diff --git a/Cabal/src/Distribution/Simple/Compiler.hs b/Cabal/src/Distribution/Simple/Compiler.hs
index 38478a71b4c..805de301644 100644
--- a/Cabal/src/Distribution/Simple/Compiler.hs
+++ b/Cabal/src/Distribution/Simple/Compiler.hs
@@ -86,6 +86,7 @@ module Distribution.Simple.Compiler
   , libraryDynDirSupported
   , libraryVisibilitySupported
   , jsemSupported
+  , jsemVersion
   , reexportedAsSupported
 
     -- * Support for profiling detail levels
@@ -480,6 +481,17 @@ jsemSupported comp = case compilerFlavor comp of
   where
     v = compilerVersion comp
 
+-- | What semaphore protocol version does this compiler use?
+--
+-- Returns @Nothing@ for compilers that don't report a "Semaphore version"
+-- field in @ghc --info@ (i.e. GHC 9.8–9.14, which use v1).
+jsemVersion :: Compiler -> Maybe Int
+jsemVersion comp = case compilerFlavor comp of
+  GHC -> case Map.lookup "Semaphore version" (compilerProperties comp) of
+    Just verStr | [(v, "")] <- reads verStr -> Just v
+    _ -> Nothing
+  _ -> Nothing
+
 -- | Does the compiler support the -reexported-modules "A as B" syntax
 reexportedAsSupported :: Compiler -> Bool
 reexportedAsSupported comp = case compilerFlavor comp of
diff --git a/cabal-install/cabal-install.cabal b/cabal-install/cabal-install.cabal
index 933e361be30..33d512538de 100644
--- a/cabal-install/cabal-install.cabal
+++ b/cabal-install/cabal-install.cabal
@@ -270,7 +270,7 @@ library
       , regex-base  >= 0.94.0.0 && <0.95
       , regex-posix >= 0.96.0.0 && <0.97
       , safe-exceptions >= 0.1.7.0 && < 0.2
-      , semaphore-compat >= 1.0.0 && < 1.1
+      , semaphore-compat >= 2.0.0 && < 2.1
 
     if flag(native-dns)
       if os(windows)
diff --git a/cabal-install/src/Distribution/Client/JobControl.hs b/cabal-install/src/Distribution/Client/JobControl.hs
index 280916fdf6c..9df6bd96841 100644
--- a/cabal-install/src/Distribution/Client/JobControl.hs
+++ b/cabal-install/src/Distribution/Client/JobControl.hs
@@ -183,26 +183,34 @@ newSemaphoreJobControl _ n
   | n < 1 || n > 1000 =
       error $ "newParallelJobControl: not a sensible number of jobs: " ++ show n
 newSemaphoreJobControl verbosity maxJobLimit = do
-  sem <- freshSemaphore "cabal_semaphore" maxJobLimit
-  info verbosity $
-    "Created semaphore called "
-      ++ getSemaphoreName (semaphoreName sem)
-      ++ " with "
-      ++ show maxJobLimit
-      ++ " slots."
-  outqVar <- newTChanIO
-  inqVar <- newTChanIO
-  countVar <- newTVarIO 0
-  void (forkIO (worker sem inqVar outqVar))
-  return
-    JobControl
-      { spawnJob = spawn inqVar countVar
-      , collectJob = collect outqVar countVar
-      , remainingJobs = remaining countVar
-      , cancelJobs = cancel inqVar countVar
-      , cleanupJobControl = destroySemaphore sem
-      , jobControlSemaphore = Just (semaphoreName sem)
-      }
+  mbServer <- freshSemaphore "cabal_semaphore" maxJobLimit
+  case mbServer of
+    Left err -> do
+      warn verbosity $
+        "Failed to create semaphore: " ++ show err
+          ++ "; falling back to normal parallelism control."
+      newParallelJobControl maxJobLimit
+    Right server -> do
+      let sem = serverSemaphore server
+      info verbosity $
+        "Created semaphore called "
+          ++ getSemaphoreName (semaphoreName sem)
+          ++ " with "
+          ++ show maxJobLimit
+          ++ " slots."
+      outqVar <- newTChanIO
+      inqVar <- newTChanIO
+      countVar <- newTVarIO 0
+      void (forkIO (worker sem inqVar outqVar))
+      return
+        JobControl
+          { spawnJob = spawn inqVar countVar
+          , collectJob = collect outqVar countVar
+          , remainingJobs = remaining countVar
+          , cancelJobs = cancel inqVar countVar
+          , cleanupJobControl = destroySemaphoreServer server
+          , jobControlSemaphore = Just (semaphoreName sem)
+          }
   where
     worker :: Semaphore -> TChan (IO a) -> TChan (Either SomeException a) -> IO ()
     worker sem inqVar outqVar =
@@ -291,8 +299,18 @@ newJobControlFromParStrat verbosity mcompiler parStrat numJobsCap = case parStra
   UseSem n ->
     case mcompiler of
       Just compiler
-        | jsemSupported compiler ->
+        | jsemSupported compiler
+        , isJsemCompatible compiler ->
             newSemaphoreJobControl verbosity (capJobs n)
+        | jsemSupported compiler ->
+            do
+              warn verbosity $
+                "Semaphore version mismatch (cabal-install uses v"
+                ++ show semaphoreVersion
+                ++ ", but the selected GHC reports "
+                ++ maybe "no version (assumed v1)" (\v -> "v" ++ show v) (jsemVersion compiler)
+                ++ "); not using -jsem, GHC will be invoked without semaphore-based parallelism."
+              newParallelJobControl (capJobs n)
         | otherwise ->
             do
               warn verbosity "-jsem is not supported by the selected compiler, falling back to normal parallelism control."
@@ -303,6 +321,17 @@ newJobControlFromParStrat verbosity mcompiler parStrat numJobsCap = case parStra
   where
     capJobs n = min (fromMaybe maxBound numJobsCap) n
 
+-- | Check if the compiler's semaphore version is compatible with ours.
+--
+-- If the compiler doesn't report a "Semaphore version" field (GHC 9.8–9.14),
+-- we assume v1. On POSIX, v1 and v2 are incompatible (different mechanisms).
+-- On Windows, all versions are compatible (same Win32 API).
+isJsemCompatible :: Compiler -> Bool
+isJsemCompatible compiler =
+  case jsemVersion compiler of
+    Just v  -> versionsAreCompatible v semaphoreVersion
+    Nothing -> versionsAreCompatible 1 semaphoreVersion
+
 withJobControl :: IO (JobControl IO a) -> (JobControl IO a -> IO b) -> IO b
 withJobControl mkJC = bracket mkJC cleanupJobControl
 
diff --git a/cabal.project b/cabal.project
index a2075cfdc29..de451969cc0 100644
--- a/cabal.project
+++ b/cabal.project
@@ -2,6 +2,7 @@ import: project-cabal/ghc-options.config
 import: project-cabal/ghc-latest.config
 import: project-cabal/pkgs.config
 import: project-cabal/constraints.config
+import: project-cabal/source-deps.config
 
 tests: True
 
diff --git a/cabal.validate-libonly.project b/cabal.validate-libonly.project
index 7c5bd38ab6b..c750b8c31e7 100644
--- a/cabal.validate-libonly.project
+++ b/cabal.validate-libonly.project
@@ -3,6 +3,7 @@ import: project-cabal/pkgs/cabal.config
 import: project-cabal/pkgs/tests.config
 import: project-cabal/pkgs/integration-tests.config
 import: project-cabal/constraints.config
+import: project-cabal/source-deps.config
 
 tests: True
 write-ghc-environment-files: never
diff --git a/cabal.validate.project b/cabal.validate.project
index bf2552af2d0..6081f292725 100644
--- a/cabal.validate.project
+++ b/cabal.validate.project
@@ -2,6 +2,7 @@ import: project-cabal/ghc-options.config
 import: project-cabal/ghc-latest.config
 import: project-cabal/pkgs.config
 import: project-cabal/constraints.config
+import: project-cabal/source-deps.config
 
 tests: True
 write-ghc-environment-files: never
diff --git a/changelog.d/semaphore-version-compat.md b/changelog.d/semaphore-version-compat.md
new file mode 100644
index 00000000000..fcd2a1a1d97
--- /dev/null
+++ b/changelog.d/semaphore-version-compat.md
@@ -0,0 +1,21 @@
+---
+synopsis: Detect semaphore version mismatch between cabal-install and GHC
+packages: [Cabal, cabal-install]
+prs: 0000
+issues: 0000
+significance: significant
+---
+
+When using `--semaphore`, cabal-install now checks whether the selected GHC's
+semaphore protocol version is compatible before passing `-jsem`.  If the GHC
+reports no `Semaphore version` field (GHC 9.8–9.14, which use v1) and
+cabal-install uses v2, a warning is emitted and cabal-install falls back to
+normal parallelism control instead of passing an incompatible semaphore name.
+
+On Windows, v1 and v2 are always compatible (same Win32 API), so semaphore
+coordination is preserved across all version combinations.
+
+- `Cabal`: add `jsemVersion :: Compiler -> Maybe Int` to read the
+  `Semaphore version` field from `ghc --info`.
+- `cabal-install`: add `isJsemCompatible` check in `newJobControlFromParStrat`;
+  emit a warning and fall back to `-jN` when versions are incompatible.
diff --git a/project-cabal/ghc-options.config b/project-cabal/ghc-options.config
index 4be19aa246d..99794c17465 100644
--- a/project-cabal/ghc-options.config
+++ b/project-cabal/ghc-options.config
@@ -9,4 +9,3 @@ if impl(ghc >= 9)
     ghc-options: -Wunused-packages
   package cabal-testsuite
     ghc-options: -Wwarn=unused-packages
-
diff --git a/project-cabal/source-deps.config b/project-cabal/source-deps.config
new file mode 100644
index 00000000000..0c7b5969362
--- /dev/null
+++ b/project-cabal/source-deps.config
@@ -0,0 +1,4 @@
+source-repository-package
+    type: git
+    location: https://gitlab.haskell.org/ghc/semaphore-compat.git
+    tag: efb04e774623672b28adb7d8038fb1c40cd5d202