diff --git a/cabal-install-solver/src/Distribution/Solver/Modular.hs b/cabal-install-solver/src/Distribution/Solver/Modular.hs
index b653d1459ed..12ac50a2698 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular.hs
@@ -145,7 +145,7 @@ solve' sc cinfo idx pkgConfigDB pprefs gcs pns =
     runSolver :: Bool -> SolverConfig
               -> RetryLog SummarizedMessage SolverFailure (Assignment, RevDepMap)
     runSolver keepLog sc' =
-        displayLogMessages keepLog $
+        displayLogMessages (reposSearched sc') keepLog $
         solve sc' cinfo idx pkgConfigDB pprefs gcs pns
 
     createErrorMsg :: SolverFailure
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Log.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Log.hs
index 1eb759bd263..327bc0ea0f7 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/Log.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/Log.hs
@@ -23,12 +23,13 @@ data SolverFailure =
 -- | Postprocesses a log file. This function discards all log messages and
 -- avoids calling 'showMessages' if the log isn't needed (specified by
 -- 'keepLog'), for efficiency.
-displayLogMessages :: Bool
+displayLogMessages :: [String]
+                   -> Bool
                    -> RetryLog Message SolverFailure a
                    -> RetryLog SummarizedMessage SolverFailure a
-displayLogMessages keepLog lg = fromProgress $
+displayLogMessages repos keepLog lg = fromProgress $
     if keepLog
-    then summarizeMessages progress
+    then summarizeMessages repos progress
     else foldProgress (const id) Fail Done progress
   where
     progress = toProgress lg
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs
index 16a1b1ff13b..30976fb0f72 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs
@@ -85,7 +85,8 @@ displayMessage (EntryTryingF qfn b) = "trying: " ++ showQFNBool qfn b
 displayMessage (EntryTryingP qpn i) = "trying: " ++ showOption qpn i
 displayMessage (EntryTryingNewP qpn i gr) = "trying: " ++ showOption qpn i ++ showGR gr
 displayMessage (EntryTryingS qsn b) = "trying: " ++ showQSNBool qsn b
-displayMessage (EntryUnknownPackage qpn gr) = "unknown package: " ++ showQPN qpn ++ showGR gr
+displayMessage (EntryUnknownPackage qpn gr repos) =
+  "unknown package: " ++ showQPN qpn ++ showGR gr ++ showReposSearched repos
 displayMessage EntrySuccess = "done"
 displayMessage (EntryFailure c fr) = "fail" ++ showFR c fr
 displayMessage (EntrySkipMany qsn b cs) = "skipping: " ++ showOptions qsn b ++ " " ++ showConflicts cs
@@ -99,8 +100,8 @@ displayMessage (EntryRejectMany qpn is c fr) = "rejecting: " ++ showOptions qpn
 -- The log contains level numbers, which are useful for any trace that involves
 -- backtracking, because only the level numbers will allow to keep track of
 -- backjumps.
-summarizeMessages :: Progress Message a b -> Progress SummarizedMessage a b
-summarizeMessages = go 0
+summarizeMessages :: [String] -> Progress Message a b -> Progress SummarizedMessage a b
+summarizeMessages repos = go 0
   where
     -- 'go' increments the level for a recursive call when it encounters
     -- 'TryP', 'TryF', or 'TryS' and decrements the level when it encounters 'Leave'.
@@ -126,7 +127,7 @@ summarizeMessages = go 0
         Step (SummarizedMsg $ AtLevel l (EntryTryingNewP qpn' i gr)) (go l ms)
 
     go !l (Step (Next (Goal (P qpn) gr)) (Step (Failure _c UnknownPackage) ms)) =
-        Step (SummarizedMsg $ AtLevel l (EntryUnknownPackage qpn gr)) (go l ms)
+        Step (SummarizedMsg $ AtLevel l (EntryUnknownPackage qpn gr repos)) (go l ms)
 
     -- standard display
     go !l (Step Enter                    ms) = go (l+1) ms
@@ -295,6 +296,13 @@ showGR :: QGoalReason -> String
 showGR UserGoal            = " (user goal)"
 showGR (DependencyGoal dr) = " (dependency of " ++ showDependencyReason dr ++ ")"
 
+-- | List the repositories that were searched for a package that turned out to
+-- be unknown. This helps the user notice when @active-repositories@ is set to
+-- a nonstandard value that excludes a package from the search.
+showReposSearched :: [String] -> String
+showReposSearched [] = ""
+showReposSearched repos = "; searched repositories: " ++ L.intercalate ", " repos
+
 showFR :: ConflictSet -> FailReason -> String
 showFR _ (UnsupportedExtension ext)       = " (conflict: requires " ++ showUnsupportedExtension ext ++ ")"
 showFR _ (UnsupportedLanguage lang)       = " (conflict: requires " ++ showUnsupportedLanguage lang ++ ")"
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Solver.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Solver.hs
index f2197379076..bdfde81d5bb 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/Solver.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/Solver.hs
@@ -74,7 +74,10 @@ data SolverConfig = SolverConfig {
   solveExecutables       :: SolveExecutables,
   goalOrder              :: Maybe (Variable QPN -> Variable QPN -> Ordering),
   solverVerbosity        :: VerbosityLevel,
-  pruneAfterFirstSuccess :: PruneAfterFirstSuccess
+  pruneAfterFirstSuccess :: PruneAfterFirstSuccess,
+  -- | The names of the repositories that were searched for packages. Used to
+  -- enrich the @unknown package@ failure message.
+  reposSearched          :: [String]
 }
 
 -- | Whether to remove all choices after the first successful choice at each
diff --git a/cabal-install-solver/src/Distribution/Solver/Types/SummarizedMessage.hs b/cabal-install-solver/src/Distribution/Solver/Types/SummarizedMessage.hs
index 747db0a88ae..39d61fe6512 100644
--- a/cabal-install-solver/src/Distribution/Solver/Types/SummarizedMessage.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Types/SummarizedMessage.hs
@@ -37,7 +37,7 @@ data Entry
   | EntryTryingS QSN Bool
   | EntryRejectMany QPN [POption] ConflictSet FailReason
   | EntrySkipMany QPN [POption] (Set CS.Conflict)
-  | EntryUnknownPackage QPN (GoalReason QPN)
+  | EntryUnknownPackage QPN (GoalReason QPN) [String]
   | EntrySuccess
   | EntryFailure ConflictSet FailReason
 
diff --git a/cabal-install/src/Distribution/Client/Dependency.hs b/cabal-install/src/Distribution/Client/Dependency.hs
index fc447247d69..1503db176a7 100644
--- a/cabal-install/src/Distribution/Client/Dependency.hs
+++ b/cabal-install/src/Distribution/Client/Dependency.hs
@@ -57,6 +57,7 @@ module Distribution.Client.Dependency
   , setSolveExecutables
   , setGoalOrder
   , setSolverVerbosity
+  , setActiveRepositories
   , removeLowerBounds
   , removeUpperBounds
   , addDefaultSetupDependencies
@@ -82,6 +83,7 @@ import Distribution.Client.Types
   , RelaxDepSubject (..)
   , RelaxDeps (..)
   , RelaxedDep (..)
+  , RepoName (..)
   , SourcePackageDb (SourcePackageDb)
   , UnresolvedPkgLoc
   , UnresolvedSourcePackage
@@ -212,6 +214,9 @@ data DepResolverParams = DepResolverParams
   , depResolverGoalOrder :: Maybe (Variable QPN -> Variable QPN -> Ordering)
   -- ^ Function to override the solver's goal-ordering heuristics.
   , depResolverVerbosity :: VerbosityLevel
+  , depResolverActiveRepos :: [RepoName]
+  -- ^ The names of the repositories that were searched for packages. Used to
+  -- enrich the @unknown package@ solver error message.
   }
 
 showDepResolverParams :: DepResolverParams -> String
@@ -309,6 +314,7 @@ basicDepResolverParams installedPkgIndex sourcePkgIndex =
     , depResolverSolveExecutables = SolveExecutables True
     , depResolverGoalOrder = Nothing
     , depResolverVerbosity = Normal
+    , depResolverActiveRepos = []
     }
 
 addTargets
@@ -444,6 +450,12 @@ setSolverVerbosity verbosity params =
     { depResolverVerbosity = verbosity
     }
 
+setActiveRepositories :: [RepoName] -> DepResolverParams -> DepResolverParams
+setActiveRepositories repos params =
+  params
+    { depResolverActiveRepos = repos
+    }
+
 dependOnWiredIns :: CompilerInfo -> DepResolverParams -> DepResolverParams
 dependOnWiredIns compiler params = addConstraints extraConstraints params
   where
@@ -882,6 +894,7 @@ resolveDependencies platform comp pkgConfigDB params = do
             order
             verbosity
             (PruneAfterFirstSuccess False)
+            (map unRepoName activeRepos)
         )
         platform
         comp
@@ -915,6 +928,7 @@ resolveDependencies platform comp pkgConfigDB params = do
                     solveExes
                     order
                     verbosity
+                    activeRepos
                   ) =
         if isJust (compilerInfoWiredInUnitIds comp) || asBool (depResolverAllowBootLibInstalls params)
           then dependOnWiredIns comp params
@@ -1236,6 +1250,7 @@ resolveWithoutDependencies
       _onlyConstrained
       _order
       _verbosity
+      _activeRepos
     ) =
     collectEithers $ map selectPackage (Set.toList targets)
     where
diff --git a/cabal-install/src/Distribution/Client/IndexUtils.hs b/cabal-install/src/Distribution/Client/IndexUtils.hs
index 70f1119c1ea..2aac83ab522 100644
--- a/cabal-install/src/Distribution/Client/IndexUtils.hs
+++ b/cabal-install/src/Distribution/Client/IndexUtils.hs
@@ -24,6 +24,7 @@ module Distribution.Client.IndexUtils
   , getSourcePackagesAtIndexState
   , ActiveRepos
   , filterSkippedActiveRepos
+  , activeReposNames
   , applyStrategy
   , addIndex
   , deprecationAwareStrategy
diff --git a/cabal-install/src/Distribution/Client/IndexUtils/ActiveRepos.hs b/cabal-install/src/Distribution/Client/IndexUtils/ActiveRepos.hs
index 3698e0638ad..245193260b7 100644
--- a/cabal-install/src/Distribution/Client/IndexUtils/ActiveRepos.hs
+++ b/cabal-install/src/Distribution/Client/IndexUtils/ActiveRepos.hs
@@ -4,6 +4,7 @@ module Distribution.Client.IndexUtils.ActiveRepos
   ( ActiveRepos (..)
   , defaultActiveRepos
   , filterSkippedActiveRepos
+  , activeReposNames
   , ActiveRepoEntry (..)
   , CombineStrategy (..)
   , organizeByRepos
@@ -45,6 +46,9 @@ filterSkippedActiveRepos repos@(ActiveRepos entries)
     notSkipped (ActiveRepo _ CombineStrategySkip) = False
     notSkipped _ = True
 
+activeReposNames :: ActiveRepos -> [RepoName]
+activeReposNames (ActiveRepos entries) = [r | ActiveRepo r _ <- entries]
+
 instance Binary ActiveRepos
 instance Structured ActiveRepos
 instance NFData ActiveRepos
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning.hs b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
index 336411c9158..6cca4e97679 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
@@ -854,6 +854,7 @@ rebuildInstallPlan
                       pkgConfigDB
                       localPackages
                       localPackagesEnabledStanzas
+                      (IndexUtils.activeReposNames ar)
                 case planOrError of
                   Left msg -> do
                     reportPlanningFailure projectConfig compiler platform localPackages
@@ -1335,6 +1336,7 @@ planPackages
   -> Maybe PkgConfigDb
   -> [PackageSpecifier UnresolvedSourcePackage]
   -> Map PackageName (Map OptionalStanza Bool)
+  -> [RepoName]
   -> Progress String String SolverInstallPlan
 planPackages
   verbosity
@@ -1345,7 +1347,8 @@ planPackages
   sourcePkgDb
   pkgConfigDB
   localPackages
-  pkgStanzasEnable =
+  pkgStanzasEnable
+  activeRepos =
     resolveDependencies
       platform
       (compilerInfo comp)
@@ -1375,6 +1378,7 @@ planPackages
           . setAllowBootLibInstalls solverSettingAllowBootLibInstalls
           . setOnlyConstrained solverSettingOnlyConstrained
           . setSolverVerbosity (verbosityLevel verbosity)
+          . setActiveRepositories activeRepos
           . setPreferenceDefault
             ( case solverSettingPreferVersion of
                 PreferOldest -> PreferAllOldest
diff --git a/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/cabal.project b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/cabal.project
new file mode 100644
index 00000000000..295806d7e8a
--- /dev/null
+++ b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/cabal.project
@@ -0,0 +1,2 @@
+packages: fake-pkg
+active-repositories: test-local-repo
diff --git a/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/cabal.test.hs b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/cabal.test.hs
new file mode 100644
index 00000000000..0849c9413bf
--- /dev/null
+++ b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/cabal.test.hs
@@ -0,0 +1,7 @@
+import Test.Cabal.Prelude
+
+main = cabalTest . recordMode DoNotRecord $ do
+  withRepo "repo" $ do
+    res <- fails $ cabal' "v2-build" ["fake-pkg"]
+    assertOutputContains "unknown package: p" res
+    assertOutputContains "searched repositories: test-local-repo" res
diff --git a/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/fake-pkg/Lib.hs b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/fake-pkg/Lib.hs
new file mode 100644
index 00000000000..6d85a26fe10
--- /dev/null
+++ b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/fake-pkg/Lib.hs
@@ -0,0 +1 @@
+module Lib where
diff --git a/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/fake-pkg/fake-pkg.cabal b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/fake-pkg/fake-pkg.cabal
new file mode 100644
index 00000000000..dc74772ca32
--- /dev/null
+++ b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/fake-pkg/fake-pkg.cabal
@@ -0,0 +1,8 @@
+cabal-version: 1.12
+name: fake-pkg
+version: 1.0
+build-type: Simple
+
+library
+  build-depends: base, p
+  default-language: Haskell2010
diff --git a/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/repo/q-1.0/Foo.hs b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/repo/q-1.0/Foo.hs
new file mode 100644
index 00000000000..efbf93bbde8
--- /dev/null
+++ b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/repo/q-1.0/Foo.hs
@@ -0,0 +1 @@
+module Foo where
diff --git a/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/repo/q-1.0/q.cabal b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/repo/q-1.0/q.cabal
new file mode 100644
index 00000000000..6c6da3c6962
--- /dev/null
+++ b/cabal-testsuite/PackageTests/NewBuild/UnknownPackageRepos/repo/q-1.0/q.cabal
@@ -0,0 +1,9 @@
+cabal-version: 1.12
+name: q
+version: 1.0
+build-type: Simple
+
+library
+  exposed-modules: Foo
+  build-depends: base
+  default-language: Haskell2010
diff --git a/changelog.d/12278.md b/changelog.d/12278.md
new file mode 100644
index 00000000000..a32406870e9
--- /dev/null
+++ b/changelog.d/12278.md
@@ -0,0 +1,20 @@
+---
+synopsis: List searched repositories in "unknown package" solver errors
+packages: [cabal-install]
+prs: 12278
+issues: 7661
+---
+
+When dependency solving fails because a package cannot be found, the solver
+now reports which repositories were searched alongside the usual "unknown
+package" message:
+
+```diff
+- [__1] unknown package: badpackage (dependency of fake-package)
++ [__1] unknown package: badpackage (dependency of fake-package); searched repositories: hackage.haskell.org
+```
+
+This makes it easier to notice when `active-repositories` is set to a
+nonstandard value that excludes a package from the search even though it is
+available on Hackage. When the default repositories are in use, the message
+simply lists them, so there is no extra noise beyond a short suffix.