Full contest integration" $ do
+      it "Alice can solve all challenges and reach the maximum grade" $
+        \Server{serverPort} -> do
+          -- Load the same challenges.json the server loaded so we know the
+          -- expected final grade.
+          challenges <-
+            eitherDecodeFileStrict "challenges.json" >>= \case
+              Left err -> fail $ "Could not load challenges.json: " <> err
+              Right cs -> pure (cs :: [Challenge])
+
+          let maxGrade = length challenges
+
+          env <- mkEnv serverPort
+
+          -- Register Alice
+          token <- runC env $ registerClient (User "alice")
+
+          -- Solve every challenge in order
+          mapM_ (solveChallenge env token) challenges
+
+          -- Verify final grade equals number of challenges
+          Score{grade} <- runC env $ scoreClient (Just token)
+          grade `shouldBe` maxGrade
+
+          -- Clean up
+          _ <- runC env $ unregisterClient (Just token)
+          pure ()
+
+-- | Fetch the current challenge for the token, post the known answer, and
+-- assert the server accepted it (200 OK / no exception).
+solveChallenge :: ClientEnv -> Token -> Challenge -> IO ()
+solveChallenge env token Challenge{level, answer} = do
+  -- Confirm the server gives us the challenge we expect at this grade
+  Challenge{level = serverLevel} <- runC env $ challengeClient (Just token)
+  serverLevel `shouldBe` level
+  -- Post the correct answer
+  runC env $ answerClient (Just token) answer
+
+-- ---------------------------------------------------------------------------
+-- Server lifecycle
+-- ---------------------------------------------------------------------------
+
+-- | Spawn the real server (which itself reads challenges.json and picks a
+-- free port) and tear it down after the test.
+withServer :: (Server -> IO ()) -> IO ()
+withServer = bracket (startServer 0) stopServer