diff --git a/lib/Eclair/Error.hs b/lib/Eclair/Error.hs
index 742f1bd..f1be9b2 100644
--- a/lib/Eclair/Error.hs
+++ b/lib/Eclair/Error.hs
@@ -92,7 +92,6 @@ posToSourcePos (Pos l c) =
 
 -- Actual location in the code (a range).
 -- Contains the file, start and end of the position.
--- TODO make this the leading location type instead of SourceSpan
 data Location
   = Location
   { locationFile :: FilePath
diff --git a/lib/Eclair/LSP/Handlers/Diagnostics.hs b/lib/Eclair/LSP/Handlers/Diagnostics.hs
index 024904c..a06250a 100644
--- a/lib/Eclair/LSP/Handlers/Diagnostics.hs
+++ b/lib/Eclair/LSP/Handlers/Diagnostics.hs
@@ -37,7 +37,7 @@ diagnosticsHandler path = do
   mFileContents <- lift $ vfsLookupFile path
   case mFileContents of
     Nothing ->
-      pure $ DiagnosticsError path Nothing "Failed to read file from VFS!"
+      pure $ DiagnosticsError path Nothing "File not found in VFS!"
     Just fileContents -> do
       errs <- liftLSP $ emitDiagnostics params path
       diagnostics <- mconcat <$> traverse (errorToDiagnostics fileContents) errs
diff --git a/tests/eclair/Test/Eclair/LSP/HandlersSpec.hs b/tests/eclair/Test/Eclair/LSP/HandlersSpec.hs
index ea9e60c..03b3006 100644
--- a/tests/eclair/Test/Eclair/LSP/HandlersSpec.hs
+++ b/tests/eclair/Test/Eclair/LSP/HandlersSpec.hs
@@ -14,9 +14,7 @@ spec :: Spec
 spec = describe "LSP handlers" $ parallel $ do
   hoverSpec
   documentHighlightSpec
-
-  describe "diagnostics" $ parallel $ do
-    it "" pending
+  diagnosticsSpec
 
 hoverSpec :: Spec
 hoverSpec = describe "Hover action" $ do
@@ -98,6 +96,31 @@ documentHighlightSpec = describe "Document highlight action" $ do
     result <- withLSP (Just file) $ documentHighlightHandler file srcPos
     result `shouldBe` DocHLError file srcPos "Failed to get highlight information!"
 
+diagnosticsSpec :: Spec
+diagnosticsSpec = describe "Diagnostics action" $ parallel $ do
+  it "reports nothing if file is OK" pending
+
+  it "reports invalid syntax" $ do
+    let file = fixture "invalid_syntax.eclair"
+    DiagnosticsOk diags <- withLSP (Just file) $ diagnosticsHandler file
+    length diags `shouldBe` 2
+
+  it "returns an error if file not found in vfs" $ do
+    let file = "not_found.eclair"
+    result <- withLSP Nothing $ diagnosticsHandler file
+    result `shouldBe` DiagnosticsError file Nothing "File not found in VFS!"
+
+  it "reports semantic errors" $ do
+    let file = fixture "semantic_errors.eclair"
+    DiagnosticsOk [diag] <- withLSP (Just file) $ diagnosticsHandler file
+    let (Diagnostic _ _ _ msg) = diag
+    toString msg `shouldContain` "Wildcard in top level fact"
+
+  it "reports type errors" $ do
+    let file = fixture "type_errors.eclair"
+    DiagnosticsOk (_:_:diag:_) <- withLSP (Just file) $ diagnosticsHandler file
+    let (Diagnostic _ _ _ msg) = diag
+    toString msg `shouldContain` "Type mismatch"
 
 fixture :: FilePath -> FilePath
 fixture file =
diff --git a/tests/eclair/fixtures/lsp/invalid_syntax.eclair b/tests/eclair/fixtures/lsp/invalid_syntax.eclair
new file mode 100644
index 0000000..53bf2d0
--- /dev/null
+++ b/tests/eclair/fixtures/lsp/invalid_syntax.eclair
@@ -0,0 +1,5 @@
+@def number(u32) output.
+
+number(123, ).
+number(456).
+number(789, ).
diff --git a/tests/eclair/fixtures/lsp/semantic_errors.eclair b/tests/eclair/fixtures/lsp/semantic_errors.eclair
new file mode 100644
index 0000000..e15c8b5
--- /dev/null
+++ b/tests/eclair/fixtures/lsp/semantic_errors.eclair
@@ -0,0 +1,3 @@
+@def wildcard_in_fact(u32) output.
+
+wildcard_in_fact(_).