diff --git a/src/code_completion.cc b/src/code_completion.cc
index 556295bc..e5e09713 100644
--- a/src/code_completion.cc
+++ b/src/code_completion.cc
@@ -42,71 +42,86 @@ int GetCompletionPriority(const CXCompletionString& str, CXCursorKind result_kin
   return priority;
 }
 
+bool IsCallKind(CXCursorKind kind) {
+  switch (kind) {
+    case CXCursor_ObjCInstanceMethodDecl:
+    case CXCursor_CXXMethod:
+    case CXCursor_FunctionTemplate:
+    case CXCursor_FunctionDecl:
+    case CXCursor_Constructor:
+    case CXCursor_Destructor:
+    case CXCursor_ConversionFunction:
+      return true;
+    default:
+      return false;
+  }
+}
+
 lsCompletionItemKind GetCompletionKind(CXCursorKind cursor_kind) {
   switch (cursor_kind) {
 
-  case CXCursor_ObjCInstanceMethodDecl:
-  case CXCursor_CXXMethod:
-    return lsCompletionItemKind::Method;
+    case CXCursor_ObjCInstanceMethodDecl:
+    case CXCursor_CXXMethod:
+      return lsCompletionItemKind::Method;
 
-  case CXCursor_FunctionTemplate:
-  case CXCursor_FunctionDecl:
-    return lsCompletionItemKind::Function;
+    case CXCursor_FunctionTemplate:
+    case CXCursor_FunctionDecl:
+      return lsCompletionItemKind::Function;
 
-  case CXCursor_Constructor:
-  case CXCursor_Destructor:
-  case CXCursor_ConversionFunction:
-    return lsCompletionItemKind::Constructor;
+    case CXCursor_Constructor:
+    case CXCursor_Destructor:
+    case CXCursor_ConversionFunction:
+      return lsCompletionItemKind::Constructor;
 
-  case CXCursor_FieldDecl:
-    return lsCompletionItemKind::Field;
+    case CXCursor_FieldDecl:
+      return lsCompletionItemKind::Field;
 
-  case CXCursor_VarDecl:
-  case CXCursor_ParmDecl:
-    return lsCompletionItemKind::Variable;
+    case CXCursor_VarDecl:
+    case CXCursor_ParmDecl:
+      return lsCompletionItemKind::Variable;
 
-  case CXCursor_UnionDecl:
-  case CXCursor_ClassTemplate:
-  case CXCursor_ClassTemplatePartialSpecialization:
-  case CXCursor_ClassDecl:
-  case CXCursor_StructDecl:
-  case CXCursor_UsingDeclaration:
-  case CXCursor_TypedefDecl:
-  case CXCursor_TypeAliasDecl:
-  case CXCursor_TypeAliasTemplateDecl:
-    return lsCompletionItemKind::Class;
+    case CXCursor_UnionDecl:
+    case CXCursor_ClassTemplate:
+    case CXCursor_ClassTemplatePartialSpecialization:
+    case CXCursor_ClassDecl:
+    case CXCursor_StructDecl:
+    case CXCursor_UsingDeclaration:
+    case CXCursor_TypedefDecl:
+    case CXCursor_TypeAliasDecl:
+    case CXCursor_TypeAliasTemplateDecl:
+      return lsCompletionItemKind::Class;
 
-  case CXCursor_EnumConstantDecl:
-  case CXCursor_EnumDecl:
-    return lsCompletionItemKind::Enum;
+    case CXCursor_EnumConstantDecl:
+    case CXCursor_EnumDecl:
+      return lsCompletionItemKind::Enum;
 
-  case CXCursor_MacroInstantiation:
-  case CXCursor_MacroDefinition:
-    return lsCompletionItemKind::Interface;
+    case CXCursor_MacroInstantiation:
+    case CXCursor_MacroDefinition:
+      return lsCompletionItemKind::Interface;
 
-  case CXCursor_Namespace:
-  case CXCursor_NamespaceAlias:
-  case CXCursor_NamespaceRef:
-    return lsCompletionItemKind::Module;
+    case CXCursor_Namespace:
+    case CXCursor_NamespaceAlias:
+    case CXCursor_NamespaceRef:
+      return lsCompletionItemKind::Module;
 
-  case CXCursor_MemberRef:
-  case CXCursor_TypeRef:
-    return lsCompletionItemKind::Reference;
+    case CXCursor_MemberRef:
+    case CXCursor_TypeRef:
+      return lsCompletionItemKind::Reference;
 
-    //return lsCompletionItemKind::Property;
-    //return lsCompletionItemKind::Unit;
-    //return lsCompletionItemKind::Value;
-    //return lsCompletionItemKind::Keyword;
-    //return lsCompletionItemKind::Snippet;
-    //return lsCompletionItemKind::Color;
-    //return lsCompletionItemKind::File;
+      //return lsCompletionItemKind::Property;
+      //return lsCompletionItemKind::Unit;
+      //return lsCompletionItemKind::Value;
+      //return lsCompletionItemKind::Keyword;
+      //return lsCompletionItemKind::Snippet;
+      //return lsCompletionItemKind::Color;
+      //return lsCompletionItemKind::File;
 
-  case CXCursor_NotImplemented:
-    return lsCompletionItemKind::Text;
+    case CXCursor_NotImplemented:
+      return lsCompletionItemKind::Text;
 
-  default:
-    std::cerr << "[complete] Unhandled completion kind " << cursor_kind << std::endl;
-    return lsCompletionItemKind::Text;
+    default:
+      std::cerr << "[complete] Unhandled completion kind " << cursor_kind << std::endl;
+      return lsCompletionItemKind::Text;
   }
 }
 
@@ -125,9 +140,7 @@ std::string BuildLabelString(CXCompletionString completion_string) {
   return label;
 }
 
-std::string BuildDetailString(CXCompletionString completion_string) {
-  std::string detail;
-
+void BuildDetailString(CXCompletionString completion_string, std::string& detail, std::string& insert, std::vector<std::string>* parameters) {
   int num_chunks = clang_getNumCompletionChunks(completion_string);
   for (int i = 0; i < num_chunks; ++i) {
     CXCompletionChunkKind kind = clang_getCompletionChunkKind(completion_string, i);
@@ -135,23 +148,29 @@ std::string BuildDetailString(CXCompletionString completion_string) {
     switch (kind) {
     case CXCompletionChunk_Optional: {
       CXCompletionString nested = clang_getCompletionChunkCompletionString(completion_string, i);
-      detail += BuildDetailString(nested);
+      BuildDetailString(nested, detail, insert, parameters);
       break;
     }
 
     case CXCompletionChunk_Placeholder: {
-      // TODO: send this info to vscode.
-      CXString text = clang_getCompletionChunkText(completion_string, i);
-      detail += clang::ToString(text);
+      std::string text = clang::ToString(clang_getCompletionChunkText(completion_string, i));
+      parameters->push_back(text);
+      detail += text;
+      insert += "${" + std::to_string(parameters->size()) + ":" + text + "}";
       break;
     }
 
+    case CXCompletionChunk_CurrentParameter:
+      // We have our own parsing logic for active parameter. This doesn't seem
+      // to be very reliable.
+      break;
+
     case CXCompletionChunk_TypedText:
     case CXCompletionChunk_Text:
-    case CXCompletionChunk_Informative:
-    case CXCompletionChunk_CurrentParameter: {
-      CXString text = clang_getCompletionChunkText(completion_string, i);
-      detail += clang::ToString(text);
+    case CXCompletionChunk_Informative: {
+      std::string text = clang::ToString(clang_getCompletionChunkText(completion_string, i));
+      detail += text;
+      insert += text;
       break;
     }
 
@@ -164,48 +183,59 @@ std::string BuildDetailString(CXCompletionString completion_string) {
 
     case CXCompletionChunk_LeftParen:
       detail += "(";
+      insert += "(";
       break;
     case CXCompletionChunk_RightParen:
       detail += ")";
+      insert += ")";
       break;
     case CXCompletionChunk_LeftBracket:
-      detail += "]";
+      detail += "[";
+      insert += "[";
       break;
     case CXCompletionChunk_RightBracket:
-      detail += "[";
+      detail += "]";
+      insert += "]";
       break;
     case CXCompletionChunk_LeftBrace:
       detail += "{";
+      insert += "{";
       break;
     case CXCompletionChunk_RightBrace:
       detail += "}";
+      insert += "}";
       break;
     case CXCompletionChunk_LeftAngle:
       detail += "<";
+      insert += "<";
       break;
     case CXCompletionChunk_RightAngle:
       detail += ">";
+      insert += ">";
       break;
     case CXCompletionChunk_Comma:
       detail += ", ";
+      insert += ", ";
       break;
     case CXCompletionChunk_Colon:
       detail += ":";
+      insert += ":";
       break;
     case CXCompletionChunk_SemiColon:
       detail += ";";
+      insert += ";";
       break;
     case CXCompletionChunk_Equal:
       detail += "=";
+      insert += "=";
       break;
     case CXCompletionChunk_HorizontalSpace:
     case CXCompletionChunk_VerticalSpace:
       detail += " ";
+      insert += " ";
       break;
     }
   }
-
-  return detail;
 }
 
 void EnsureDocumentParsed(CompletionSession* session,
@@ -288,6 +318,16 @@ void CompletionQueryMain(CompletionManager* completion_manager) {
     timer.Reset();
     for (unsigned i = 0; i < cx_results->NumResults; ++i) {
       CXCompletionResult& result = cx_results->Results[i];
+      
+      // TODO: Try to figure out how we can hide base method calls without also
+      // hiding method implementation assistance, ie,
+      //
+      //    void Foo::* {
+      //    }
+      //
+
+      if (clang_getCompletionAvailability(result.CompletionString) == CXAvailability_NotAvailable)
+        continue;
 
       // TODO: fill in more data
       lsCompletionItem ls_completion_item;
@@ -295,9 +335,13 @@ void CompletionQueryMain(CompletionManager* completion_manager) {
       // kind/label/detail/docs/sortText
       ls_completion_item.kind = GetCompletionKind(result.CursorKind);
       ls_completion_item.label = BuildLabelString(result.CompletionString);
-      ls_completion_item.detail = BuildDetailString(result.CompletionString);
+      BuildDetailString(result.CompletionString, ls_completion_item.detail, ls_completion_item.insertText, &ls_completion_item.parameters_);
       ls_completion_item.documentation = clang::ToString(clang_getCompletionBriefComment(result.CompletionString));
       ls_completion_item.sortText = uint64_t(GetCompletionPriority(result.CompletionString, result.CursorKind, ls_completion_item.label));
+      
+      // If this function is slow we can skip building insertText at the cost of some code duplication.
+      if (!IsCallKind(result.CursorKind))
+        ls_completion_item.insertText = "";
 
       ls_result.push_back(ls_completion_item);
     }
diff --git a/src/command_line.cc b/src/command_line.cc
index e2560072..100bddb0 100644
--- a/src/command_line.cc
+++ b/src/command_line.cc
@@ -812,6 +812,7 @@ void RegisterMessageTypes() {
   MessageRegistry::instance()->Register<Ipc_TextDocumentDidSave>();
   MessageRegistry::instance()->Register<Ipc_TextDocumentRename>();
   MessageRegistry::instance()->Register<Ipc_TextDocumentComplete>();
+  MessageRegistry::instance()->Register<Ipc_TextDocumentSignatureHelp>();
   MessageRegistry::instance()->Register<Ipc_TextDocumentDefinition>();
   MessageRegistry::instance()->Register<Ipc_TextDocumentDocumentHighlight>();
   MessageRegistry::instance()->Register<Ipc_TextDocumentHover>();
@@ -1342,6 +1343,11 @@ bool QueryDbMainLoop(
         // See https://github.com/Microsoft/language-server-protocol/issues/138.
         response.result.capabilities.completionProvider->triggerCharacters = { ".", ":", ">" };
 
+        response.result.capabilities.signatureHelpProvider = lsSignatureHelpOptions();
+        // NOTE: If updating signature help tokens make sure to also update
+        // WorkingFile::FindClosestCallNameInBuffer.
+        response.result.capabilities.signatureHelpProvider->triggerCharacters = { "(", "," };
+
         response.result.capabilities.codeLensProvider = lsCodeLensOptions();
         response.result.capabilities.codeLensProvider->resolveProvider = false;
 
@@ -1599,6 +1605,76 @@ bool QueryDbMainLoop(
         break;
       }
 
+      case IpcId::TextDocumentSignatureHelp: {
+        auto msg = static_cast<Ipc_TextDocumentSignatureHelp*>(message.get());
+        lsTextDocumentPositionParams params = msg->params;
+        //std::cerr << "!! SignatureHelp @ " << msg->params.position.line << ":" << msg->params.position.character << std::endl;
+
+        WorkingFile* file = working_files->GetFileByFilename(params.textDocument.uri.GetPath());
+        std::string search;
+        int active_param = 0;
+        if (file) {
+          lsPosition completion_position;
+          search = file->FindClosestCallNameInBuffer(params.position, &active_param, &completion_position);
+          // Move completion position back by one; completer will automatically increment it by one to deal with -> and . accesses.
+          if (completion_position.character > 0)
+            completion_position.character -= 1;
+          std::cerr << "[completion] Changing completion position from " << params.position.ToString() << " to " << completion_position.ToString() << std::endl;
+          params.position = completion_position;
+        }
+        std::cerr << "[completion] Returning signatures for " << search << std::endl;
+        if (search.empty())
+          break;
+
+        CompletionManager::OnComplete callback = std::bind([](BaseIpcMessage* message, std::string search, int active_param, const NonElidedVector<lsCompletionItem>& results) {
+          auto msg = static_cast<Ipc_TextDocumentSignatureHelp*>(message);
+          auto ipc = IpcManager::instance();
+
+          Out_TextDocumentSignatureHelp response;
+          response.id = msg->id;
+
+          for (auto& result : results) {
+            if (result.label != search)
+              continue;
+
+            lsSignatureInformation signature;
+            signature.label = result.detail;
+            for (auto& parameter : result.parameters_) {
+              lsParameterInformation ls_param;
+              ls_param.label = parameter;
+              signature.parameters.push_back(ls_param);
+            }
+            response.result.signatures.push_back(signature);
+          }
+
+          // Guess the signature the user wants based on available parameter
+          // count.
+          response.result.activeSignature = 0;
+          for (size_t i = 0; i < response.result.signatures.size(); ++i) {
+            if (active_param < response.result.signatures.size()) {
+              response.result.activeSignature = i;
+              break;
+            }
+          }
+
+          // Set signature to what we parsed from the working file.
+          response.result.activeParameter = active_param;
+
+          response.Write(std::cerr);
+          std::cerr << std::endl;
+
+          Timer timer;
+          ipc->SendOutMessageToClient(IpcId::TextDocumentSignatureHelp, response);
+          timer.ResetAndPrint("[complete] Writing signature help results");
+
+          delete message;
+        }, message.release(), search, active_param, std::placeholders::_1);
+
+        completion_manager->CodeComplete(params, std::move(callback));
+
+        break;
+      }
+
       case IpcId::TextDocumentDefinition: {
         auto msg = static_cast<Ipc_TextDocumentDefinition*>(message.get());
 
@@ -2144,6 +2220,7 @@ void LanguageServerStdinLoop(IndexerConfig* config, std::unordered_map<IpcId, Ti
     case IpcId::TextDocumentDidSave:
     case IpcId::TextDocumentRename:
     case IpcId::TextDocumentCompletion:
+    case IpcId::TextDocumentSignatureHelp:
     case IpcId::TextDocumentDefinition:
     case IpcId::TextDocumentDocumentHighlight:
     case IpcId::TextDocumentHover:
diff --git a/src/ipc.cc b/src/ipc.cc
index 040f2ebd..609dd7a5 100644
--- a/src/ipc.cc
+++ b/src/ipc.cc
@@ -26,6 +26,8 @@ const char* IpcIdToString(IpcId id) {
     return "textDocument/rename";
   case IpcId::TextDocumentCompletion:
     return "textDocument/completion";
+  case IpcId::TextDocumentSignatureHelp:
+    return "textDocument/signatureHelp";
   case IpcId::TextDocumentDefinition:
     return "textDocument/definition";
   case IpcId::TextDocumentDocumentHighlight:
diff --git a/src/ipc.h b/src/ipc.h
index 8a0ce65b..55d6fe27 100644
--- a/src/ipc.h
+++ b/src/ipc.h
@@ -18,6 +18,7 @@ enum class IpcId : int {
   TextDocumentPublishDiagnostics,
   TextDocumentRename,
   TextDocumentCompletion,
+  TextDocumentSignatureHelp,
   TextDocumentDefinition,
   TextDocumentDocumentHighlight,
   TextDocumentHover,
diff --git a/src/language_server_api.cc b/src/language_server_api.cc
index 6d7e6f3e..3c94d42b 100644
--- a/src/language_server_api.cc
+++ b/src/language_server_api.cc
@@ -189,6 +189,10 @@ bool lsPosition::operator==(const lsPosition& other) const {
   return line == other.line && character == other.character;
 }
 
+std::string lsPosition::ToString() const {
+  return std::to_string(line) + ":" + std::to_string(character);
+}
+
 lsRange::lsRange() {}
 lsRange::lsRange(lsPosition start, lsPosition end) : start(start), end(end) {}
 
diff --git a/src/language_server_api.h b/src/language_server_api.h
index 25468f13..cc510f23 100644
--- a/src/language_server_api.h
+++ b/src/language_server_api.h
@@ -260,6 +260,8 @@ struct lsPosition {
 
   bool operator==(const lsPosition& other) const;
 
+  std::string ToString() const;
+
   // Note: these are 0-based.
   int line = 0;
   int character = 0;
@@ -421,6 +423,9 @@ enum class lsCompletionItemKind {
 MAKE_REFLECT_TYPE_PROXY(lsCompletionItemKind, int);
 
 struct lsCompletionItem {
+  // A set of function parameters. Used internally for signature help. Not sent to vscode.
+  std::vector<std::string> parameters_;
+
   // The label of this completion item. By default
   // also the text that is inserted when selecting
   // this completion.
@@ -447,11 +452,11 @@ struct lsCompletionItem {
 
   // A string that should be inserted a document when selecting
   // this completion. When `falsy` the label is used.
-  //std::string insertText;
+  std::string insertText;
 
   // The format of the insert text. The format applies to both the `insertText` property
   // and the `newText` property of a provided `textEdit`.
-  //lsInsertTextFormat insertTextFormat;
+  lsInsertTextFormat insertTextFormat = lsInsertTextFormat::Snippet;
 
   // An edit which is applied to a document when selecting this completion. When an edit is provided the value of
   // `insertText` is ignored.
@@ -479,7 +484,9 @@ MAKE_REFLECT_STRUCT(lsCompletionItem,
   kind,
   detail,
   documentation,
-  sortText);
+  sortText,
+  insertText,
+  insertTextFormat);
 
 
 struct lsTextDocumentItem {
@@ -937,7 +944,7 @@ MAKE_REFLECT_STRUCT(lsCompletionOptions, resolveProvider, triggerCharacters);
 // Signature help options.
 struct lsSignatureHelpOptions {
   // The characters that trigger signature help automatically.
-  NonElidedVector<std::string> triggerCharacters;
+  std::vector<std::string> triggerCharacters;
 };
 MAKE_REFLECT_STRUCT(lsSignatureHelpOptions, triggerCharacters);
 
@@ -1273,6 +1280,74 @@ struct Out_TextDocumentComplete : public lsOutMessage<Out_TextDocumentComplete>
 };
 MAKE_REFLECT_STRUCT(Out_TextDocumentComplete, jsonrpc, id, result);
 
+// Signature help.
+struct Ipc_TextDocumentSignatureHelp : public IpcMessage<Ipc_TextDocumentSignatureHelp> {
+  const static IpcId kIpcId = IpcId::TextDocumentSignatureHelp;
+
+  lsRequestId id;
+  lsTextDocumentPositionParams params;
+};
+MAKE_REFLECT_STRUCT(Ipc_TextDocumentSignatureHelp, id, params);
+// Represents a parameter of a callable-signature. A parameter can
+// have a label and a doc-comment.
+struct lsParameterInformation {
+  // The label of this parameter. Will be shown in
+  // the UI.
+  std::string label;
+
+  // The human-readable doc-comment of this parameter. Will be shown
+  // in the UI but can be omitted.
+  optional<std::string> documentation;
+};
+MAKE_REFLECT_STRUCT(lsParameterInformation, label, documentation);
+// Represents the signature of something callable. A signature
+// can have a label, like a function-name, a doc-comment, and
+// a set of parameters.
+struct lsSignatureInformation {
+  // The label of this signature. Will be shown in
+  // the UI.
+  std::string label;
+
+  // The human-readable doc-comment of this signature. Will be shown
+  // in the UI but can be omitted.
+  optional<std::string> documentation;
+
+  // The parameters of this signature.
+  std::vector<lsParameterInformation> parameters;
+};
+MAKE_REFLECT_STRUCT(lsSignatureInformation, label, documentation, parameters);
+// Signature help represents the signature of something
+// callable. There can be multiple signature but only one
+// active and only one active parameter.
+struct lsSignatureHelp {
+  // One or more signatures.
+  NonElidedVector<lsSignatureInformation> signatures;
+
+  // The active signature. If omitted or the value lies outside the
+  // range of `signatures` the value defaults to zero or is ignored if
+  // `signatures.length === 0`. Whenever possible implementors should 
+  // make an active decision about the active signature and shouldn't 
+  // rely on a default value.
+  // In future version of the protocol this property might become
+  // mandantory to better express this.
+  optional<int> activeSignature;
+
+  // The active parameter of the active signature. If omitted or the value
+  // lies outside the range of `signatures[activeSignature].parameters` 
+  // defaults to 0 if the active signature has parameters. If 
+  // the active signature has no parameters it is ignored. 
+  // In future version of the protocol this property might become
+  // mandantory to better express the active parameter if the
+  // active signature does have any.
+  optional<int> activeParameter;
+};
+MAKE_REFLECT_STRUCT(lsSignatureHelp, signatures, activeSignature, activeParameter);
+struct Out_TextDocumentSignatureHelp : public lsOutMessage<Out_TextDocumentSignatureHelp> {
+  lsRequestId id;
+  lsSignatureHelp result;
+};
+MAKE_REFLECT_STRUCT(Out_TextDocumentSignatureHelp, jsonrpc, id, result);
+
 // Goto definition
 struct Ipc_TextDocumentDefinition : public IpcMessage<Ipc_TextDocumentDefinition> {
   const static IpcId kIpcId = IpcId::TextDocumentDefinition;
diff --git a/src/working_files.cc b/src/working_files.cc
index 3d30f15f..4162aed3 100644
--- a/src/working_files.cc
+++ b/src/working_files.cc
@@ -2,10 +2,32 @@
 
 #include "position.h"
 
+#include <doctest/doctest.h>
+
 #include <climits>
 
 namespace {
 
+lsPosition GetPositionForOffset(const std::string& content, int offset) {
+  if (offset >= content.size())
+    offset = content.size() - 1;
+
+  lsPosition result;
+  int i = 0;
+  while (i < offset) {
+    if (content[i] == '\n') {
+      result.line += 1;
+      result.character = 0;
+    }
+    else {
+      result.character += 1;
+    }
+    ++i;
+  }
+
+  return result;
+}
+
 int GetOffsetForPosition(lsPosition position, const std::string& content) {
   int offset = 0;
 
@@ -142,6 +164,50 @@ optional<int> WorkingFile::GetIndexLineFromBufferLine(int buffer_line) const {
   return closest_index_line;
 }
 
+std::string WorkingFile::FindClosestCallNameInBuffer(lsPosition position, int* active_parameter, lsPosition* completion_position) const {
+  *active_parameter = 0;
+
+  int offset = GetOffsetForPosition(position, buffer_content);
+
+  // If vscode auto-inserts closing ')' we will begin on ')' token in foo()
+  // which will make the below algorithm think it's a nested call.
+  if (offset > 0 && buffer_content[offset] == ')')
+    --offset;
+
+  // Scan back out of call context.
+  int balance = 0;
+  while (offset > 0) {
+    char c = buffer_content[offset];
+    if (c == ')') ++balance;
+    else if (c == '(') --balance;
+
+    if (balance == 0 && c == ',')
+      *active_parameter += 1;
+
+    --offset;
+
+    if (balance == -1)
+      break;
+  }
+
+  if (offset < 0)
+    return "";
+
+  // Scan back entire identifier.
+  int start_offset = offset;
+  while (offset > 0) {
+    char c = buffer_content[offset - 1];
+    if (isalnum(c) == false && c != '_')
+      break;
+    --offset;
+  }
+
+  if (completion_position)
+    *completion_position = GetPositionForOffset(buffer_content, offset);
+
+  return buffer_content.substr(offset, start_offset - offset + 1);
+}
+
 CXUnsavedFile WorkingFile::AsUnsavedFile() const {
   CXUnsavedFile result;
   result.Filename = filename.c_str();
@@ -237,3 +303,73 @@ std::vector<CXUnsavedFile> WorkingFiles::AsUnsavedFiles() {
     result.push_back(file->AsUnsavedFile());
   return result;
 }
+
+TEST_SUITE("WorkingFile");
+
+lsPosition CharPos(const WorkingFile& file, char character, int character_offset = 0) {
+  const std::string& search = file.buffer_content;
+
+  lsPosition result;
+  int index = 0;
+  while (index < search.size()) {
+    char c = search[index];
+    if (c == character)
+      break;
+    if (c == '\n') {
+      result.line += 1;
+      result.character = 0;
+    }
+    else {
+      result.character += 1;
+    }
+    ++index;
+  }
+  REQUIRE(index < search.size());
+  result.character += character_offset;
+  return result;
+}
+
+TEST_CASE("simple call") {
+  WorkingFile f("foo.cc", "abcd(1, 2");
+  int active_param = 0;
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, '('), &active_param) == "abcd");
+  REQUIRE(active_param == 0);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, '1'), &active_param) == "abcd");
+  REQUIRE(active_param == 0);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, ','), &active_param) == "abcd");
+  REQUIRE(active_param == 1);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, ' '), &active_param) == "abcd");
+  REQUIRE(active_param == 1);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, '2'), &active_param) == "abcd");
+  REQUIRE(active_param == 1);
+}
+
+TEST_CASE("nested call") {
+  WorkingFile f("foo.cc", "abcd(efg(), 2");
+  int active_param = 0;
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, '('), &active_param) == "abcd");
+  REQUIRE(active_param == 0);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, 'e'), &active_param) == "abcd");
+  REQUIRE(active_param == 0);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, 'f'), &active_param) == "abcd");
+  REQUIRE(active_param == 0);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, 'g'), &active_param) == "abcd");
+  REQUIRE(active_param == 0);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, 'g', 1), &active_param) == "efg");
+  REQUIRE(active_param == 0);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, 'g', 2), &active_param) == "efg");
+  REQUIRE(active_param == 0);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, ','), &active_param) == "abcd");
+  REQUIRE(active_param == 1);
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, ' '), &active_param) == "abcd");
+  REQUIRE(active_param == 1);
+}
+
+TEST_CASE("auto-insert )") {
+  WorkingFile f("foo.cc", "abc()");
+  int active_param = 0;
+  REQUIRE(f.FindClosestCallNameInBuffer(CharPos(f, ')'), &active_param) == "abc");
+  REQUIRE(active_param == 0);
+}
+
+TEST_SUITE_END();
\ No newline at end of file
diff --git a/src/working_files.h b/src/working_files.h
index 7547d67b..79e9230f 100644
--- a/src/working_files.h
+++ b/src/working_files.h
@@ -49,6 +49,13 @@ struct WorkingFile {
   // accepts and returns 1-based lines.
   optional<int> GetIndexLineFromBufferLine(int buffer_line) const;
 
+  // Finds the closest 'callable' name prior to position. This is used for
+  // signature help to filter code completion results.
+  //
+  // |completion_position| will be point to a good code completion location to
+  // for fetching signatures.
+  std::string FindClosestCallNameInBuffer(lsPosition position, int* active_parameter, lsPosition* completion_position = nullptr) const;
+
   CXUnsavedFile AsUnsavedFile() const;
 };