diff --git a/clang-tools-extra/clangd/Diagnostics.h b/clang-tools-extra/clangd/Diagnostics.h
--- a/clang-tools-extra/clangd/Diagnostics.h
+++ b/clang-tools-extra/clangd/Diagnostics.h
@@ -126,6 +126,10 @@
 /// Convert from clang diagnostic level to LSP severity.
 int getSeverity(DiagnosticsEngine::Level L);
 
+/// Returns a URI providing more information about a particular diagnostic.
+llvm::Optional<std::string> getDiagnosticDocURI(Diag::DiagSource, unsigned ID,
+                                                llvm::StringRef Name);
+
 /// StoreDiags collects the diagnostics that can later be reported by
 /// clangd. It groups all notes for a diagnostic into a single Diag
 /// and filters out diagnostics that don't mention the main file (i.e. neither
diff --git a/clang-tools-extra/clangd/Diagnostics.cpp b/clang-tools-extra/clangd/Diagnostics.cpp
--- a/clang-tools-extra/clangd/Diagnostics.cpp
+++ b/clang-tools-extra/clangd/Diagnostics.cpp
@@ -477,6 +477,10 @@
   }
 
   Main.code = D.Name;
+  if (auto URI = getDiagnosticDocURI(D.Source, D.ID, D.Name)) {
+    Main.codeDescription.emplace();
+    Main.codeDescription->href = std::move(*URI);
+  }
   switch (D.Source) {
   case Diag::Clang:
     Main.source = "clang";
@@ -903,5 +907,31 @@
   return Code;
 }
 
+llvm::Optional<std::string> getDiagnosticDocURI(Diag::DiagSource Source,
+                                                unsigned ID,
+                                                llvm::StringRef Name) {
+  switch (Source) {
+  case Diag::Unknown:
+    break;
+  case Diag::Clang:
+    // There is a page listing many warning flags, but it provides too little
+    // information to be worth linking.
+    // https://clang.llvm.org/docs/DiagnosticsReference.html
+    break;
+  case Diag::ClangTidy:
+    return {("https://clang.llvm.org/extra/clang-tidy/checks/" + Name + ".html")
+                .str()};
+  case Diag::Clangd:
+    if (Name == "unused-includes")
+      return {"https://clangd.llvm.org/guides/include-cleaner"};
+    break;
+  case Diag::ClangdConfig:
+    // FIXME: we should link to https://clangd.llvm.org/config
+    // However we have no diagnostic codes, which the link should describe!
+    break;
+  }
+  return llvm::None;
+}
+
 } // namespace clangd
 } // namespace clang
diff --git a/clang-tools-extra/clangd/Protocol.h b/clang-tools-extra/clangd/Protocol.h
--- a/clang-tools-extra/clangd/Protocol.h
+++ b/clang-tools-extra/clangd/Protocol.h
@@ -835,6 +835,13 @@
 };
 llvm::json::Value toJSON(DiagnosticTag Tag);
 
+/// Structure to capture a description for an error code.
+struct CodeDescription {
+  /// An URI to open with more information about the diagnostic error.
+  std::string href;
+};
+llvm::json::Value toJSON(const CodeDescription &);
+
 struct CodeAction;
 struct Diagnostic {
   /// The range at which the message applies.
@@ -847,6 +854,9 @@
   /// The diagnostic's code. Can be omitted.
   std::string code;
 
+  /// An optional property to describe the error code.
+  llvm::Optional<CodeDescription> codeDescription;
+
   /// A human-readable string describing the source of this
   /// diagnostic, e.g. 'typescript' or 'super lint'.
   std::string source;
@@ -874,7 +884,7 @@
 
   /// A data entry field that is preserved between a
   /// `textDocument/publishDiagnostics` notification
-  /// and`textDocument/codeAction` request.
+  /// and `textDocument/codeAction` request.
   /// Mutating users should associate their data with a unique key they can use
   /// to retrieve later on.
   llvm::json::Object data;
diff --git a/clang-tools-extra/clangd/Protocol.cpp b/clang-tools-extra/clangd/Protocol.cpp
--- a/clang-tools-extra/clangd/Protocol.cpp
+++ b/clang-tools-extra/clangd/Protocol.cpp
@@ -592,6 +592,10 @@
 
 llvm::json::Value toJSON(DiagnosticTag Tag) { return static_cast<int>(Tag); }
 
+llvm::json::Value toJSON(const CodeDescription &D) {
+  return llvm::json::Object{{"href", D.href}};
+}
+
 llvm::json::Value toJSON(const Diagnostic &D) {
   llvm::json::Object Diag{
       {"range", D.range},
@@ -604,6 +608,8 @@
     Diag["codeActions"] = D.codeActions;
   if (!D.code.empty())
     Diag["code"] = D.code;
+  if (D.codeDescription.hasValue())
+    Diag["codeDescription"] = *D.codeDescription;
   if (!D.source.empty())
     Diag["source"] = D.source;
   if (D.relatedInformation)
diff --git a/clang-tools-extra/clangd/test/diagnostics-tidy.test b/clang-tools-extra/clangd/test/diagnostics-tidy.test
--- a/clang-tools-extra/clangd/test/diagnostics-tidy.test
+++ b/clang-tools-extra/clangd/test/diagnostics-tidy.test
@@ -8,6 +8,9 @@
 # CHECK-NEXT:    "diagnostics": [
 # CHECK-NEXT:      {
 # CHECK-NEXT:        "code": "bugprone-sizeof-expression",
+# CHECK-NEXT:        "codeDescription": {
+# CHECK-NEXT:          "href": "https://clang.llvm.org/extra/clang-tidy/checks/bugprone-sizeof-expression.html"
+# CHECK-NEXT:        },
 # CHECK-NEXT:        "message": "Suspicious usage of 'sizeof(K)'; did you mean 'K'?",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
diff --git a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
--- a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
@@ -1828,13 +1828,17 @@
   Cfg.Diagnostics.Includes.IgnoreHeader.emplace_back(
       [](llvm::StringRef Header) { return Header.endswith("ignore.h"); });
   WithContextValue WithCfg(Config::Key, std::move(Cfg));
+  auto AST = TU.build();
   EXPECT_THAT(
-      *TU.build().getDiagnostics(),
+      *AST.getDiagnostics(),
       UnorderedElementsAre(AllOf(
           Diag(Test.range("diag"),
                "included header unused.h is not used directly"),
           withTag(DiagnosticTag::Unnecessary), diagSource(Diag::Clangd),
           withFix(Fix(Test.range("fix"), "", "remove #include directive")))));
+  auto &Diag = AST.getDiagnostics()->front();
+  EXPECT_EQ(getDiagnosticDocURI(Diag.Source, Diag.ID, Diag.Name),
+            std::string("https://clangd.llvm.org/guides/include-cleaner"));
   Cfg.Diagnostics.SuppressAll = true;
   WithContextValue SuppressAllWithCfg(Config::Key, std::move(Cfg));
   EXPECT_THAT(*TU.build().getDiagnostics(), IsEmpty());