diff --git a/ghc-worker/ghc-worker.cabal b/ghc-worker/ghc-worker.cabal
index 8532f2c1..2fbdaf15 100644
--- a/ghc-worker/ghc-worker.cabal
+++ b/ghc-worker/ghc-worker.cabal
@@ -108,7 +108,8 @@ test-suite ghc-worker-test
     ProjectBuildTest,
     Resource.Measure,
     Resource.BasicTest,
-    ScheduleTest
+    ScheduleTest,
+    StaleUnitTest
   ghc-options: -threaded -rtsopts "-with-rtsopts=-N -T"
   build-depends:
     buck-worker-internal,
diff --git a/ghc-worker/test/Main.hs b/ghc-worker/test/Main.hs
index 278dbf7f..1fb9fe9a 100644
--- a/ghc-worker/test/Main.hs
+++ b/ghc-worker/test/Main.hs
@@ -10,6 +10,7 @@ import InterfacePathTest (test_interfacePath)
 import ProjectBuildTest (test_projectBuild)
 import Resource.BasicTest (test_resources)
 import ScheduleTest (test_sortScheduleOrder)
+import StaleUnitTest (test_staleUnit)
 import System.IO (hSetEncoding, stderr, stdout, utf8)
 import Test.Data.Env (testConfigOptions)
 import Test.Tasty (
@@ -42,7 +43,8 @@ testsGeneral =
     test_depFiles,
     test_interfacePath,
     test_sortScheduleOrder,
-    test_projectBuild
+    test_projectBuild,
+    test_staleUnit
   ] <> if fullTest then [
     BuildPlanTest.Test1.test_buildPlan,
     BuildPlanTest.Test2.test_buildPlan
diff --git a/ghc-worker/test/StaleUnitTest.hs b/ghc-worker/test/StaleUnitTest.hs
new file mode 100644
index 00000000..c4594d67
--- /dev/null
+++ b/ghc-worker/test/StaleUnitTest.hs
@@ -0,0 +1,250 @@
+-- | Description: A worker that is not restarted between builds serves the first build's unit state to the second.
+module StaleUnitTest where
+
+import Control.Exception (SomeException, displayException, try)
+import Control.Monad.IO.Class (liftIO)
+import Data.ByteString (ByteString)
+import qualified Data.ByteString.Char8 as ByteString
+import Data.Foldable (for_, toList)
+import Data.IORef (readIORef)
+import Data.List (intercalate, sort)
+import Data.List.NonEmpty (NonEmpty, nonEmpty)
+import qualified Data.List.NonEmpty as NonEmpty
+import Data.Maybe (isJust)
+import Data.Traversable (for)
+import GHC (getSession)
+import GHC.Driver.Env (HscEnv (..))
+import GHC.Driver.Session (DynFlags (..), GhcMode (..), targetProfile)
+import GHC.Iface.Binary (CheckHiWay (IgnoreHiWay), TraceBinIFace (QuietBinIFace), readBinIface)
+import GHC.Types.Avail (availNames)
+import GHC.Types.Name (getOccString)
+import GHC.Unit (stringToUnitId)
+import GHC.Unit.Module.ModIface (mi_exports)
+import Hedgehog (TestT, footnote, (===))
+import Hedgehog.Internal.Property (Failure (..), Journal (..), Log (..), failWith, runTestT)
+import Internal.AbiHash (showAbiHash)
+import Internal.Compile.Make (compileModuleWithDepsInHpt)
+import Internal.DynFlags (modifyGlobalFlags)
+import Internal.Metadata (computeMetadata)
+import Internal.Session (withGhcMakeModule)
+import Prelude hiding (log)
+import System.Directory.Extra (createDirectoryIfMissing)
+import System.IO (hPutStrLn, stderr)
+import System.OsPath.Extra (OsPath, fromOsPath, osp, (<.>), (</>))
+import Test.Build (compileTarget, metadataArgs)
+import Test.Data.Env (SessionEnv (..), TestEnv (..))
+import Test.Data.Project (BuildModule (..), GenUnit (..), ModuleKey (..), UnitKey (..))
+import Test.Data.TestLog (DiagnosticEntry (..), TestLog (..))
+import Test.Env (newSessionEnv, withTestEnv)
+import Test.Log (withTestLog)
+import Test.PackageDb (ModuleSpec (..))
+import Test.Path (compileTmpDir, moduleName, moduleOutputBase, unitName, unitTmpDir)
+import Test.Run (transientSession, unitTest)
+import Test.Target (fileTarget)
+import Test.Tasty (TestTree, testGroup)
+import Types.Args (Args (..))
+import Types.BuckArgs (IsInterpreted (Compiled))
+import Types.Env (Env (..))
+import Types.Target (TargetSpec (..))
+
+-- | One build as Buck sends it to a worker it did not restart: the unit's extra GHC args, every module's source, then
+-- the compiles in order.
+data Build =
+  Build {
+    extraArgs :: [String],
+    sources :: [(BuildModule, ByteString)],
+    compiles :: [ModuleKey]
+  }
+
+-- | What one build step produced. Failures carry GHC's own text so the test output quotes the panic.
+data Step =
+  Step {
+    label :: String,
+    ok :: Bool,
+    output :: [String]
+  }
+
+-- | The produced interface of one module.
+data Iface =
+  Iface {
+    exports :: [String],
+    abi :: String
+  }
+  deriving stock (Eq, Show)
+
+-- | Flip to False to see the stale sequences fail on their own terms.
+expectFailures :: Bool
+expectFailures = True
+
+-- | The sequence is red by design until the worker evicts or fingerprints unit state. Passes while the inner
+-- assertions fail, printing their report to stderr so the stale output stays visible in a green run; fails, naming
+-- itself, once the sequence comes out fresh, which is the signal to delete the wrapper.
+stillStale :: String -> TestT IO () -> TestT IO ()
+stillStale name inner
+  | expectFailures = do
+      (result, Journal logs) <- liftIO (runTestT inner)
+      case result of
+        Left (Failure location message _) ->
+          liftIO $ hPutStrLn stderr $ unlines $
+            ("still stale: " ++ name) : foldMap (\ l -> [show l]) location ++ message : [note | Footnote note <- logs]
+        Right () ->
+          failWith Nothing ("stillStale: " ++ name ++ " came out fresh; the worker no longer serves stale state here, delete the wrapper")
+  | otherwise = inner
+
+-- | Run one worker task with its own log, keeping the diagnostics and fatal errors so a failure can quote them.
+-- The task's args replace the env's, as the server does, so only the task directory is prepared here.
+runStep :: SessionEnv -> String -> OsPath -> (Env -> IO Bool) -> IO Step
+runStep env label taskDir action =
+  withTestLog False label \ (log, logVar) -> do
+    createDirectoryIfMissing True (fromOsPath (env.tempDir </> taskDir))
+    result <- try (action env.env {log})
+    TestLog {diagnostics, fatal} <- readIORef logVar
+    let logged = [d.rendered | d <- diagnostics] ++ fatal
+    pure case result of
+      Right ok -> Step {label, ok, output = logged}
+      Left (e :: SomeException) -> Step {label, ok = False, output = logged ++ [displayException e]}
+
+runBuild :: SessionEnv -> UnitKey -> Build -> IO [Step]
+runBuild env unit Build {extraArgs, sources, compiles} = do
+  for_ sources \ (BuildModule {key}, content) ->
+    fileTarget (fromOsPath env.sourceDir) (stringToUnitId (unitName unit)) ModuleSpec {name = moduleName key, content, boot = False}
+  metadata <- runStep env "metadata" (unitTmpDir unit) \ taskEnv ->
+    fst <$> computeMetadata taskEnv {args = unitArgs {ghcOptions = unitArgs.ghcOptions ++ extraArgs}}
+  compiled <- for compiles \ key ->
+    runStep env ("compile " ++ moduleName key) (compileTmpDir key) \ taskEnv -> do
+      let compileEnv = taskEnv {args = env.shared.baseArgs}
+          target = compileTarget key
+      result <- withGhcMakeModule Compiled target compileEnv \ _targetSpec -> do
+        modifyGlobalFlags \ d -> d {ghcMode = CompManager}
+        compileModuleWithDepsInHpt compileEnv.log (TargetModule target)
+      pure (isJust result)
+  pure (metadata : compiled)
+  where
+    unitArgs = metadataArgs env GenUnit {key = unit, depUnits = [], modules = map fst sources}
+
+readIface :: SessionEnv -> ModuleKey -> TestT IO Iface
+readIface env key =
+  transientSession [] do
+    hsc_env@HscEnv {hsc_dflags, hsc_NC} <- getSession
+    iface <- liftIO (readBinIface (targetProfile hsc_dflags) hsc_NC IgnoreHiWay QuietBinIFace path)
+    pure Iface {
+      exports = sort (concatMap (map getOccString . availNames) (mi_exports iface)),
+      abi = showAbiHash hsc_env iface
+    }
+  where
+    path = fromOsPath (env.tempDir </> moduleOutputBase key <.> [osp|dyn_hi|])
+
+-- | One worker state gets every build in order; a fresh one gets only the last. The last build's module must come out
+-- of both the same, and the fresh one is checked against the expected export list so the reference itself is sound.
+staleSequence :: IO TestEnv -> String -> ModuleKey -> [String] -> NonEmpty Build -> TestT IO ()
+staleSequence testEnv name key expectedExports builds =
+  stillStale name do
+    shared <- liftIO testEnv
+    long <- liftIO (newSessionEnv shared)
+    fresh <- liftIO (newSessionEnv shared)
+    longSteps <- liftIO (concat <$> traverse (runBuild long unit1) builds)
+    freshSteps <- liftIO (runBuild fresh unit1 (NonEmpty.last builds))
+    checkSteps "long-lived worker" longSteps
+    checkSteps "fresh worker" freshSteps
+    longIface <- readIface long key
+    freshIface <- readIface fresh key
+    footnote ("long-lived worker: " ++ show longIface)
+    footnote ("fresh worker: " ++ show freshIface)
+    freshIface.exports === expectedExports
+    longIface.exports === freshIface.exports
+    longIface.abi === freshIface.abi
+  where
+    checkSteps worker steps =
+      for_ (nonEmpty [s | s <- steps, not s.ok]) \ failed ->
+        failWith Nothing $ intercalate "\n" $ concat [(worker ++ ": " ++ s.label ++ " failed") : s.output | s <- toList failed]
+
+unit1 :: UnitKey
+unit1 = UnitKey 1
+
+k, m, k2 :: ModuleKey
+k = ModuleKey {unit = unit1, number = 1, errorVariant = Nothing}
+m = ModuleKey {unit = unit1, number = 2, errorVariant = Nothing}
+k2 = ModuleKey {unit = unit1, number = 3, errorVariant = Nothing}
+
+plain :: ModuleKey -> BuildModule
+plain key = BuildModule {key, deps = [], th = False, bindings = 1, extDeps = []}
+
+source :: [ByteString] -> ByteString
+source = ByteString.unlines
+
+kValue :: Int -> ByteString
+kValue n = source ["module Unit1Module1 where", "value_1_1 :: Int", "value_1_1 = " <> ByteString.pack (show n)]
+
+kValueAndExtra :: ByteString
+kValueAndExtra = kValue 1 <> source ["value_1_1_1 :: Int", "value_1_1_1 = 100"]
+
+kCpp :: ByteString
+kCpp = source [
+  "{-# LANGUAGE CPP #-}",
+  "module Unit1Module1 where",
+  "value_1_1 :: Int",
+  "value_1_1 = 1",
+  "#ifdef FOO",
+  "value_1_1_foo :: Int",
+  "value_1_1_foo = 42",
+  "#endif"
+  ]
+
+-- | The splice puts K's value into a declaration name, because a value-only change does not move the ABI hash.
+mSplice :: ByteString
+mSplice = source [
+  "{-# LANGUAGE TemplateHaskell #-}",
+  "module Unit1Module2 where",
+  "import Language.Haskell.TH (mkName, sigD, valD, varP, normalB, conT)",
+  "import Unit1Module1 (value_1_1)",
+  "$(let n = mkName (\"spliced_\" ++ show value_1_1) in sequence [sigD n (conT ''Int), valD (varP n) (normalB [| value_1_1 |]) []])"
+  ]
+
+mImportsK :: ByteString
+mImportsK = source ["module Unit1Module2 where", "import Unit1Module1", "value_1_2 :: Int", "value_1_2 = value_1_1 + 1"]
+
+k2Source :: ByteString
+k2Source = source ["module Unit1Module3 where", "value_1_3 :: Int", "value_1_3 = 5"]
+
+-- | The importer also gains an export that needs the new module, because a recompile from its old source text would
+-- otherwise produce the same export list and, the change being value-only, the same ABI hash.
+mImportsKAndK2 :: ByteString
+mImportsKAndK2 = source [
+  "module Unit1Module2 where",
+  "import Unit1Module1",
+  "import Unit1Module3",
+  "value_1_2 :: Int",
+  "value_1_2 = value_1_1 + value_1_3",
+  "value_1_2_3 :: Int",
+  "value_1_2_3 = value_1_3"
+  ]
+
+test_staleUnit :: TestTree
+test_staleUnit =
+  withTestEnv \ testEnv ->
+    testGroup "stale unit state across builds" [
+      unitTest "source changes, same args: the second build exports the new binding" $
+        staleSequence testEnv "source change" k ["value_1_1", "value_1_1_1"] [
+          Build {extraArgs = [], sources = [(plain k, kValue 1)], compiles = [k]},
+          Build {extraArgs = [], sources = [(plain k, kValueAndExtra)], compiles = [k]}
+        ],
+      unitTest "unit args change: -DFOO added, the second build exports the CPP-gated binding" $
+        staleSequence testEnv "unit args change" k ["value_1_1", "value_1_1_foo"] [
+          Build {extraArgs = [], sources = [(plain k, kCpp)], compiles = [k]},
+          Build {extraArgs = ["-DFOO"], sources = [(plain k, kCpp)], compiles = [k]}
+        ],
+      unitTest "TH splice reads a changed module: the second build's splice sees the new value" $
+        staleSequence testEnv "TH splice" m ["spliced_100"] [
+          Build {extraArgs = [], sources = [(plain k, kValue 1), ((plain m) {th = True}, mSplice)], compiles = [k, m]},
+          Build {extraArgs = [], sources = [(plain k, kValue 100), ((plain m) {th = True}, mSplice)], compiles = [k, m]}
+        ],
+      unitTest "module added to a known unit: the importer's second build sees the new module's binding" $
+        staleSequence testEnv "module added" m ["value_1_2", "value_1_2_3"] [
+          Build {extraArgs = [], sources = [(plain k, kValue 1), (plain m, mImportsK)], compiles = [k, m]},
+          Build {
+            extraArgs = [],
+            sources = [(plain k, kValue 1), (plain k2, k2Source), (plain m, mImportsKAndK2)],
+            compiles = [k, k2, m]
+          }
+        ]
+    ]