diff --git a/Sources/SourceKitLSP/Swift/Diagnostic.swift b/Sources/SourceKitLSP/Swift/Diagnostic.swift
index 7495407de..b3e32aaf0 100644
--- a/Sources/SourceKitLSP/Swift/Diagnostic.swift
+++ b/Sources/SourceKitLSP/Swift/Diagnostic.swift
@@ -125,6 +125,19 @@ extension TextEdit {
   }
 }
 
+fileprivate extension String {
+  /// Returns this string with the first letter uppercased.
+  ///
+  /// If the string does not start with a letter, no change is made to it.
+  func withFirstLetterUppercased() -> String {
+    if let firstLetter = self.first {
+      return firstLetter.uppercased() + self.dropFirst()
+    } else {
+      return self
+    }
+  }
+}
+
 extension Diagnostic {
 
   /// Creates a diagnostic from a sourcekitd response dictionary.
@@ -138,7 +151,7 @@ extension Diagnostic {
     let keys = diag.sourcekitd.keys
     let values = diag.sourcekitd.values
 
-    guard let message: String = diag[keys.description] else { return nil }
+    guard let message: String = diag[keys.description]?.withFirstLetterUppercased() else { return nil }
 
     var range: Range<Position>? = nil
     if let line: Int = diag[keys.line],
@@ -309,7 +322,7 @@ extension DiagnosticRelatedInformation {
       return nil
     }
 
-    guard let message: String = diag[keys.description] else { return nil }
+    guard let message: String = diag[keys.description]?.withFirstLetterUppercased() else { return nil }
 
     var actions: [CodeAction]? = nil
     if let skfixits: SKDResponseArray = diag[keys.fixits],
diff --git a/Tests/SourceKitLSPTests/LocalSwiftTests.swift b/Tests/SourceKitLSPTests/LocalSwiftTests.swift
index eabdbf7ca..d039cf969 100644
--- a/Tests/SourceKitLSPTests/LocalSwiftTests.swift
+++ b/Tests/SourceKitLSPTests/LocalSwiftTests.swift
@@ -447,7 +447,7 @@ final class LocalSwiftTests: XCTestCase {
         XCTAssertEqual(
           fixit,
           CodeAction(
-            title: "chain the optional using '?' to access member 'bigEndian' only for non-'nil' base values",
+            title: "Chain the optional using '?' to access member 'bigEndian' only for non-'nil' base values",
             kind: .quickFix,
             diagnostics: nil,
             edit: WorkspaceEdit(changes: [uri: [expectedTextEdit]], documentChanges: nil),
@@ -469,7 +469,7 @@ final class LocalSwiftTests: XCTestCase {
         XCTAssertEqual(
           fixit,
           CodeAction(
-            title: "force-unwrap using '!' to abort execution if the optional value contains 'nil'",
+            title: "Force-unwrap using '!' to abort execution if the optional value contains 'nil'",
             kind: .quickFix,
             diagnostics: nil,
             edit: WorkspaceEdit(changes: [uri: [expectedTextEdit]], documentChanges: nil),
@@ -627,11 +627,11 @@ final class LocalSwiftTests: XCTestCase {
 
     for fixit in quickFixes {
       if fixit.title.contains("!") {
-        XCTAssert(fixit.title.starts(with: "force-unwrap using '!'"))
+        XCTAssert(fixit.title.starts(with: "Force-unwrap using '!'"))
         expectedTextEdit.newText = "!"
         XCTAssertEqual(fixit.edit, WorkspaceEdit(changes: [uri: [expectedTextEdit]], documentChanges: nil))
       } else {
-        XCTAssert(fixit.title.starts(with: "chain the optional using '?'"))
+        XCTAssert(fixit.title.starts(with: "Chain the optional using '?'"))
         expectedTextEdit.newText = "?"
         XCTAssertEqual(fixit.edit, WorkspaceEdit(changes: [uri: [expectedTextEdit]], documentChanges: nil))
       }
@@ -639,7 +639,7 @@ final class LocalSwiftTests: XCTestCase {
       XCTAssertEqual(fixit.diagnostics?.count, 1)
       XCTAssertEqual(fixit.diagnostics?.first?.severity, .error)
       XCTAssertEqual(fixit.diagnostics?.first?.range, Range(Position(line: 1, utf16index: 6)))
-      XCTAssert(fixit.diagnostics?.first?.message.starts(with: "value of optional type") == true)
+      XCTAssert(fixit.diagnostics?.first?.message.starts(with: "Value of optional type") == true)
     }
   }
 
@@ -729,7 +729,7 @@ final class LocalSwiftTests: XCTestCase {
     XCTAssertEqual(quickFixes.count, 1)
     guard let fixit = quickFixes.first else { return }
 
-    XCTAssertEqual(fixit.title, "use 'new(_:hotness:)' instead")
+    XCTAssertEqual(fixit.title, "Use 'new(_:hotness:)' instead")
     XCTAssertEqual(fixit.diagnostics?.count, 1)
     XCTAssert(fixit.diagnostics?.first?.message.contains("is deprecated") == true)
     XCTAssertEqual(
@@ -1627,7 +1627,7 @@ final class LocalSwiftTests: XCTestCase {
 
     let diagnostic = try await testClient.nextDiagnosticsNotification()
     let diag = try XCTUnwrap(diagnostic.diagnostics.first)
-    XCTAssertEqual(diag.message, "cannot find 'bar' in scope")
+    XCTAssertEqual(diag.message, "Cannot find 'bar' in scope")
 
     // Ensure that we don't get a second `PublishDiagnosticsNotification`
     await assertThrowsError(try await testClient.nextDiagnosticsNotification(timeout: 2))
diff --git a/Tests/SourceKitLSPTests/PullDiagnosticsTests.swift b/Tests/SourceKitLSPTests/PullDiagnosticsTests.swift
index 4eb62c25e..76620e395 100644
--- a/Tests/SourceKitLSPTests/PullDiagnosticsTests.swift
+++ b/Tests/SourceKitLSPTests/PullDiagnosticsTests.swift
@@ -101,8 +101,8 @@ final class PullDiagnosticsTests: XCTestCase {
     // toolchains that don't contain the change yet.
     XCTAssert(
       [
-        "add stubs for conformance",
-        "do you want to add protocol stubs?",
+        "Add stubs for conformance",
+        "Do you want to add protocol stubs?",
       ].contains(action.title)
     )
   }
diff --git a/Tests/SourceKitLSPTests/SourceKitTests.swift b/Tests/SourceKitLSPTests/SourceKitTests.swift
index 6be73a700..d68440805 100644
--- a/Tests/SourceKitLSPTests/SourceKitTests.swift
+++ b/Tests/SourceKitLSPTests/SourceKitTests.swift
@@ -287,8 +287,8 @@ final class SKTests: XCTestCase {
       // FIXME: The error message for the missing module is misleading on Darwin
       // https://github.com/apple/swift-package-manager/issues/5925
       XCTAssert(
-        diagnostic.message.contains("could not build Objective-C module")
-          || diagnostic.message.contains("no such module"),
+        diagnostic.message.contains("Could not build Objective-C module")
+          || diagnostic.message.contains("No such module"),
         "expected module import error but found \"\(diagnostic.message)\""
       )
     }