diff --git a/include/swift/IDE/CompletionInstance.h b/include/swift/IDE/CompletionInstance.h
index b928fa997e79c..8eb345fee94da 100644
--- a/include/swift/IDE/CompletionInstance.h
+++ b/include/swift/IDE/CompletionInstance.h
@@ -18,6 +18,7 @@
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Chrono.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/VirtualFileSystem.h"
 
@@ -38,13 +39,22 @@ makeCodeCompletionMemoryBuffer(const llvm::MemoryBuffer *origBuf,
 /// Manages \c CompilerInstance for completion like operations.
 class CompletionInstance {
   unsigned MaxASTReuseCount = 100;
+  unsigned DependencyCheckIntervalSecond = 5;
 
   std::mutex mtx;
 
   std::unique_ptr<CompilerInstance> CachedCI;
+  ModuleDecl *CurrentModule = nullptr;
   llvm::hash_code CachedArgHash;
+  llvm::sys::TimePoint<> DependencyCheckedTimestamp;
+  llvm::StringMap<llvm::hash_code> InMemoryDependencyHash;
   unsigned CachedReuseCount = 0;
 
+  void cacheCompilerInstance(std::unique_ptr<CompilerInstance> CI,
+                             llvm::hash_code ArgsHash);
+
+  bool shouldCheckDependencies() const;
+
   /// Calls \p Callback with cached \c CompilerInstance if it's usable for the
   /// specified completion request.
   /// Returns \c if the callback was called. Returns \c false if the compiler
@@ -52,6 +62,7 @@ class CompletionInstance {
   /// in function bodies, or the interface hash of the file has changed.
   bool performCachedOperationIfPossible(
       const swift::CompilerInvocation &Invocation, llvm::hash_code ArgsHash,
+      llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FileSystem,
       llvm::MemoryBuffer *completionBuffer, unsigned int Offset,
       DiagnosticConsumer *DiagC,
       llvm::function_ref<void(CompilerInstance &, bool)> Callback);
@@ -69,6 +80,8 @@ class CompletionInstance {
       llvm::function_ref<void(CompilerInstance &, bool)> Callback);
 
 public:
+  void setDependencyCheckIntervalSecond(unsigned Value);
+
   /// Calls \p Callback with a \c CompilerInstance which is prepared for the
   /// second pass. \p Callback is resposible to perform the second pass on it.
   /// The \c CompilerInstance may be reused from the previous completions,
diff --git a/lib/IDE/CompletionInstance.cpp b/lib/IDE/CompletionInstance.cpp
index cb3f8c965b577..67955f93407dc 100644
--- a/lib/IDE/CompletionInstance.cpp
+++ b/lib/IDE/CompletionInstance.cpp
@@ -21,11 +21,14 @@
 #include "swift/Basic/LangOptions.h"
 #include "swift/Basic/PrettyStackTrace.h"
 #include "swift/Basic/SourceManager.h"
+#include "swift/ClangImporter/ClangModule.h"
 #include "swift/Driver/FrontendUtil.h"
 #include "swift/Frontend/Frontend.h"
 #include "swift/Parse/Lexer.h"
 #include "swift/Parse/PersistentParserState.h"
+#include "swift/Serialization/SerializedModuleLoader.h"
 #include "swift/Subsystems.h"
+#include "clang/AST/ASTContext.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/Support/MemoryBuffer.h"
 
@@ -162,10 +165,118 @@ static DeclContext *getEquivalentDeclContextFromSourceFile(DeclContext *DC,
   return newDC;
 }
 
+/// For each dependency file in \p CI, run \p callback until the callback
+/// returns \c true. Returns \c true if any callback call returns \c true, \c
+/// false otherwise.
+static bool
+forEachDependencyUntilTrue(CompilerInstance &CI, ModuleDecl *CurrentModule,
+                           unsigned excludeBufferID,
+                           llvm::function_ref<bool(StringRef)> callback) {
+  // Check files in the current module.
+  for (FileUnit *file : CurrentModule->getFiles()) {
+    StringRef filename;
+    if (auto SF = dyn_cast<SourceFile>(file)) {
+      if (SF->getBufferID() == excludeBufferID)
+        continue;
+      filename = SF->getFilename();
+    } else if (auto LF = dyn_cast<LoadedFile>(file))
+      filename = LF->getFilename();
+    else
+      continue;
+
+    // Ignore synthesized files.
+    if (filename.empty() || filename.front() == '<')
+      continue;
+
+    if (callback(filename))
+      return true;
+  }
+
+  // Check other non-system depenencies (e.g. modules, headers).
+  for (auto &dep : CI.getDependencyTracker()->getDependencies()) {
+    if (callback(dep))
+      return true;
+  }
+
+  return false;
+}
+
+/// Collect hash codes of the dependencies into \c Map.
+static void cacheDependencyHashIfNeeded(CompilerInstance &CI,
+                                        ModuleDecl *CurrentModule,
+                                        unsigned excludeBufferID,
+                                        llvm::StringMap<llvm::hash_code> &Map) {
+  auto &FS = CI.getFileSystem();
+  forEachDependencyUntilTrue(
+      CI, CurrentModule, excludeBufferID, [&](StringRef filename) {
+        if (Map.count(filename))
+          return false;
+
+        auto stat = FS.status(filename);
+        if (!stat)
+          return false;
+
+        // We will check the hash only if the modification time of the dependecy
+        // is zero. See 'areAnyDependentFilesInvalidated() below'.
+        if (stat->getLastModificationTime() != llvm::sys::TimePoint<>())
+          return false;
+
+        auto buf = FS.getBufferForFile(filename);
+        Map[filename] = llvm::hash_value(buf.get()->getBuffer());
+        return false;
+      });
+}
+
+/// Check if any dependent files are modified since \p timestamp.
+static bool areAnyDependentFilesInvalidated(
+    CompilerInstance &CI, ModuleDecl *CurrentModule, llvm::vfs::FileSystem &FS,
+    unsigned excludeBufferID, llvm::sys::TimePoint<> timestamp,
+    llvm::StringMap<llvm::hash_code> &Map) {
+
+  return forEachDependencyUntilTrue(
+      CI, CurrentModule, excludeBufferID, [&](StringRef filePath) {
+        auto stat = FS.status(filePath);
+        if (!stat)
+          // Missing.
+          return true;
+
+        auto lastModTime = stat->getLastModificationTime();
+        if (lastModTime > timestamp)
+          // Modified.
+          return true;
+
+        // If the last modification time is zero, this file is probably from a
+        // virtual file system. We need to check the content.
+        if (lastModTime == llvm::sys::TimePoint<>()) {
+          // Get the hash code of the last content.
+          auto oldHashEntry = Map.find(filePath);
+          if (oldHashEntry == Map.end())
+            // Unreachable? Not virtual in old filesystem, but virtual in new
+            // one.
+            return true;
+          auto oldHash = oldHashEntry->second;
+
+          // Calculate the hash code of the current content.
+          auto newContent = FS.getBufferForFile(filePath);
+          if (!newContent)
+            // Unreachable? stat succeeded, but coundn't get the content.
+            return true;
+
+          auto newHash = llvm::hash_value(newContent.get()->getBuffer());
+
+          if (oldHash != newHash)
+            return true;
+        }
+
+        return false;
+      });
+}
+
 } // namespace
 
 bool CompletionInstance::performCachedOperationIfPossible(
     const swift::CompilerInvocation &Invocation, llvm::hash_code ArgsHash,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FileSystem,
     llvm::MemoryBuffer *completionBuffer, unsigned int Offset,
     DiagnosticConsumer *DiagC,
     llvm::function_ref<void(CompilerInstance &, bool)> Callback) {
@@ -187,10 +298,18 @@ bool CompletionInstance::performCachedOperationIfPossible(
   auto &oldInfo = oldState->getCodeCompletionDelayedDeclState();
 
   auto &SM = CI.getSourceMgr();
-  if (SM.getIdentifierForBuffer(SM.getCodeCompletionBufferID()) !=
-      completionBuffer->getBufferIdentifier())
+  auto bufferName = completionBuffer->getBufferIdentifier();
+  if (SM.getIdentifierForBuffer(SM.getCodeCompletionBufferID()) != bufferName)
     return false;
 
+  if (shouldCheckDependencies()) {
+    if (areAnyDependentFilesInvalidated(
+            CI, CurrentModule, *FileSystem, SM.getCodeCompletionBufferID(),
+            DependencyCheckedTimestamp, InMemoryDependencyHash))
+      return false;
+    DependencyCheckedTimestamp = std::chrono::system_clock::now();
+  }
+
   // Parse the new buffer into temporary SourceFile.
   SourceManager tmpSM;
   auto tmpBufferID = tmpSM.addMemBufferCopy(completionBuffer);
@@ -263,8 +382,7 @@ bool CompletionInstance::performCachedOperationIfPossible(
         completionBuffer->getBuffer().slice(startOffset, endOffset);
     auto newOffset = Offset - startOffset;
 
-    newBufferID = SM.addMemBufferCopy(sourceText,
-                                      completionBuffer->getBufferIdentifier());
+    newBufferID = SM.addMemBufferCopy(sourceText, bufferName);
     SM.openVirtualFile(SM.getLocForBufferStart(newBufferID),
                        tmpSM.getDisplayNameForLoc(startLoc),
                        tmpSM.getLineAndColumn(startLoc).first - 1);
@@ -310,8 +428,7 @@ bool CompletionInstance::performCachedOperationIfPossible(
       endOffset = tmpSM.getLocOffsetInBuffer(endLoc, tmpBufferID);
       sourceText = sourceText.slice(0, endOffset);
     }
-    newBufferID = SM.addMemBufferCopy(sourceText,
-                                      completionBuffer->getBufferIdentifier());
+    newBufferID = SM.addMemBufferCopy(sourceText, bufferName);
     SM.setCodeCompletionPoint(newBufferID, Offset);
 
     // Create a new module and a source file using the current AST context.
@@ -331,6 +448,7 @@ bool CompletionInstance::performCachedOperationIfPossible(
     performImportResolution(*newSF);
     bindExtensions(*newSF);
 
+    CurrentModule = newM;
     traceDC = newM;
 #ifndef NDEBUG
     const auto *reparsedState = newSF->getDelayedParserState();
@@ -357,6 +475,8 @@ bool CompletionInstance::performCachedOperationIfPossible(
   }
 
   CachedReuseCount += 1;
+  cacheDependencyHashIfNeeded(CI, CurrentModule, SM.getCodeCompletionBufferID(),
+                              InMemoryDependencyHash);
 
   return true;
 }
@@ -369,7 +489,15 @@ bool CompletionInstance::performNewOperation(
     llvm::function_ref<void(CompilerInstance &, bool)> Callback) {
   llvm::PrettyStackTraceString trace("While performing new completion");
 
+  auto isCachedCompletionRequested = ArgsHash.hasValue();
+
   auto TheInstance = std::make_unique<CompilerInstance>();
+
+  // Track dependencies in fast-completion mode to invalidate the compiler
+  // instance if any dependent files are modified.
+  if (isCachedCompletionRequested)
+    TheInstance->createDependencyTracker(false);
+
   {
     auto &CI = *TheInstance;
     if (DiagC)
@@ -407,15 +535,41 @@ bool CompletionInstance::performNewOperation(
     Callback(CI, /*reusingASTContext=*/false);
   }
 
-  if (ArgsHash.hasValue()) {
-    CachedCI = std::move(TheInstance);
-    CachedArgHash = *ArgsHash;
-    CachedReuseCount = 0;
-  }
+  // Cache the compiler instance if fast completion is enabled.
+  if (isCachedCompletionRequested)
+    cacheCompilerInstance(std::move(TheInstance), *ArgsHash);
 
   return true;
 }
 
+void CompletionInstance::cacheCompilerInstance(
+    std::unique_ptr<CompilerInstance> CI, llvm::hash_code ArgsHash) {
+  CachedCI = std::move(CI);
+  CurrentModule = CachedCI->getMainModule();
+  CachedArgHash = ArgsHash;
+  auto now = std::chrono::system_clock::now();
+  DependencyCheckedTimestamp = now;
+  CachedReuseCount = 0;
+  InMemoryDependencyHash.clear();
+  cacheDependencyHashIfNeeded(
+      *CachedCI, CurrentModule,
+      CachedCI->getASTContext().SourceMgr.getCodeCompletionBufferID(),
+      InMemoryDependencyHash);
+}
+
+bool CompletionInstance::shouldCheckDependencies() const {
+  assert(CachedCI);
+  using namespace std::chrono;
+  auto now = system_clock::now();
+  return DependencyCheckedTimestamp + seconds(DependencyCheckIntervalSecond) <
+         now;
+}
+
+void CompletionInstance::setDependencyCheckIntervalSecond(unsigned Value) {
+  std::lock_guard<std::mutex> lock(mtx);
+  DependencyCheckIntervalSecond = Value;
+}
+
 bool swift::ide::CompletionInstance::performOperation(
     swift::CompilerInvocation &Invocation, llvm::ArrayRef<const char *> Args,
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FileSystem,
@@ -451,8 +605,9 @@ bool swift::ide::CompletionInstance::performOperation(
     // the cached completion instance.
     std::lock_guard<std::mutex> lock(mtx);
 
-    if (performCachedOperationIfPossible(Invocation, ArgsHash, completionBuffer,
-                                         Offset, DiagC, Callback))
+    if (performCachedOperationIfPossible(Invocation, ArgsHash, FileSystem,
+                                         completionBuffer, Offset, DiagC,
+                                         Callback))
       return true;
 
     if (performNewOperation(ArgsHash, Invocation, FileSystem, completionBuffer,
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Headers/ClangFW.h b/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Headers/ClangFW.h
new file mode 100644
index 0000000000000..3eab67881eee9
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Headers/ClangFW.h
@@ -0,0 +1,2 @@
+
+#import <ClangFW/Funcs.h>
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Headers/Funcs.h b/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Headers/Funcs.h
new file mode 100644
index 0000000000000..d2a3d166d5c15
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Headers/Funcs.h
@@ -0,0 +1,4 @@
+#ifndef CLANGFW_FUNCS_H
+#define CLANGFW_FUNCS_H
+int clangFWFunc();
+#endif
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Modules/module.modulemap b/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Modules/module.modulemap
new file mode 100644
index 0000000000000..c1720d160f034
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework/Modules/module.modulemap
@@ -0,0 +1,4 @@
+framework module ClangFW {
+  umbrella header "ClangFW.h"
+  export *
+}
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework_mod/Headers/Funcs.h b/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework_mod/Headers/Funcs.h
new file mode 100644
index 0000000000000..09c4450451ecd
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/ClangFW.framework_mod/Headers/Funcs.h
@@ -0,0 +1,5 @@
+#ifndef CLANGFW_FUNCS_H
+#define CLANGFW_FUNCS_H
+int clangFWFunc_mod();
+#endif
+
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/Bridging.h b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/Bridging.h
new file mode 100644
index 0000000000000..aebbbe5cf5564
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/Bridging.h
@@ -0,0 +1 @@
+#import "LocalCFunc.h"
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/Library.swift b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/Library.swift
new file mode 100644
index 0000000000000..7e27f53052063
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/Library.swift
@@ -0,0 +1,5 @@
+func localSwiftFunc() -> Int {}
+
+struct MyStruct {
+  func myStructMethod() {}
+}
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/LibraryExt.swift b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/LibraryExt.swift
new file mode 100644
index 0000000000000..67919b0f97b56
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/LibraryExt.swift
@@ -0,0 +1,3 @@
+extension MyStruct {
+  func extensionMethod() {}
+}
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/LocalCFunc.h b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/LocalCFunc.h
new file mode 100644
index 0000000000000..12daa10870eaf
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject/LocalCFunc.h
@@ -0,0 +1,6 @@
+#ifndef MYPROJECT_LOCALCFUNC_H
+#define MYPROJECT_LOCALCFUNC_H
+
+int localClangFunc();
+
+#endif
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject_mod/Library.swift b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject_mod/Library.swift
new file mode 100644
index 0000000000000..1e071aa1cc52b
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject_mod/Library.swift
@@ -0,0 +1,5 @@
+func localSwiftFunc_mod() -> Int {}
+
+struct MyStruct {
+  func myStructMethod_mod() {}
+}
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject_mod/LocalCFunc.h b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject_mod/LocalCFunc.h
new file mode 100644
index 0000000000000..65ee9fd3abb10
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/MyProject_mod/LocalCFunc.h
@@ -0,0 +1,7 @@
+#ifndef MYPROJECT_LOCALCFUNC_H
+#define MYPROJECT_LOCALCFUNC_H
+
+int localClangFunc_mod();
+
+#endif
+
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/SwiftFW_src/Funcs.swift b/test/SourceKit/CodeComplete/Inputs/checkdeps/SwiftFW_src/Funcs.swift
new file mode 100644
index 0000000000000..6ff2c2e73ee0f
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/SwiftFW_src/Funcs.swift
@@ -0,0 +1,2 @@
+public func swiftFWFunc() -> Int { return 1 }
+
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/SwiftFW_src_mod/Funcs.swift b/test/SourceKit/CodeComplete/Inputs/checkdeps/SwiftFW_src_mod/Funcs.swift
new file mode 100644
index 0000000000000..98800199b2574
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/SwiftFW_src_mod/Funcs.swift
@@ -0,0 +1 @@
+public func swiftFWFunc_mod() -> Int { return 1 }
diff --git a/test/SourceKit/CodeComplete/Inputs/checkdeps/test.swift b/test/SourceKit/CodeComplete/Inputs/checkdeps/test.swift
new file mode 100644
index 0000000000000..54e03f90fdef0
--- /dev/null
+++ b/test/SourceKit/CodeComplete/Inputs/checkdeps/test.swift
@@ -0,0 +1,5 @@
+import Foundation
+import ClangFW // < 500 headers framework
+func foo() {
+  /* HERE */
+}
diff --git a/test/SourceKit/CodeComplete/complete_checkdeps_bridged.swift b/test/SourceKit/CodeComplete/complete_checkdeps_bridged.swift
new file mode 100644
index 0000000000000..2675887fc0dbf
--- /dev/null
+++ b/test/SourceKit/CodeComplete/complete_checkdeps_bridged.swift
@@ -0,0 +1,74 @@
+import ClangFW
+import SwiftFW
+
+func foo() {
+  /*HERE*/
+}
+
+// REQUIRES: shell
+
+// RUN: %empty-directory(%t/Frameworks)
+// RUN: %empty-directory(%t/MyProject)
+
+// RUN: COMPILER_ARGS=( \
+// RUN:   -target %target-triple \
+// RUN:   -module-name MyProject \
+// RUN:   -F %t/Frameworks \
+// RUN:   -I %t/MyProject \
+// RUN:   -import-objc-header %t/MyProject/Bridging.h \
+// RUN:   %t/MyProject/Library.swift \
+// RUN:   %s \
+// RUN: )
+// RUN: INPUT_DIR=%S/Inputs/checkdeps
+// RUN: DEPCHECK_INTERVAL=1
+// RUN: SLEEP_TIME=2
+
+// RUN: cp -R $INPUT_DIR/MyProject %t/
+// RUN: cp -R $INPUT_DIR/ClangFW.framework %t/Frameworks/
+// RUN: %empty-directory(%t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule)
+// RUN: %target-swift-frontend -emit-module -module-name SwiftFW -o %t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule/%target-swiftmodule-name $INPUT_DIR/SwiftFW_src/Funcs.swift
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=global-config -completion-check-dependency-interval ${DEPCHECK_INTERVAL} == \
+
+// RUN:   -shell -- echo "### Initial" == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} == \
+
+// RUN:   -shell -- echo '### Modify bridging header library file' == \
+// RUN:   -shell -- cp -R $INPUT_DIR/MyProject_mod/LocalCFunc.h %t/MyProject/ == \
+// RUN:   -shell -- sleep $SLEEP_TIME == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} == \
+
+// RUN:   -shell -- echo '### Fast completion' == \
+// RUN:   -shell -- sleep $SLEEP_TIME == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} \
+
+// RUN:   | %FileCheck %s
+
+
+// CHECK-LABEL: ### Initial
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Modify bridging header library file
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc_mod()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Fast completion
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc_mod()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK: key.reusingastcontext: 1
diff --git a/test/SourceKit/CodeComplete/complete_checkdeps_clangmodule.swift b/test/SourceKit/CodeComplete/complete_checkdeps_clangmodule.swift
new file mode 100644
index 0000000000000..68965b5e9a97d
--- /dev/null
+++ b/test/SourceKit/CodeComplete/complete_checkdeps_clangmodule.swift
@@ -0,0 +1,74 @@
+import ClangFW
+import SwiftFW
+
+func foo() {
+  /*HERE*/
+}
+
+// REQUIRES: shell
+
+// RUN: %empty-directory(%t/Frameworks)
+// RUN: %empty-directory(%t/MyProject)
+
+// RUN: COMPILER_ARGS=( \
+// RUN:   -target %target-triple \
+// RUN:   -module-name MyProject \
+// RUN:   -F %t/Frameworks \
+// RUN:   -I %t/MyProject \
+// RUN:   -import-objc-header %t/MyProject/Bridging.h \
+// RUN:   %t/MyProject/Library.swift \
+// RUN:   %s \
+// RUN: )
+// RUN: INPUT_DIR=%S/Inputs/checkdeps
+// RUN: DEPCHECK_INTERVAL=1
+// RUN: SLEEP_TIME=2
+
+// RUN: cp -R $INPUT_DIR/MyProject %t/
+// RUN: cp -R $INPUT_DIR/ClangFW.framework %t/Frameworks/
+// RUN: %empty-directory(%t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule)
+// RUN: %target-swift-frontend -emit-module -module-name SwiftFW -o %t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule/%target-swiftmodule-name $INPUT_DIR/SwiftFW_src/Funcs.swift
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=global-config -completion-check-dependency-interval ${DEPCHECK_INTERVAL} == \
+
+// RUN:   -shell -- echo "### Initial" == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} == \
+
+// RUN:   -shell -- echo '### Modify framework (c)' == \
+// RUN:   -shell -- cp -R $INPUT_DIR/ClangFW.framework_mod/* %t/Frameworks/ClangFW.framework/ == \
+// RUN:   -shell -- sleep $SLEEP_TIME == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} == \
+
+// RUN:   -shell -- echo '### Fast completion' == \
+// RUN:   -shell -- sleep $SLEEP_TIME == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} \
+
+// RUN:   | %FileCheck %s
+
+
+// CHECK-LABEL: ### Initial
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Modify framework (c)
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc_mod()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Fast completion
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc_mod()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK: key.reusingastcontext: 1
diff --git a/test/SourceKit/CodeComplete/complete_checkdeps_otherfile.swift b/test/SourceKit/CodeComplete/complete_checkdeps_otherfile.swift
new file mode 100644
index 0000000000000..d9d6f90daf3d6
--- /dev/null
+++ b/test/SourceKit/CodeComplete/complete_checkdeps_otherfile.swift
@@ -0,0 +1,73 @@
+import ClangFW
+import SwiftFW
+
+func foo() {
+  /*HERE*/
+}
+
+// REQUIRES: shell
+
+// RUN: %empty-directory(%t/Frameworks)
+// RUN: %empty-directory(%t/MyProject)
+
+// RUN: COMPILER_ARGS=( \
+// RUN:   -target %target-triple \
+// RUN:   -module-name MyProject \
+// RUN:   -F %t/Frameworks \
+// RUN:   -I %t/MyProject \
+// RUN:   -import-objc-header %t/MyProject/Bridging.h \
+// RUN:   %t/MyProject/Library.swift \
+// RUN:   %s \
+// RUN: )
+// RUN: INPUT_DIR=%S/Inputs/checkdeps
+// RUN: DEPCHECK_INTERVAL=1
+// RUN: SLEEP_TIME=2
+
+// RUN: cp -R $INPUT_DIR/MyProject %t/
+// RUN: cp -R $INPUT_DIR/ClangFW.framework %t/Frameworks/
+// RUN: %empty-directory(%t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule)
+// RUN: %target-swift-frontend -emit-module -module-name SwiftFW -o %t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule/%target-swiftmodule-name $INPUT_DIR/SwiftFW_src/Funcs.swift
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=global-config -completion-check-dependency-interval ${DEPCHECK_INTERVAL} == \
+
+// RUN:   -shell -- echo "### Initial" == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} == \
+
+// RUN:   -shell -- echo "### Modify local library file" == \
+// RUN:   -shell -- cp -R $INPUT_DIR/MyProject_mod/Library.swift %t/MyProject/ == \
+// RUN:   -shell -- sleep $SLEEP_TIME == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} == \
+
+// RUN:   -shell -- echo '### Fast completion' == \
+// RUN:   -shell -- sleep $SLEEP_TIME == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} \
+
+// RUN:   | %FileCheck %s
+
+// CHECK-LABEL: ### Initial
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Modify local library file
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc_mod()"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Fast completion
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc_mod()"
+// CHECK: ]
+// CHECK: key.reusingastcontext: 1
diff --git a/test/SourceKit/CodeComplete/complete_checkdeps_swiftmodule.swift b/test/SourceKit/CodeComplete/complete_checkdeps_swiftmodule.swift
new file mode 100644
index 0000000000000..30cbd7b97a37f
--- /dev/null
+++ b/test/SourceKit/CodeComplete/complete_checkdeps_swiftmodule.swift
@@ -0,0 +1,73 @@
+import ClangFW
+import SwiftFW
+
+func foo() {
+  /*HERE*/
+}
+
+// REQUIRES: shell
+
+// RUN: %empty-directory(%t/Frameworks)
+// RUN: %empty-directory(%t/MyProject)
+
+// RUN: COMPILER_ARGS=( \
+// RUN:   -target %target-triple \
+// RUN:   -module-name MyProject \
+// RUN:   -F %t/Frameworks \
+// RUN:   -I %t/MyProject \
+// RUN:   -import-objc-header %t/MyProject/Bridging.h \
+// RUN:   %t/MyProject/Library.swift \
+// RUN:   %s \
+// RUN: )
+// RUN: INPUT_DIR=%S/Inputs/checkdeps
+// RUN: DEPCHECK_INTERVAL=1
+// RUN: SLEEP_TIME=2
+
+// RUN: cp -R $INPUT_DIR/MyProject %t/
+// RUN: cp -R $INPUT_DIR/ClangFW.framework %t/Frameworks/
+// RUN: %empty-directory(%t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule)
+// RUN: %target-swift-frontend -emit-module -module-name SwiftFW -o %t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule/%target-swiftmodule-name $INPUT_DIR/SwiftFW_src/Funcs.swift
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=global-config -completion-check-dependency-interval ${DEPCHECK_INTERVAL} == \
+
+// RUN:   -shell -- echo "### Initial" == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} == \
+
+// RUN:   -shell -- echo '### Modify framework (s)' == \
+// RUN:   -shell --  %target-swift-frontend -emit-module -module-name SwiftFW -o %t/Frameworks/SwiftFW.framework/Modules/SwiftFW.swiftmodule/%target-swiftmodule-name $INPUT_DIR/SwiftFW_src_mod/Funcs.swift == \
+// RUN:   -shell -- sleep $SLEEP_TIME == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} == \
+
+// RUN:   -shell -- echo '### Fast completion' == \
+// RUN:   -shell -- sleep $SLEEP_TIME == \
+// RUN:   -req=complete -pos=5:3 %s -- ${COMPILER_ARGS[@]} \
+
+// RUN:   | %FileCheck %s
+
+// CHECK-LABEL: ### Initial
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Modify framework (s)
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc_mod()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Fast completion
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "clangFWFunc()"
+// CHECK-DAG: key.description: "swiftFWFunc_mod()"
+// CHECK-DAG: key.description: "localClangFunc()"
+// CHECK-DAG: key.description: "localSwiftFunc()"
+// CHECK: ]
+// CHECK: key.reusingastcontext: 1
diff --git a/test/SourceKit/CodeComplete/complete_checkdeps_vfs.swift b/test/SourceKit/CodeComplete/complete_checkdeps_vfs.swift
new file mode 100644
index 0000000000000..388c09e57af33
--- /dev/null
+++ b/test/SourceKit/CodeComplete/complete_checkdeps_vfs.swift
@@ -0,0 +1,52 @@
+func foo(value: MyStruct) {
+  value./*HERE*/
+}
+
+// REQUIRES: shell
+
+// RUN: DEPCHECK_INTERVAL=1
+// RUN: SLEEP_TIME=2
+
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/VFS)
+// RUN: cp %S/Inputs/checkdeps/MyProject/LibraryExt.swift %t/VFS/
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=global-config -completion-check-dependency-interval ${DEPCHECK_INTERVAL} == \
+
+// RUN:   -shell -- echo "### Initial" == \
+// RUN:   -req=complete -pos=2:9 -pass-as-sourcetext -vfs-files=%t/VFS/Main.swift=@%s,%t/VFS/Library.swift=@%S/Inputs/checkdeps/MyProject/Library.swift %t/VFS/Main.swift -- -target %target-triple %t/VFS/Main.swift %t/VFS/LibraryExt.swift %t/VFS/Library.swift == \
+
+// RUN:   -shell -- echo "### Modify" == \
+// RUN:   -shell -- sleep ${SLEEP_TIME} == \
+// RUN:   -req=complete -pos=2:9 -pass-as-sourcetext -vfs-files=%t/VFS/Main.swift=@%s,%t/VFS/Library.swift=@%S/Inputs/checkdeps/MyProject_mod/Library.swift %t/VFS/Main.swift -- -target %target-triple %t/VFS/Main.swift %t/VFS/LibraryExt.swift %t/VFS/Library.swift == \
+
+// RUN:   -shell -- echo "### Keep" == \
+// RUN:   -shell -- sleep ${SLEEP_TIME} == \
+// RUN:   -req=complete -pos=2:9 -pass-as-sourcetext -vfs-files=%t/VFS/Main.swift=@%s,%t/VFS/Library.swift=@%S/Inputs/checkdeps/MyProject_mod/Library.swift %t/VFS/Main.swift -- -target %target-triple %t/VFS/Main.swift %t/VFS/LibraryExt.swift %t/VFS/Library.swift \
+
+// RUN:   |  %FileCheck %s
+
+// CHECK-LABEL: ### Initial
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "myStructMethod()"
+// CHECK-DAG: key.description: "extensionMethod()"
+// CHECK-DAG: key.description: "self"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Modify
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "myStructMethod_mod()"
+// CHECK-DAG: key.description: "extensionMethod()"
+// CHECK-DAG: key.description: "self"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Keep
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "myStructMethod_mod()"
+// CHECK-DAG: key.description: "extensionMethod()"
+// CHECK-DAG: key.description: "self"
+// CHECK: ]
+// CHECK: key.reusingastcontext: 1
diff --git a/test/SourceKit/CodeComplete/complete_checkdeps_vfs_open.swift b/test/SourceKit/CodeComplete/complete_checkdeps_vfs_open.swift
new file mode 100644
index 0000000000000..1a8916dc0aceb
--- /dev/null
+++ b/test/SourceKit/CodeComplete/complete_checkdeps_vfs_open.swift
@@ -0,0 +1,54 @@
+func foo(value: MyStruct) {
+  value./*HERE*/
+}
+
+// REQUIRES: shell
+
+// RUN: DEPCHECK_INTERVAL=1
+// RUN: SLEEP_TIME=2
+
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/VFS)
+// RUN: cp %S/Inputs/checkdeps/MyProject/LibraryExt.swift %t/VFS/
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=global-config -completion-check-dependency-interval ${DEPCHECK_INTERVAL} == \
+
+// RUN:   -shell -- echo "### Initial" == \
+// RUN:   -req=complete.open -pos=2:9 -pass-as-sourcetext -vfs-files=%t/VFS/Main.swift=@%s,%t/VFS/Library.swift=@%S/Inputs/checkdeps/MyProject/Library.swift %t/VFS/Main.swift -- -target %target-triple %t/VFS/Main.swift %t/VFS/LibraryExt.swift %t/VFS/Library.swift == \
+// RUN:   -req=complete.close -pos=2:9 -name %t/VFS/Main.swift %s == \
+
+// RUN:   -shell -- echo "### Modify" == \
+// RUN:   -shell -- sleep ${SLEEP_TIME} == \
+// RUN:   -req=complete.open -pos=2:9 -pass-as-sourcetext -vfs-files=%t/VFS/Main.swift=@%s,%t/VFS/Library.swift=@%S/Inputs/checkdeps/MyProject_mod/Library.swift %t/VFS/Main.swift -- -target %target-triple %t/VFS/Main.swift %t/VFS/LibraryExt.swift %t/VFS/Library.swift == \
+// RUN:   -req=complete.close -pos=2:9 -name %t/VFS/Main.swift %s == \
+
+// RUN:   -shell -- echo "### Keep" == \
+// RUN:   -req=complete.open -pos=2:9 -pass-as-sourcetext -vfs-files=%t/VFS/Main.swift=@%s,%t/VFS/Library.swift=@%S/Inputs/checkdeps/MyProject_mod/Library.swift %t/VFS/Main.swift -- -target %target-triple %t/VFS/Main.swift %t/VFS/LibraryExt.swift %t/VFS/Library.swift == \
+// RUN:   -req=complete.close -pos=2:9 -name %t/VFS/Main.swift %s \
+
+// RUN:   | tee %t/trace | %FileCheck %s
+
+// CHECK-LABEL: ### Initial
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "myStructMethod()"
+// CHECK-DAG: key.description: "extensionMethod()"
+// CHECK-DAG: key.description: "self"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Modify
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "myStructMethod_mod()"
+// CHECK-DAG: key.description: "extensionMethod()"
+// CHECK-DAG: key.description: "self"
+// CHECK: ]
+// CHECK-NOT: key.reusingastcontext: 1
+
+// CHECK-LABEL: ### Keep
+// CHECK: key.results: [
+// CHECK-DAG: key.description: "myStructMethod_mod()"
+// CHECK-DAG: key.description: "extensionMethod()"
+// CHECK-DAG: key.description: "self"
+// CHECK: ]
+// CHECK: key.reusingastcontext: 1
diff --git a/tools/SourceKit/include/SourceKit/Core/Context.h b/tools/SourceKit/include/SourceKit/Core/Context.h
index 056e727fe7e2e..cbfc287320924 100644
--- a/tools/SourceKit/include/SourceKit/Core/Context.h
+++ b/tools/SourceKit/include/SourceKit/Core/Context.h
@@ -36,6 +36,9 @@ class GlobalConfig {
     ///
     /// At the time of writing this just means ignoring .swiftsourceinfo files.
     bool OptimizeForIDE = false;
+
+    /// Interval second for checking dependencies in fast code completion.
+    unsigned CompletionCheckDependencyInterval = 5;
   };
 
 private:
@@ -43,8 +46,10 @@ class GlobalConfig {
   mutable llvm::sys::Mutex Mtx;
 
 public:
-  Settings update(Optional<bool> OptimizeForIDE);
+  Settings update(Optional<bool> OptimizeForIDE,
+                  Optional<unsigned> CompletionCheckDependencyInterval);
   bool shouldOptimizeForIDE() const;
+  unsigned getCompletionCheckDependencyInterval() const;
 };
 
 class Context {
diff --git a/tools/SourceKit/include/SourceKit/Core/LangSupport.h b/tools/SourceKit/include/SourceKit/Core/LangSupport.h
index e7c5707da4305..cb3b13d3fabd5 100644
--- a/tools/SourceKit/include/SourceKit/Core/LangSupport.h
+++ b/tools/SourceKit/include/SourceKit/Core/LangSupport.h
@@ -37,6 +37,7 @@ class SourceFileSyntax;
 } // namespace swift
 
 namespace SourceKit {
+class GlobalConfig;
 
 struct EntityInfo {
   UIdent Kind;
@@ -145,7 +146,7 @@ class CodeCompletionConsumer {
   virtual void failed(StringRef ErrDescription) = 0;
 
   virtual void setCompletionKind(UIdent kind) {};
-  virtual void setReusingASTContext(bool) {};
+  virtual void setReusingASTContext(bool) = 0;
   virtual bool handleResult(const CodeCompletionInfo &Info) = 0;
 };
 
@@ -651,6 +652,8 @@ class LangSupport {
 
   virtual ~LangSupport() { }
 
+  virtual void globalConfigurationUpdated(std::shared_ptr<GlobalConfig> Config) {};
+
   virtual void indexSource(StringRef Filename,
                            IndexingConsumer &Consumer,
                            ArrayRef<const char *> Args) = 0;
diff --git a/tools/SourceKit/lib/Core/Context.cpp b/tools/SourceKit/lib/Core/Context.cpp
index 65a245283bd46..d93c510a85f7a 100644
--- a/tools/SourceKit/lib/Core/Context.cpp
+++ b/tools/SourceKit/lib/Core/Context.cpp
@@ -17,10 +17,13 @@
 using namespace SourceKit;
 
 GlobalConfig::Settings
-GlobalConfig::update(Optional<bool> OptimizeForIDE) {
+GlobalConfig::update(Optional<bool> OptimizeForIDE,
+                     Optional<unsigned> CompletionCheckDependencyInterval) {
   llvm::sys::ScopedLock L(Mtx);
   if (OptimizeForIDE.hasValue())
     State.OptimizeForIDE = *OptimizeForIDE;
+  if (CompletionCheckDependencyInterval.hasValue())
+    State.CompletionCheckDependencyInterval = *CompletionCheckDependencyInterval;
   return State;
 };
 
@@ -28,6 +31,10 @@ bool GlobalConfig::shouldOptimizeForIDE() const {
   llvm::sys::ScopedLock L(Mtx);
   return State.OptimizeForIDE;
 }
+unsigned GlobalConfig::getCompletionCheckDependencyInterval() const {
+  llvm::sys::ScopedLock L(Mtx);
+  return State.CompletionCheckDependencyInterval;
+}
 
 SourceKit::Context::Context(StringRef RuntimeLibPath,
     llvm::function_ref<std::unique_ptr<LangSupport>(Context &)>
diff --git a/tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp b/tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp
index 1c8c8ccdb6edc..73a1e7b7aecc6 100644
--- a/tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp
+++ b/tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp
@@ -1221,6 +1221,7 @@ void SwiftLangSupport::codeCompleteOpen(
         completionKind = completionCtx.CodeCompletionKind;
         typeContextKind = completionCtx.typeContextKind;
         mayUseImplicitMemberExpr = completionCtx.MayUseImplicitMemberExpr;
+        consumer.setReusingASTContext(completionCtx.ReusingASTContext);
         completions =
             extendCompletions(results, sink, info, nameToPopularity, CCOpts);
       });
diff --git a/tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp b/tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp
index fcb4b4c5eadea..373306076dbaf 100644
--- a/tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp
+++ b/tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp
@@ -257,6 +257,13 @@ class InMemoryFileSystemProvider: public SourceKit::FileSystemProvider {
 };
 }
 
+static void
+configureCompletionInstance(std::shared_ptr<CompletionInstance> CompletionInst,
+                            std::shared_ptr<GlobalConfig> GlobalConfig) {
+  CompletionInst->setDependencyCheckIntervalSecond(
+      GlobalConfig->getCompletionCheckDependencyInterval());
+}
+
 SwiftLangSupport::SwiftLangSupport(SourceKit::Context &SKCtx)
     : NotificationCtr(SKCtx.getNotificationCenter()),
       CCCache(new SwiftCompletionCache) {
@@ -270,7 +277,8 @@ SwiftLangSupport::SwiftLangSupport(SourceKit::Context &SKCtx)
                                              SKCtx.getGlobalConfiguration(),
                                              Stats, RuntimeResourcePath);
 
-  CompletionInst = std::make_unique<CompletionInstance>();
+  CompletionInst = std::make_shared<CompletionInstance>();
+  configureCompletionInstance(CompletionInst, SKCtx.getGlobalConfiguration());
 
   // By default, just use the in-memory cache.
   CCCache->inMemory = std::make_unique<ide::CodeCompletionCache>();
@@ -282,6 +290,11 @@ SwiftLangSupport::SwiftLangSupport(SourceKit::Context &SKCtx)
 SwiftLangSupport::~SwiftLangSupport() {
 }
 
+void SwiftLangSupport::globalConfigurationUpdated(
+    std::shared_ptr<GlobalConfig> Config) {
+  configureCompletionInstance(CompletionInst, Config);
+}
+
 UIdent SwiftLangSupport::getUIDForDecl(const Decl *D, bool IsRef) {
   return UIdentVisitor(IsRef).visit(const_cast<Decl*>(D));
 }
diff --git a/tools/SourceKit/lib/SwiftLang/SwiftLangSupport.h b/tools/SourceKit/lib/SwiftLang/SwiftLangSupport.h
index 06cba4f8b89d9..b29e59a6b665c 100644
--- a/tools/SourceKit/lib/SwiftLang/SwiftLangSupport.h
+++ b/tools/SourceKit/lib/SwiftLang/SwiftLangSupport.h
@@ -450,6 +450,8 @@ class SwiftLangSupport : public LangSupport {
   // LangSupport Interface
   //==========================================================================//
 
+  void globalConfigurationUpdated(std::shared_ptr<GlobalConfig> Config) override;
+
   void indexSource(StringRef Filename, IndexingConsumer &Consumer,
                    ArrayRef<const char *> Args) override;
 
diff --git a/tools/SourceKit/tools/sourcekitd-test/Options.td b/tools/SourceKit/tools/sourcekitd-test/Options.td
index 48b9d2101ecfa..5119a2e169470 100644
--- a/tools/SourceKit/tools/sourcekitd-test/Options.td
+++ b/tools/SourceKit/tools/sourcekitd-test/Options.td
@@ -143,6 +143,11 @@ def vfs_name : Separate<["-"], "vfs-name">,
 def optimize_for_ide : Joined<["-"], "for-ide=">,
   HelpText<"Value for the OptimizeForIde global configuration setting">;
 
+def completion_check_dependency_interval : Separate<["-"], "completion-check-dependency-interval">,
+  HelpText<"Inteval seconds for checking dependencies in fast completion">;
+def completion_check_dependency_interval_EQ : Joined<["-"], "completion-check-dependency-interval=">,
+  Alias<completion_check_dependency_interval>;
+
 def suppress_config_request : Flag<["-"], "suppress-config-request">,
   HelpText<"Suppress the default global configuration request, that is otherwise sent before any other request (except for the global-config request itself)">;
 
diff --git a/tools/SourceKit/tools/sourcekitd-test/TestOptions.cpp b/tools/SourceKit/tools/sourcekitd-test/TestOptions.cpp
index 48330f4ac9080..ee577379698a8 100644
--- a/tools/SourceKit/tools/sourcekitd-test/TestOptions.cpp
+++ b/tools/SourceKit/tools/sourcekitd-test/TestOptions.cpp
@@ -381,6 +381,19 @@ bool TestOptions::parseArgs(llvm::ArrayRef<const char *> Args) {
       break;
     }
 
+    case OPT_completion_check_dependency_interval: {
+      int64_t Value;
+      if (StringRef(InputArg->getValue()).getAsInteger(10, Value)) {
+        llvm::errs() << "error: expected number for inteval\n";
+        return true;
+      } else if (Value < 0) {
+        llvm::errs() << "error: completion-check-dependency-interval must be > 0\n";
+        return true;
+      }
+      CompletionCheckDependencyInterval = Value;
+      break;
+    }
+
     case OPT_suppress_config_request:
       SuppressDefaultConfigRequest = true;
       break;
diff --git a/tools/SourceKit/tools/sourcekitd-test/TestOptions.h b/tools/SourceKit/tools/sourcekitd-test/TestOptions.h
index 7551ab48afee5..a0c6769bca1c0 100644
--- a/tools/SourceKit/tools/sourcekitd-test/TestOptions.h
+++ b/tools/SourceKit/tools/sourcekitd-test/TestOptions.h
@@ -114,6 +114,7 @@ struct TestOptions {
   bool timeRequest = false;
   llvm::Optional<bool> OptimizeForIde;
   bool SuppressDefaultConfigRequest = false;
+  llvm::Optional<unsigned> CompletionCheckDependencyInterval;
   unsigned repeatRequest = 1;
   struct VFSFile {
     std::string path;
diff --git a/tools/SourceKit/tools/sourcekitd-test/sourcekitd-test.cpp b/tools/SourceKit/tools/sourcekitd-test/sourcekitd-test.cpp
index 748a4d4e072b8..5bbc258a859b4 100644
--- a/tools/SourceKit/tools/sourcekitd-test/sourcekitd-test.cpp
+++ b/tools/SourceKit/tools/sourcekitd-test/sourcekitd-test.cpp
@@ -548,7 +548,14 @@ static int handleTestInvocation(TestOptions Opts, TestOptions &InitOpts) {
   case SourceKitRequest::GlobalConfiguration:
     sourcekitd_request_dictionary_set_uid(Req, KeyRequest, RequestGlobalConfiguration);
     if (Opts.OptimizeForIde.hasValue())
-      sourcekitd_request_dictionary_set_int64(Req, KeyOptimizeForIDE, static_cast<int64_t>(Opts.OptimizeForIde.getValue()));
+      sourcekitd_request_dictionary_set_int64(
+          Req, KeyOptimizeForIDE,
+          static_cast<int64_t>(Opts.OptimizeForIde.getValue()));
+    if (Opts.CompletionCheckDependencyInterval.hasValue())
+      sourcekitd_request_dictionary_set_int64(
+          Req, KeyCompletionCheckDependencyInterval,
+          static_cast<int64_t>(
+              Opts.CompletionCheckDependencyInterval.getValue()));
     break;
 
   case SourceKitRequest::ProtocolVersion:
diff --git a/tools/SourceKit/tools/sourcekitd/lib/API/Requests.cpp b/tools/SourceKit/tools/sourcekitd/lib/API/Requests.cpp
index 66d465e0ffbef..739fb0b436f26 100644
--- a/tools/SourceKit/tools/sourcekitd/lib/API/Requests.cpp
+++ b/tools/SourceKit/tools/sourcekitd/lib/API/Requests.cpp
@@ -438,9 +438,21 @@ void handleRequestImpl(sourcekitd_object_t ReqObj, ResponseReceiver Rec) {
     if (!Req.getInt64(KeyOptimizeForIDE, EditorMode, true)) {
       OptimizeForIDE = EditorMode;
     }
+    Optional<unsigned> CompletionCheckDependencyInterval;
+    int64_t IntervalValue = 0;
+    if (!Req.getInt64(KeyCompletionCheckDependencyInterval,
+                      IntervalValue, /*isOptional=*/true))
+      CompletionCheckDependencyInterval = IntervalValue;
+
+    GlobalConfig::Settings UpdatedConfig = Config->update(
+        OptimizeForIDE, CompletionCheckDependencyInterval);
+
+    getGlobalContext().getSwiftLangSupport().globalConfigurationUpdated(Config);
 
-    GlobalConfig::Settings UpdatedConfig = Config->update(OptimizeForIDE);
     dict.set(KeyOptimizeForIDE, UpdatedConfig.OptimizeForIDE);
+    dict.set(KeyCompletionCheckDependencyInterval,
+             UpdatedConfig.CompletionCheckDependencyInterval);
+
     return Rec(RB.createResponse());
   }
   if (ReqUID == RequestProtocolVersion) {
@@ -2022,6 +2034,7 @@ class SKGroupedCodeCompletionConsumer : public GroupedCodeCompletionConsumer {
   void startGroup(UIdent kind, StringRef name) override;
   void endGroup() override;
   void setNextRequestStart(unsigned offset) override;
+  void setReusingASTContext(bool flag) override;
 };
 } // end anonymous namespace
 
@@ -2219,6 +2232,10 @@ void SKGroupedCodeCompletionConsumer::setNextRequestStart(unsigned offset) {
   assert(!Response.isNull());
   Response.set(KeyNextRequestStart, offset);
 }
+void SKGroupedCodeCompletionConsumer::setReusingASTContext(bool flag) {
+  if (flag)
+    RespBuilder.getDictionary().setBool(KeyReusingASTContext, flag);
+}
 
 //===----------------------------------------------------------------------===//
 // Type Context Info
diff --git a/utils/gyb_sourcekit_support/UIDs.py b/utils/gyb_sourcekit_support/UIDs.py
index 9bd7cb711ac86..d69e00ff7c62c 100644
--- a/utils/gyb_sourcekit_support/UIDs.py
+++ b/utils/gyb_sourcekit_support/UIDs.py
@@ -179,6 +179,8 @@ def __init__(self, internal_name, external_name):
     KEY('OptimizeForIDE', 'key.optimize_for_ide'),
     KEY('RequiredBystanders', 'key.required_bystanders'),
     KEY('ReusingASTContext', 'key.reusingastcontext'),
+    KEY('CompletionCheckDependencyInterval',
+        'key.completion_check_dependency_interval'),
 ]