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
@@ -27,6 +27,19 @@
 
 namespace clang {
 namespace clangd {
+namespace {
+// Helper to ignore type mismatches on optional fields.
+template <typename T>
+bool mapOptionalOrLog(llvm::json::ObjectMapper &O, llvm::StringLiteral Prop,
+                      T &Out) {
+  if (!O.mapOptional(Prop, Out)) {
+    elog("Unexpected value for optional field '{0}', ignoring", Prop);
+    Out = T{}; // Default-initialize Out to ensure we don't have a
+               // half-constructed object.
+  }
+  return true;
+}
+} // namespace
 
 char LSPError::ID;
 
@@ -490,7 +503,7 @@
   return O && O.map("textDocument", R.textDocument) &&
          O.map("contentChanges", R.contentChanges) &&
          O.map("wantDiagnostics", R.wantDiagnostics) &&
-         O.mapOptional("forceRebuild", R.forceRebuild);
+         mapOptionalOrLog(O, "forceRebuild", R.forceRebuild);
 }
 
 bool fromJSON(const llvm::json::Value &E, FileChangeType &Out,
@@ -580,10 +593,10 @@
               llvm::json::Path P) {
   llvm::json::ObjectMapper O(Params, P);
   return O && O.map("range", R.range) && O.map("message", R.message) &&
-         O.mapOptional("severity", R.severity) &&
-         O.mapOptional("category", R.category) &&
-         O.mapOptional("code", R.code) && O.mapOptional("source", R.source);
-  return true;
+         mapOptionalOrLog(O, "severity", R.severity) &&
+         mapOptionalOrLog(O, "category", R.category) &&
+         mapOptionalOrLog(O, "code", R.code) &&
+         mapOptionalOrLog(O, "source", R.source);
 }
 
 llvm::json::Value toJSON(const PublishDiagnosticsParams &PDP) {
@@ -817,9 +830,9 @@
               llvm::json::Path P) {
   llvm::json::ObjectMapper O(Params, P);
   int TriggerKind;
-  if (!O || !O.map("triggerKind", TriggerKind) ||
-      !O.mapOptional("triggerCharacter", R.triggerCharacter))
+  if (!O || !O.map("triggerKind", TriggerKind))
     return false;
+  mapOptionalOrLog(O, "triggerCharacter", R.triggerCharacter);
   R.triggerKind = static_cast<CompletionTriggerKind>(TriggerKind);
   return true;
 }
@@ -1121,8 +1134,9 @@
   llvm::json::ObjectMapper O(Params, P);
   if (!O)
     return true; // 'any' type in LSP.
-  return O.mapOptional("compilationDatabaseChanges",
-                       S.compilationDatabaseChanges);
+  mapOptionalOrLog(O, "compilationDatabaseChanges",
+                   S.compilationDatabaseChanges);
+  return true;
 }
 
 bool fromJSON(const llvm::json::Value &Params, InitializationOptions &Opts,
@@ -1133,8 +1147,8 @@
 
   return fromJSON(Params, Opts.ConfigSettings, P) &&
          O.map("compilationDatabasePath", Opts.compilationDatabasePath) &&
-         O.mapOptional("fallbackFlags", Opts.fallbackFlags) &&
-         O.mapOptional("clangdFileStatus", Opts.FileStatus);
+         mapOptionalOrLog(O, "fallbackFlags", Opts.fallbackFlags) &&
+         mapOptionalOrLog(O, "clangdFileStatus", Opts.FileStatus);
 }
 
 bool fromJSON(const llvm::json::Value &E, TypeHierarchyDirection &Out,
@@ -1190,10 +1204,11 @@
   return O && O.map("name", I.name) && O.map("kind", I.kind) &&
          O.map("uri", I.uri) && O.map("range", I.range) &&
          O.map("selectionRange", I.selectionRange) &&
-         O.mapOptional("detail", I.detail) &&
-         O.mapOptional("deprecated", I.deprecated) &&
-         O.mapOptional("parents", I.parents) &&
-         O.mapOptional("children", I.children) && O.mapOptional("data", I.data);
+         mapOptionalOrLog(O, "detail", I.detail) &&
+         mapOptionalOrLog(O, "deprecated", I.deprecated) &&
+         mapOptionalOrLog(O, "parents", I.parents) &&
+         mapOptionalOrLog(O, "children", I.children) &&
+         mapOptionalOrLog(O, "data", I.data);
 }
 
 bool fromJSON(const llvm::json::Value &Params,
@@ -1238,7 +1253,7 @@
   return O && O.map("name", I.name) && O.map("kind", I.kind) &&
          O.map("uri", I.uri) && O.map("range", I.range) &&
          O.map("selectionRange", I.selectionRange) &&
-         O.mapOptional("data", I.data);
+         mapOptionalOrLog(O, "data", I.data);
 }
 
 bool fromJSON(const llvm::json::Value &Params,