diff --git a/src/core/lib/promise/activity.h b/src/core/lib/promise/activity.h
index 98b42841820..b1d62f41419 100644
--- a/src/core/lib/promise/activity.h
+++ b/src/core/lib/promise/activity.h
@@ -364,6 +364,7 @@ class PromiseActivity final
       // Can't safely run, so ask to run later.
       wakeup_scheduler_.ScheduleWakeup(this);
     } else {
+      // Already a wakeup scheduled for later, drop ref.
       WakeupComplete();
     }
   }
diff --git a/test/core/promise/BUILD b/test/core/promise/BUILD
index 972fc677785..838f21ad1b1 100644
--- a/test/core/promise/BUILD
+++ b/test/core/promise/BUILD
@@ -278,6 +278,12 @@ grpc_proto_fuzzer(
     tags = ["no_windows"],
     uses_polling = False,
     deps = [
+        "//:activity",
+        "//:join",
+        "//:map",
+        "//:promise",
+        "//:race",
+        "//:seq",
         "//test/core/util:grpc_test_util",
     ],
 )
diff --git a/test/core/promise/promise_fuzzer.cc b/test/core/promise/promise_fuzzer.cc
index ac12411bf99..78a0da9e3a8 100644
--- a/test/core/promise/promise_fuzzer.cc
+++ b/test/core/promise/promise_fuzzer.cc
@@ -12,10 +12,306 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include "src/core/lib/promise/activity.h"
+#include "src/core/lib/promise/join.h"
+#include "src/core/lib/promise/map.h"
+#include "src/core/lib/promise/promise.h"
+#include "src/core/lib/promise/race.h"
+#include "src/core/lib/promise/seq.h"
 #include "src/libfuzzer/libfuzzer_macro.h"
 #include "test/core/promise/promise_fuzzer.pb.h"
 
 bool squelch = true;
 bool leak_check = true;
 
-DEFINE_PROTO_FUZZER(const promise_fuzzer::Msg&) {}
+namespace grpc_core {
+// Return type for infallible promises.
+// We choose this so that it's easy to construct, and will trigger asan failures
+// if misused, and is copyable.
+using IntHdl = std::shared_ptr<int>;
+
+template <typename T>
+using PromiseFactory = std::function<Promise<T>(T)>;
+
+namespace {
+class Fuzzer {
+ public:
+  void Run(const promise_fuzzer::Msg& msg) {
+    // If there's no promise we can't construct and activity and... we're done.
+    if (!msg.has_promise()) {
+      return;
+    }
+    // Construct activity.
+    activity_ = MakeActivity(
+        [msg, this] {
+          return Seq(MakePromise(msg.promise()),
+                     [] { return absl::OkStatus(); });
+        },
+        Scheduler{this},
+        [this](absl::Status status) {
+          // Must only be called once
+          GPR_ASSERT(!done_);
+          // If we became certain of the eventual status, verify it.
+          if (expected_status_.has_value()) {
+            GPR_ASSERT(status == *expected_status_);
+          }
+          // Mark ourselves done.
+          done_ = true;
+        });
+    for (int i = 0; !done_ && activity_ != nullptr && i < msg.actions_size();
+         i++) {
+      // Do some things
+      const auto& action = msg.actions(i);
+      switch (action.action_type_case()) {
+        // Force a wakeup
+        case promise_fuzzer::Action::kForceWakeup:
+          activity_->ForceWakeup();
+          break;
+        // Cancel from the outside
+        case promise_fuzzer::Action::kCancel:
+          ExpectCancelled();
+          activity_.reset();
+          break;
+        // Flush any pending wakeups
+        case promise_fuzzer::Action::kFlushWakeup:
+          if (wakeup_ != nullptr) absl::exchange(wakeup_, nullptr)();
+          break;
+        // Drop some wakeups (external system closed?)
+        case promise_fuzzer::Action::kDropWaker: {
+          int n = action.drop_waker();
+          auto v = std::move(wakers_[n]);
+          wakers_.erase(n);
+          break;
+        }
+        // Wakeup some wakeups
+        case promise_fuzzer::Action::kAwakeWaker: {
+          int n = action.awake_waker();
+          auto v = std::move(wakers_[n]);
+          wakers_.erase(n);
+          for (auto& w : v) {
+            w.Wakeup();
+          }
+          break;
+        }
+        case promise_fuzzer::Action::ACTION_TYPE_NOT_SET:
+          break;
+      }
+    }
+    ExpectCancelled();
+    activity_.reset();
+    if (wakeup_ != nullptr) absl::exchange(wakeup_, nullptr)();
+    GPR_ASSERT(done_);
+  }
+
+ private:
+  // Schedule wakeups against the fuzzer
+  struct Scheduler {
+    Fuzzer* fuzzer;
+    // Schedule a wakeup
+    template <typename ActivityType>
+    void ScheduleWakeup(ActivityType* activity) {
+      GPR_ASSERT(activity == fuzzer->activity_.get());
+      GPR_ASSERT(fuzzer->wakeup_ == nullptr);
+      fuzzer->wakeup_ = [activity]() { activity->RunScheduledWakeup(); };
+    }
+  };
+
+  // We know that if not already finished, the status when finished will be
+  // cancelled.
+  void ExpectCancelled() {
+    if (!done_ && !expected_status_.has_value()) {
+      expected_status_ = absl::CancelledError();
+    }
+  }
+
+  // Construct a promise factory from a protobuf
+  PromiseFactory<IntHdl> MakePromiseFactory(
+      const promise_fuzzer::PromiseFactory& p) {
+    switch (p.promise_factory_type_case()) {
+      case promise_fuzzer::PromiseFactory::kPromise:
+        return [p, this](IntHdl) { return MakePromise(p.promise()); };
+      case promise_fuzzer::PromiseFactory::kLast:
+        return [](IntHdl h) { return [h]() { return h; }; };
+      case promise_fuzzer::PromiseFactory::PROMISE_FACTORY_TYPE_NOT_SET:
+        break;
+    }
+    return [](IntHdl) {
+      return []() -> Poll<IntHdl> { return std::make_shared<int>(42); };
+    };
+  }
+
+  // Construct a promise from a protobuf
+  Promise<IntHdl> MakePromise(const promise_fuzzer::Promise& p) {
+    switch (p.promise_type_case()) {
+      case promise_fuzzer::Promise::kSeq:
+        switch (p.seq().promise_factories_size()) {
+          case 1:
+            return Seq(MakePromise(p.seq().first()),
+                       MakePromiseFactory(p.seq().promise_factories(0)));
+          case 2:
+            return Seq(MakePromise(p.seq().first()),
+                       MakePromiseFactory(p.seq().promise_factories(0)),
+                       MakePromiseFactory(p.seq().promise_factories(1)));
+          case 3:
+            return Seq(MakePromise(p.seq().first()),
+                       MakePromiseFactory(p.seq().promise_factories(0)),
+                       MakePromiseFactory(p.seq().promise_factories(1)),
+                       MakePromiseFactory(p.seq().promise_factories(2)));
+          case 4:
+            return Seq(MakePromise(p.seq().first()),
+                       MakePromiseFactory(p.seq().promise_factories(0)),
+                       MakePromiseFactory(p.seq().promise_factories(1)),
+                       MakePromiseFactory(p.seq().promise_factories(2)),
+                       MakePromiseFactory(p.seq().promise_factories(3)));
+          case 5:
+            return Seq(MakePromise(p.seq().first()),
+                       MakePromiseFactory(p.seq().promise_factories(0)),
+                       MakePromiseFactory(p.seq().promise_factories(1)),
+                       MakePromiseFactory(p.seq().promise_factories(2)),
+                       MakePromiseFactory(p.seq().promise_factories(3)),
+                       MakePromiseFactory(p.seq().promise_factories(4)));
+          case 6:
+            return Seq(MakePromise(p.seq().first()),
+                       MakePromiseFactory(p.seq().promise_factories(0)),
+                       MakePromiseFactory(p.seq().promise_factories(1)),
+                       MakePromiseFactory(p.seq().promise_factories(2)),
+                       MakePromiseFactory(p.seq().promise_factories(3)),
+                       MakePromiseFactory(p.seq().promise_factories(4)),
+                       MakePromiseFactory(p.seq().promise_factories(5)));
+        }
+        break;
+      case promise_fuzzer::Promise::kJoin:
+        switch (p.join().promises_size()) {
+          case 1:
+            return Map(Join(MakePromise(p.join().promises(0))),
+                       [](std::tuple<IntHdl> t) { return std::get<0>(t); });
+          case 2:
+            return Map(
+                Join(MakePromise(p.join().promises(0)),
+                     MakePromise(p.join().promises(1))),
+                [](std::tuple<IntHdl, IntHdl> t) { return std::get<0>(t); });
+          case 3:
+            return Map(Join(MakePromise(p.join().promises(0)),
+                            MakePromise(p.join().promises(1)),
+                            MakePromise(p.join().promises(2))),
+                       [](std::tuple<IntHdl, IntHdl, IntHdl> t) {
+                         return std::get<0>(t);
+                       });
+          case 4:
+            return Map(Join(MakePromise(p.join().promises(0)),
+                            MakePromise(p.join().promises(1)),
+                            MakePromise(p.join().promises(2)),
+                            MakePromise(p.join().promises(3))),
+                       [](std::tuple<IntHdl, IntHdl, IntHdl, IntHdl> t) {
+                         return std::get<0>(t);
+                       });
+          case 5:
+            return Map(
+                Join(MakePromise(p.join().promises(0)),
+                     MakePromise(p.join().promises(1)),
+                     MakePromise(p.join().promises(2)),
+                     MakePromise(p.join().promises(3)),
+                     MakePromise(p.join().promises(4))),
+                [](std::tuple<IntHdl, IntHdl, IntHdl, IntHdl, IntHdl> t) {
+                  return std::get<0>(t);
+                });
+          case 6:
+            return Map(
+                Join(MakePromise(p.join().promises(0)),
+                     MakePromise(p.join().promises(1)),
+                     MakePromise(p.join().promises(2)),
+                     MakePromise(p.join().promises(3)),
+                     MakePromise(p.join().promises(4)),
+                     MakePromise(p.join().promises(5))),
+                [](std::tuple<IntHdl, IntHdl, IntHdl, IntHdl, IntHdl, IntHdl>
+                       t) { return std::get<0>(t); });
+        }
+        break;
+      case promise_fuzzer::Promise::kRace:
+        switch (p.race().promises_size()) {
+          case 1:
+            return Race(MakePromise(p.race().promises(0)));
+          case 2:
+            return Race(MakePromise(p.race().promises(0)),
+                        MakePromise(p.race().promises(1)));
+          case 3:
+            return Race(MakePromise(p.race().promises(0)),
+                        MakePromise(p.race().promises(1)),
+                        MakePromise(p.race().promises(2)));
+          case 4:
+            return Race(MakePromise(p.race().promises(0)),
+                        MakePromise(p.race().promises(1)),
+                        MakePromise(p.race().promises(2)),
+                        MakePromise(p.race().promises(3)));
+          case 5:
+            return Race(MakePromise(p.race().promises(0)),
+                        MakePromise(p.race().promises(1)),
+                        MakePromise(p.race().promises(2)),
+                        MakePromise(p.race().promises(3)),
+                        MakePromise(p.race().promises(4)));
+          case 6:
+            return Race(MakePromise(p.race().promises(0)),
+                        MakePromise(p.race().promises(1)),
+                        MakePromise(p.race().promises(2)),
+                        MakePromise(p.race().promises(3)),
+                        MakePromise(p.race().promises(4)),
+                        MakePromise(p.race().promises(5)));
+        }
+        break;
+      case promise_fuzzer::Promise::kNever:
+        return Never<IntHdl>();
+      case promise_fuzzer::Promise::kSleepFirstN: {
+        int n = p.sleep_first_n();
+        return [n]() mutable -> Poll<IntHdl> {
+          if (n <= 0) return std::make_shared<int>(0);
+          n--;
+          return Pending{};
+        };
+      }
+      case promise_fuzzer::Promise::kCancelFromInside:
+        return [this]() -> Poll<IntHdl> {
+          this->activity_.reset();
+          return Pending{};
+        };
+      case promise_fuzzer::Promise::kWaitOnceOnWaker: {
+        bool called = false;
+        auto config = p.wait_once_on_waker();
+        return [this, config, called]() mutable -> Poll<IntHdl> {
+          if (!called) {
+            if (config.owning()) {
+              wakers_[config.waker()].push_back(
+                  Activity::current()->MakeOwningWaker());
+            } else {
+              wakers_[config.waker()].push_back(
+                  Activity::current()->MakeNonOwningWaker());
+            }
+            return Pending();
+          }
+          return std::make_shared<int>(3);
+        };
+      }
+      case promise_fuzzer::Promise::PromiseTypeCase::PROMISE_TYPE_NOT_SET:
+        break;
+    }
+    return [] { return std::make_shared<int>(42); };
+  }
+
+  // Activity under test
+  ActivityPtr activity_;
+  // Scheduled wakeup (may be nullptr if no wakeup scheduled)
+  std::function<void()> wakeup_;
+  // If we are certain of the final status, then that. Otherwise, nullopt if we
+  // don't know.
+  absl::optional<absl::Status> expected_status_;
+  // Has on_done been called?
+  bool done_ = false;
+  // Wakers that may be scheduled
+  std::map<int, std::vector<Waker>> wakers_;
+};
+}  // namespace
+
+}  // namespace grpc_core
+
+DEFINE_PROTO_FUZZER(const promise_fuzzer::Msg& msg) {
+  grpc_core::Fuzzer().Run(msg);
+}
diff --git a/test/core/promise/promise_fuzzer.proto b/test/core/promise/promise_fuzzer.proto
index 90a72582685..3b236d8df5f 100644
--- a/test/core/promise/promise_fuzzer.proto
+++ b/test/core/promise/promise_fuzzer.proto
@@ -16,5 +16,76 @@ syntax = "proto3";
 
 package promise_fuzzer;
 
+message Seq {
+    Promise first = 1;
+    repeated PromiseFactory promise_factories = 2;
+}
+
+message Join {
+    repeated Promise promises = 1;
+}
+
+message Race {
+    repeated Promise promises = 1;
+}
+
+message Last {}
+
+message PromiseFactory {
+    oneof promise_factory_type {
+        // Return a specific promise
+        Promise promise = 1;
+        // Return the result of the last thing
+        Last last = 2;
+    }
+}
+
+message Never {}
+
+message ScheduleWaker {
+  bool owning = 1;
+  int32 waker = 2;
+}
+
+message Promise {
+    oneof promise_type {
+        // Seq combinator
+        Seq seq = 1;
+        // Join combinator
+        Join join = 2;
+        // Race combinator
+        Race race = 3;
+        // Never complete
+        Never never = 4;
+        // Sleep n times, then wakeup
+        int32 sleep_first_n = 5;
+        // Cancel and be pending
+        Cancel cancel_from_inside = 6;
+        // Wait for waker n, then continue
+        ScheduleWaker wait_once_on_waker = 7;
+    }
+}
+
+message Cancel {}
+
+message Wakeup {}
+
+message Action {
+    oneof action_type {
+        // Activity::ForceWakeup
+        Wakeup force_wakeup = 1;
+        // Cancel the activity
+        Cancel cancel = 2;
+        // Flush any pending scheduled wakeups
+        Wakeup flush_wakeup = 3;
+        // Awake waker n if it exists
+        int32 awake_waker = 4;
+        // Drop waker n if it exists
+        int32 drop_waker = 5;
+    }
+}
+
 message Msg {
+    Promise promise = 1;
+    repeated Action actions = 2;
 }
diff --git a/test/core/promise/promise_fuzzer_corpus/crash-4758aec0c3e382d4fb3e398d634d8da72ed0d574 b/test/core/promise/promise_fuzzer_corpus/crash-4758aec0c3e382d4fb3e398d634d8da72ed0d574
new file mode 100644
index 00000000000..9b9d8c0f473
--- /dev/null
+++ b/test/core/promise/promise_fuzzer_corpus/crash-4758aec0c3e382d4fb3e398d634d8da72ed0d574
@@ -0,0 +1,20 @@
+promise {
+  race {
+    promises {
+      cancel_from_inside {
+      }
+    }
+    promises {
+      sleep_first_n: 0
+    }
+    promises {
+      sleep_first_n: 2097151999
+    }
+    promises {
+    }
+  }
+}
+actions {
+  flush_wakeup {
+  }
+}
diff --git a/test/core/promise/promise_fuzzer_corpus/leak-90b749c0ca79c7a11d25f4b6f0594af99fe4a78f b/test/core/promise/promise_fuzzer_corpus/leak-90b749c0ca79c7a11d25f4b6f0594af99fe4a78f
new file mode 100644
index 00000000000..623431d4c6a
--- /dev/null
+++ b/test/core/promise/promise_fuzzer_corpus/leak-90b749c0ca79c7a11d25f4b6f0594af99fe4a78f
@@ -0,0 +1,12 @@
+promise {
+  never {
+  }
+}
+actions {
+  force_wakeup {
+  }
+}
+actions {
+  force_wakeup {
+  }
+}