diff --git a/Stackage/BuildConstraints.hs b/Stackage/BuildConstraints.hs
index 3b3f0d01..674ac395 100644
--- a/Stackage/BuildConstraints.hs
+++ b/Stackage/BuildConstraints.hs
@@ -10,6 +10,7 @@ module Stackage.BuildConstraints
     , SystemInfo (..)
     , getSystemInfo
     , defaultBuildConstraints
+    , toBC
     ) where
 
 import           Control.Monad.Writer.Strict (execWriter, tell)
diff --git a/Stackage/CheckBuildPlan.hs b/Stackage/CheckBuildPlan.hs
index f13265aa..ea656a9c 100644
--- a/Stackage/CheckBuildPlan.hs
+++ b/Stackage/CheckBuildPlan.hs
@@ -7,6 +7,7 @@
 -- | Confirm that a build plan has a consistent set of dependencies.
 module Stackage.CheckBuildPlan
     ( checkBuildPlan
+    , BadBuildPlan
     ) where
 
 import           Control.Monad.Writer.Strict (Writer, execWriter, tell)
diff --git a/test/Stackage/BuildPlanSpec.hs b/test/Stackage/BuildPlanSpec.hs
index 7c87d6d3..99679801 100644
--- a/test/Stackage/BuildPlanSpec.hs
+++ b/test/Stackage/BuildPlanSpec.hs
@@ -1,22 +1,56 @@
+{-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE OverloadedStrings, NoImplicitPrelude #-}
 module Stackage.BuildPlanSpec (spec) where
 
-import Stackage.BuildPlan
-import Stackage.Prelude
-import Stackage.BuildConstraints
-import Stackage.UpdateBuildPlan
-import Test.Hspec
-import qualified Data.Yaml as Y
-import Distribution.Version (anyVersion)
 import qualified Data.Map as Map
-import Network.HTTP.Client (withManager)
-import Network.HTTP.Client.TLS (tlsManagerSettings)
+import qualified Data.Map.Strict as M
+import qualified Data.Set as S
+import           Data.Yaml
+import qualified Data.Yaml as Y
+import           Distribution.Version
+import           Network.HTTP.Client
+import           Network.HTTP.Client.TLS (tlsManagerSettings)
+import           Stackage.BuildConstraints
+import           Stackage.BuildPlan
+import           Stackage.CheckBuildPlan
+import           Stackage.PackageDescription
+import           Stackage.Prelude
+import           Stackage.UpdateBuildPlan
+import           Test.Hspec
 
 spec :: Spec
-spec = it "works" $ withManager tlsManagerSettings $ \man -> do
-    bc <- defaultBuildConstraints man
-    pkgs <- getLatestAllowedPlans bc
-    bp <- newBuildPlan pkgs bc
+spec = do
+    it "simple package set" $ check testBuildConstraints $ makePackageSet
+        [("foo", [0, 0, 0], [("bar", thisV [0, 0, 0])])
+        ,("bar", [0, 0, 0], [])]
+    it "bad version range on depdendency fails" $ badBuildPlan $ makePackageSet
+        [("foo", [0, 0, 0], [("bar", thisV [1, 1, 0])])
+        ,("bar", [0, 0, 0], [])]
+    it "nonexistent package fails to check" $ badBuildPlan $ makePackageSet
+        [("foo", [0, 0, 0], [("nonexistent", thisV [0, 0, 0])])
+        ,("bar", [0, 0, 0], [])]
+    it "default package set checks ok" $ check defaultBuildConstraints getLatestAllowedPlans
+
+-- | Checking should be considered a bad build plan.
+badBuildPlan :: (BuildConstraints -> IO (Map PackageName PackagePlan))
+             -> void
+             -> IO ()
+badBuildPlan m _ = do
+    mu <- try (check testBuildConstraints m)
+    case mu of
+        Left (_ :: BadBuildPlan) ->
+            return ()
+        Right () ->
+            error "Expected bad build plan."
+
+-- | Check build plan with the given package set getter.
+check :: (Manager -> IO BuildConstraints)
+      -> (BuildConstraints -> IO (Map PackageName PackagePlan))
+      -> IO ()
+check readPlanFile getPlans = withManager tlsManagerSettings $ \man -> do
+    bc <- readPlanFile man
+    plans <- getPlans bc
+    bp <- newBuildPlan plans bc
     let bs = Y.encode bp
         ebp' = Y.decodeEither bs
 
@@ -26,14 +60,74 @@ spec = it "works" $ withManager tlsManagerSettings $ \man -> do
     forM_ allPackages $ \name ->
         (name, lookup name (bpPackages bp')) `shouldBe`
         (name, lookup name (bpPackages bp))
-
     bpGithubUsers bp' `shouldBe` bpGithubUsers bp
+
     when (bp' /= bp) $ error "bp' /= bp"
-    bp2 <- updateBuildPlan pkgs bp
+    bp2 <- updateBuildPlan plans bp
     when (dropVersionRanges bp2 /= dropVersionRanges bp) $ error "bp2 /= bp"
+    checkBuildPlan bp
   where
     dropVersionRanges bp =
         bp { bpPackages = map go $ bpPackages bp }
       where
         go pb = pb { ppConstraints = go' $ ppConstraints pb }
         go' pc = pc { pcVersionRange = anyVersion }
+
+-- | Make a package set from a convenient data structure.
+makePackageSet
+    :: [(String,[Int],[(String,VersionRange)])]
+    -> BuildConstraints
+    -> IO (Map PackageName PackagePlan)
+makePackageSet ps _ =
+    return $
+    M.fromList $
+    map
+        (\(name,ver,deps) ->
+              ( PackageName name
+              , dummyPackage ver $
+                M.fromList $
+                map
+                    (\(dname,dver) ->
+                          ( PackageName dname
+                          , DepInfo {diComponents = S.fromList
+                                             [CompLibrary]
+                                    ,diRange = dver}))
+                    deps))
+        ps
+    where
+        dummyPackage v deps =
+            PackagePlan
+                {ppVersion = Version v []
+                ,ppGithubPings = mempty
+                ,ppUsers = mempty
+                ,ppConstraints =
+                    PackageConstraints
+                        {pcVersionRange = anyV
+                        ,pcMaintainer = Nothing
+                        ,pcTests = Don'tBuild
+                        ,pcHaddocks = Don'tBuild
+                        ,pcBuildBenchmarks = False
+                        ,pcFlagOverrides = mempty
+                        ,pcEnableLibProfile = False}
+                ,ppDesc =
+                    SimpleDesc
+                        {sdPackages = deps
+                        ,sdTools = mempty
+                        ,sdProvidedExes = mempty
+                        ,sdModules = mempty}}
+
+-- | This exact version is required.
+thisV :: [Int] -> VersionRange
+thisV ver = thisVersion (Version ver [])
+
+-- | Accept any version.
+anyV :: VersionRange
+anyV = anyVersion
+
+-- | Test plan.
+testBuildConstraints :: void -> IO BuildConstraints
+testBuildConstraints _ =
+    decodeFileEither
+        (fpToString fp) >>=
+    either throwIO toBC
+    where fp = "test/test-build-constraints.yaml"
diff --git a/test/test-build-constraints.yaml b/test/test-build-constraints.yaml
new file mode 100644
index 00000000..de831eaf
--- /dev/null
+++ b/test/test-build-constraints.yaml
@@ -0,0 +1,20 @@
+packages:
+    "Test":
+       - foo
+       - bar
+
+global-flags: []
+
+skipped-tests: []
+expected-test-failures: []
+expected-haddock-failures: []
+skipped-benchmarks: []
+skipped-profiling: []
+
+github-users:
+  bar:
+    - demo
+
+package-flags:
+  foo:
+    demo: true