diff --git a/message-index/messages/GHC-15708/example1/after/FunDepsDisabled.hs b/message-index/messages/GHC-15708/example1/after/FunDepsDisabled.hs
new file mode 100644
index 00000000..9e06e512
--- /dev/null
+++ b/message-index/messages/GHC-15708/example1/after/FunDepsDisabled.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE FunctionalDependencies #-}
+
+module FunDepsDisabled where
+
+class Convert a b | a -> b where
+  convert :: a -> b
diff --git a/message-index/messages/GHC-15708/example1/before/FunDepsDisabled.hs b/message-index/messages/GHC-15708/example1/before/FunDepsDisabled.hs
new file mode 100644
index 00000000..12de7d57
--- /dev/null
+++ b/message-index/messages/GHC-15708/example1/before/FunDepsDisabled.hs
@@ -0,0 +1,4 @@
+module FunDepsDisabled where
+
+class Convert a b | a -> b where
+  convert :: a -> b
diff --git a/message-index/messages/GHC-15708/example1/index.md b/message-index/messages/GHC-15708/example1/index.md
new file mode 100644
index 00000000..c49e252b
--- /dev/null
+++ b/message-index/messages/GHC-15708/example1/index.md
@@ -0,0 +1,17 @@
+---
+title: Missing FunctionalDependencies language extension
+---
+
+Cannot specify functional dependencies for a class - `FunctionalDependencies` language extension is missing.
+
+## Error Message
+
+```
+FunDepsDisabled.hs:1:1: error: [GHC-15708]
+    • Fundeps in class ‘Convert’
+    • In the class declaration for ‘Convert’
+    Suggested fix: Perhaps you intended to use FunctionalDependencies
+  |
+1 | class Convert a b | a -> b where
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
+```
diff --git a/message-index/messages/GHC-15708/index.md b/message-index/messages/GHC-15708/index.md
new file mode 100644
index 00000000..ac63c3d4
--- /dev/null
+++ b/message-index/messages/GHC-15708/index.md
@@ -0,0 +1,10 @@
+---
+title: Missing FunctionalDependencies
+summary: Missing FunctionalDependencies language extension
+severity: error
+introduced: 9.6.1
+---
+
+The [Functional Dependencies extension](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/functional_dependencies.html) enables dependencies in multiparameter class declarations.
+
+When the extension is missing, the syntax for specifying dependencies is not recognized.