Index: clangd/Diagnostics.cpp
===================================================================
--- clangd/Diagnostics.cpp
+++ clangd/Diagnostics.cpp
@@ -145,6 +145,13 @@
   OS << diagLeveltoString(D.Severity) << ": " << D.Message;
 }
 
+/// Capitalizes the first word in the diagnostic's message.
+std::string capitalizeDiagnosticText(std::string Message) {
+  if (!Message.empty())
+    Message[0] = llvm::toUpper(Message[0]);
+  return Message;
+}
+
 /// Returns a message sent to LSP for the main diagnostic in \p D.
 /// The message includes all the notes with their corresponding locations.
 /// However, notes with fix-its are excluded as those usually only contain a
@@ -166,7 +173,7 @@
     printDiag(OS, Note);
   }
   OS.flush();
-  return Result;
+  return capitalizeDiagnosticText(std::move(Result));
 }
 
 /// Returns a message sent to LSP for the note of the main diagnostic.
@@ -179,7 +186,7 @@
   OS << "\n\n";
   printDiag(OS, Main);
   OS.flush();
-  return Result;
+  return capitalizeDiagnosticText(std::move(Result));
 }
 } // namespace
 
Index: test/clangd/capitalize-diagnostics.test
===================================================================
--- /dev/null
+++ test/clangd/capitalize-diagnostics.test
@@ -0,0 +1,42 @@
+# RUN: clangd -lit-test < %s | FileCheck -strict-whitespace %s
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///foo.c","languageId":"c","version":1,"text":"void foo() { }\nvoid foo() { }"}}}
+#      CHECK:  "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:  "params": {
+# CHECK-NEXT:    "diagnostics": [
+# CHECK-NEXT:      {
+# CHECK-NEXT:        "message": "Redefinition of 'foo'\n\nfoo.c:1:6: note: previous definition is here",
+# CHECK-NEXT:        "range": {
+# CHECK-NEXT:          "end": {
+# CHECK-NEXT:            "character": 8,
+# CHECK-NEXT:            "line": 1
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "start": {
+# CHECK-NEXT:            "character": 5,
+# CHECK-NEXT:            "line": 1
+# CHECK-NEXT:          }
+# CHECK-NEXT:        },
+# CHECK-NEXT:        "severity": 1
+# CHECK-NEXT:      },
+# CHECK-NEXT:      {
+# CHECK-NEXT:        "message": "Previous definition is here\n\nfoo.c:2:6: error: redefinition of 'foo'",
+# CHECK-NEXT:        "range": {
+# CHECK-NEXT:          "end": {
+# CHECK-NEXT:            "character": 8,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          },
+# CHECK-NEXT:          "start": {
+# CHECK-NEXT:            "character": 5,
+# CHECK-NEXT:            "line": 0
+# CHECK-NEXT:          }
+# CHECK-NEXT:        },
+# CHECK-NEXT:        "severity": 3
+# CHECK-NEXT:      }
+# CHECK-NEXT:    ],
+# CHECK-NEXT:    "uri": "file://{{.*}}/foo.c"
+# CHECK-NEXT:  }
+---
+{"jsonrpc":"2.0","id":5,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}
Index: test/clangd/diagnostics.test
===================================================================
--- test/clangd/diagnostics.test
+++ test/clangd/diagnostics.test
@@ -6,7 +6,7 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "message": "return type of 'main' is not 'int'",
+# CHECK-NEXT:        "message": "Return type of 'main' is not 'int'",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
 # CHECK-NEXT:            "character": 4,
Index: test/clangd/did-change-configuration-params.test
===================================================================
--- test/clangd/did-change-configuration-params.test
+++ test/clangd/did-change-configuration-params.test
@@ -24,7 +24,7 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "message": "variable 'i' is uninitialized when used here",
+# CHECK-NEXT:        "message": "Variable 'i' is uninitialized when used here",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
 # CHECK-NEXT:            "character": 28,
Index: test/clangd/execute-command.test
===================================================================
--- test/clangd/execute-command.test
+++ test/clangd/execute-command.test
@@ -6,7 +6,7 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "message": "using the result of an assignment as a condition without parentheses",
+# CHECK-NEXT:        "message": "Using the result of an assignment as a condition without parentheses",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
 # CHECK-NEXT:            "character": 37,
Index: test/clangd/extra-flags.test
===================================================================
--- test/clangd/extra-flags.test
+++ test/clangd/extra-flags.test
@@ -6,7 +6,7 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "message": "variable 'i' is uninitialized when used here",
+# CHECK-NEXT:        "message": "Variable 'i' is uninitialized when used here",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
 # CHECK-NEXT:            "character": 28,
@@ -28,7 +28,7 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "message": "variable 'i' is uninitialized when used here",
+# CHECK-NEXT:        "message": "Variable 'i' is uninitialized when used here",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
 # CHECK-NEXT:            "character": 28,
Index: test/clangd/fixits.test
===================================================================
--- test/clangd/fixits.test
+++ test/clangd/fixits.test
@@ -6,7 +6,7 @@
 # CHECK-NEXT:  "params": {
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
-# CHECK-NEXT:        "message": "using the result of an assignment as a condition without parentheses",
+# CHECK-NEXT:        "message": "Using the result of an assignment as a condition without parentheses",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
 # CHECK-NEXT:            "character": 37,
@@ -23,7 +23,7 @@
 # CHECK-NEXT:    "uri": "file://{{.*}}/foo.c"
 # CHECK-NEXT:  }
 ---
-{"jsonrpc":"2.0","id":2,"method":"textDocument/codeAction","params":{"textDocument":{"uri":"test:///foo.c"},"range":{"start":{"line":104,"character":13},"end":{"line":0,"character":35}},"context":{"diagnostics":[{"range":{"start": {"line": 0, "character": 32}, "end": {"line": 0, "character": 37}},"severity":2,"message":"using the result of an assignment as a condition without parentheses"}]}}}
+{"jsonrpc":"2.0","id":2,"method":"textDocument/codeAction","params":{"textDocument":{"uri":"test:///foo.c"},"range":{"start":{"line":104,"character":13},"end":{"line":0,"character":35}},"context":{"diagnostics":[{"range":{"start": {"line": 0, "character": 32}, "end": {"line": 0, "character": 37}},"severity":2,"message":"Using the result of an assignment as a condition without parentheses"}]}}}
 #      CHECK:  "id": 2,
 # CHECK-NEXT:  "jsonrpc": "2.0",
 # CHECK-NEXT:  "result": [
@@ -92,7 +92,7 @@
 # CHECK-NEXT:    }
 # CHECK-NEXT:  ]
 ---
-{"jsonrpc":"2.0","id":3,"method":"textDocument/codeAction","params":{"textDocument":{"uri":"test:///foo.c"},"range":{"start":{"line":104,"character":13},"end":{"line":0,"character":35}},"context":{"diagnostics":[{"range":{"start": {"line": 0, "character": 32}, "end": {"line": 0, "character": 37}},"severity":2,"message":"using the result of an assignment as a condition without parentheses"}]}}}
+{"jsonrpc":"2.0","id":3,"method":"textDocument/codeAction","params":{"textDocument":{"uri":"test:///foo.c"},"range":{"start":{"line":104,"character":13},"end":{"line":0,"character":35}},"context":{"diagnostics":[{"range":{"start": {"line": 0, "character": 32}, "end": {"line": 0, "character": 37}},"severity":2,"message":"Using the result of an assignment as a condition without parentheses"}]}}}
 # Make sure unused "code" and "source" fields ignored gracefully
 #      CHECK:  "id": 3,
 # CHECK-NEXT:  "jsonrpc": "2.0",