diff --git a/CMakeLists.txt b/CMakeLists.txt
index 52101c0..4cff0ee 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -268,6 +268,7 @@ if(TILE_BUILD_TESTS)
     target_sources(${PROJECT_NAME}_test_all PRIVATE ${test_file})
   endmacro()
 
+tile_add_test(fiber_detail_scheduler_test "tile/fiber/detail/scheduler_test.cc")
   tile_add_test(base_internal_meta_test "tile/base/internal/meta_test.cc")
   # tile_add_test(net_internal_http_engine_test
   # "tile/net/internal/http_engine_test.cc")
diff --git a/tile/base/internal/format.h b/tile/base/internal/format.h
index 80a5e6d..e42b093 100644
--- a/tile/base/internal/format.h
+++ b/tile/base/internal/format.h
@@ -26,6 +26,14 @@ template <typename T> struct has_to_string {
   static constexpr bool value = decltype(Test<T>(0))::value;
 };
 
+template <typename T> struct can_to_string {
+  template <typename U>
+  static auto Test(int) -> decltype(ToString(std::declval<U>()),
+                                    std::true_type());
+  template <typename...> static auto Test(...) -> std::false_type;
+  static constexpr bool value = decltype(Test<T>(0))::value;
+};
+
 template <typename Rep, typename Period>
 std::string format_as_impl(const std::chrono::duration<Rep, Period> &val,
                            const char *suffix) {
@@ -52,11 +60,21 @@ auto format_as(const T &val)
 template <typename T>
 auto format_as(const T &val)
     -> tile::enable_if_t<!tile::detail::is_streamable<std::ostream, T>::value &&
-                             tile::detail::has_to_string<T>::value,
+                             tile::detail::has_to_string<T>::value &&
+                             !tile::detail::can_to_string<T>::value,
                          std::string> {
   return val.ToString();
 }
 
+template <typename T>
+auto format_as(const T &val)
+    -> tile::enable_if_t<!tile::detail::is_streamable<std::ostream, T>::value &&
+                             !tile::detail::has_to_string<T>::value &&
+                             tile::detail::can_to_string<T>::value,
+                         std::string> {
+  return ToString(val);
+}
+
 template <typename T>
 auto format_as(T *ptr)
     -> tile::enable_if_t<!std::is_void<tile::remove_cvref_t<T>>::value,
diff --git a/tile/fiber/detail/fiber.cc b/tile/fiber/detail/fiber.cc
index b7b7589..51fe33f 100644
--- a/tile/fiber/detail/fiber.cc
+++ b/tile/fiber/detail/fiber.cc
@@ -1,10 +1,12 @@
 #include "tile/fiber/detail/fiber.h"
 
 #include "tile/base/align.h"
+#include "tile/base/internal/index_alloc.h"
 #include "tile/base/internal/move_on_copy.h"
 #include "tile/base/logging.h"
 #include "tile/base/make_unique.h"
 #include "tile/base/object_pool.h"
+#include "tile/base/string.h"
 
 #include "nova/context/fcontext.h"
 
@@ -12,6 +14,25 @@ namespace tile {
 namespace fiber {
 namespace detail {
 
+std::string ToString(const FiberState &state) noexcept {
+  switch (state) {
+  case FiberState::Runnable:
+    return "Runnable";
+  case FiberState::Running:
+    return "Running";
+  case FiberState::Waiting:
+    return "Waiting";
+  case FiberState::Terminated:
+    return "Terminated";
+  default:
+    TILE_UNEXPECTED("");
+    return "Unknown";
+  }
+}
+std::ostream &operator<<(std::ostream &os, const FiberState &state) noexcept {
+  return os << ToString(state);
+}
+
 static thread_local Fiber *tls_current_fiber = nullptr;
 static thread_local Fiber *tls_master_fiber = nullptr;
 
@@ -32,8 +53,14 @@ void FiberEntry(fcontext_transfer_t t) {
   try {
     // From Resume()
     t = jump_fcontext(t.fctx, nullptr);
-    self = reinterpret_cast<Fiber *>(t.data);
-    self->caller_->ctx_ = t.fctx;
+    Fiber *caller = reinterpret_cast<Fiber *>(t.data);
+
+    self = GetCurrentFiber();
+    if (caller) {
+      caller->ctx_ = t.fctx;
+    } else {
+      TILE_CHECK_EQ(self, GetMasterFiber());
+    }
 
     (*fn)();
   } catch (const std::exception &e) {
@@ -41,13 +68,16 @@ void FiberEntry(fcontext_transfer_t t) {
   }
   self->state_ = FiberState::Terminated;
 
-  if (GetMasterFiber() != GetCurrentFiber()) {
-    GetMasterFiber()->Resume();
-  } else {
+  if (GetMasterFiber() == self) {
     // master fiber end
     jump_fcontext(t.fctx, GetMasterFiber());
+  } else {
+    TILE_CHECK(GetMasterFiber() != nullptr);
+    TILE_CHECK_NE(GetMasterFiber()->state(), FiberState::Terminated);
+    GetMasterFiber()->Resume();
   }
 }
+
 fcontext_transfer_t FiberOnTop(fcontext_transfer_t t) {}
 
 fcontext_t CreateFiber(void *stack, std::size_t stack_size,
@@ -70,7 +100,8 @@ std::unique_ptr<Fiber> Fiber::Create(std::function<void()> proc) noexcept {
 }
 
 Fiber::Fiber(std::function<void()> proc)
-    : data_(object_pool::Get<FiberContext>().Leak()) {
+    : id_(internal::IndexAlloc::For<Fiber>()->Next()),
+      data_(object_pool::Get<FiberContext>().Leak()) {
   TILE_CHECK(proc);
 
   data_->proc = std::move(proc);
@@ -81,15 +112,20 @@ Fiber::~Fiber() {
   if (data_) {
     object_pool::Put<FiberContext>(data_.release());
   }
+  internal::IndexAlloc::For<Fiber>()->Free(id_);
 }
 
 void Fiber::Resume() {
   auto caller = GetCurrentFiber();
-  TILE_CHECK_NE(caller, this, "Calling `ResumeOn()`, on self is undefined.");
+  TILE_CHECK_NE(caller, this, "Calling `Resume()`, on self is undefined.");
 
-  auto t = jump_fcontext(ctx_, this);
-  caller = reinterpret_cast<Fiber *>(t.data);
-  caller->ctx_ = t.fctx;
+  SetUpCurrentFiber(this);
+
+  auto t = jump_fcontext(internal::Exchange(ctx_, nullptr), caller);
+
+  if (auto from = reinterpret_cast<Fiber *>(t.data)) {
+    from->ctx_ = t.fctx;
+  }
 
   SetUpCurrentFiber(caller);
 }
@@ -97,7 +133,21 @@ void Fiber::Resume() {
 void Fiber::ResumeOn(std::function<void()> &&cb) noexcept {
   auto caller = GetCurrentFiber();
   TILE_CHECK_NE(caller, this, "Calling `ResumeOn()`, on self is undefined.");
-  ontop_fcontext(ctx_, this, FiberOnTop);
+  SetUpCurrentFiber(this);
+  auto t = ontop_fcontext(ctx_, this, FiberOnTop);
+  caller = reinterpret_cast<Fiber *>(t.data);
+  SetUpCurrentFiber(caller);
+}
+
+std::string ToString(const Fiber &fiber) noexcept { return ToString(&fiber); }
+std::string ToString(const std::unique_ptr<Fiber> &fiber) noexcept {
+  return ToString(fiber.get());
+}
+std::string ToString(Fiber const *const fiber) noexcept {
+  if (TILE_UNLIKELY(fiber == nullptr)) {
+    return "Fiber(nullptr)";
+  }
+  return Format("Fiber({})[{}]", fiber->id(), fmt::ptr(fiber));
 }
 
 } // namespace detail
diff --git a/tile/fiber/detail/fiber.h b/tile/fiber/detail/fiber.h
index b671a4d..b775abb 100644
--- a/tile/fiber/detail/fiber.h
+++ b/tile/fiber/detail/fiber.h
@@ -22,10 +22,11 @@ class Scheduler;
 enum class FiberState {
   Runnable,
   Running,
-  Suspended,
   Waiting,
   Terminated,
 };
+std::string ToString(const FiberState &state) noexcept;
+std::ostream &operator<<(std::ostream &os, const FiberState &state) noexcept;
 
 class Scheduler;
 
@@ -46,6 +47,8 @@ public:
   void Resume();
   void ResumeOn(std::function<void()> &&cb) noexcept;
   void Yield();
+  Id id() const noexcept { return id_; }
+  FiberState state() const noexcept { return state_; }
 
   Scheduler *scheduler() const { return scheduler_; }
 
@@ -61,10 +64,14 @@ private:
   Fiber(std::function<void()> proc = nullptr);
 
 private:
+  Id id_;
+
   std::unique_ptr<FiberContext> data_;
   FiberState state_{FiberState::Runnable};
+
   void *ctx_{nullptr};
-  Fiber *caller_{nullptr};
+
+  // for scheduler
   Scheduler *scheduler_{nullptr};
 };
 Fiber *GetCurrentFiber() noexcept;
@@ -75,6 +82,10 @@ void SetUpMasterFiber(Fiber *fiber) noexcept;
 
 inline bool IsFiberContext() noexcept { return GetCurrentFiber() != nullptr; }
 
+std::string ToString(Fiber const *const fiber) noexcept;
+std::string ToString(const std::unique_ptr<Fiber> &fiber) noexcept;
+std::string ToString(const Fiber &fiber) noexcept;
+
 } // namespace detail
 } // namespace fiber
 } // namespace tile
diff --git a/tile/fiber/detail/scheduler.cc b/tile/fiber/detail/scheduler.cc
index 5788e96..b1a8dac 100644
--- a/tile/fiber/detail/scheduler.cc
+++ b/tile/fiber/detail/scheduler.cc
@@ -8,7 +8,7 @@ static thread_local Scheduler *current_scheduler = nullptr;
 void Scheduler::SwitchTo(Fiber *self, Fiber *to) {
   TILE_CHECK_EQ(self, GetCurrentFiber());
 
-  TILE_CHECK(to->state_ == FiberState::Runnable,
+  TILE_CHECK(to->state_ == FiberState::Terminated,
              "Fiber `to` is not in Runnable.");
   TILE_CHECK_NE(self, to, "Switch to yourself result in U.B.");
 
diff --git a/tile/fiber/detail/scheduler_test.cc b/tile/fiber/detail/scheduler_test.cc
new file mode 100644
index 0000000..ef128d5
--- /dev/null
+++ b/tile/fiber/detail/scheduler_test.cc
@@ -0,0 +1,55 @@
+#include "tile/fiber/detail/scheduler.h"
+
+#include "tile/base/deferred.h"
+
+#include "tile/base/random.h"
+#include "gtest/gtest.h"
+
+namespace tile {
+namespace fiber {
+namespace detail {
+
+TEST(Fiber, Resume) {
+  constexpr int kFiberCount = 1000;
+  constexpr int kSwitchCount = 100000;
+
+  std::unique_ptr<Fiber> master;
+  std::unique_ptr<Fiber> worker[kFiberCount];
+
+  int cnt = 0;
+  auto end_check = tile::Deferred([&]() { ASSERT_EQ(cnt, kSwitchCount); });
+
+  master = Fiber::Create([&]() {
+    while (cnt < kSwitchCount) {
+      ASSERT_EQ(GetCurrentFiber(), master.get());
+      worker[Random(kFiberCount - 1)]->Resume();
+      ASSERT_EQ(GetCurrentFiber(), master.get());
+    }
+
+    for (int i = 0; i != kFiberCount; ++i) {
+      worker[i]->Resume();
+    }
+  });
+  for (int i = 0; i != kFiberCount; ++i) {
+    worker[i] = Fiber::Create([&, i]() {
+      while (cnt < kSwitchCount) {
+        ++cnt;
+        ASSERT_EQ(GetCurrentFiber(), worker[i].get());
+        master->Resume();
+        ASSERT_EQ(GetCurrentFiber(), worker[i].get());
+      }
+    });
+  }
+
+  SetUpMasterFiber(master.get());
+  SetUpCurrentFiber(nullptr);
+  master->Resume();
+  ASSERT_EQ(cnt, kSwitchCount);
+  for (int i = 0; i != kFiberCount; ++i) {
+    ASSERT_EQ(worker[i]->state(), FiberState::Terminated);
+  }
+}
+
+} // namespace detail
+} // namespace fiber
+} // namespace tile
diff --git a/tile/fiber/this_fiber.cc b/tile/fiber/this_fiber.cc
index 606263a..5cd3d72 100644
--- a/tile/fiber/this_fiber.cc
+++ b/tile/fiber/this_fiber.cc
@@ -18,6 +18,7 @@ void SleepFor(std::chrono::nanoseconds expires_in) {
 Fiber::Id GetId() {
   auto self = fiber::detail::GetCurrentFiber();
   TILE_CHECK(self, "GetId() should be called in a fiber.");
+  return self->id();
 }
 } // namespace this_fiber
 } // namespace tile