diff --git a/.gitignore b/.gitignore
index ee919347..e853a217 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,3 +11,6 @@ _yardoc/
 
 # rspec failure tracking
 .rspec_status
+
+# editors
+.idea/*
\ No newline at end of file
diff --git a/lib/cadence/workflow/context.rb b/lib/cadence/workflow/context.rb
index d4dc52b6..4d42a673 100644
--- a/lib/cadence/workflow/context.rb
+++ b/lib/cadence/workflow/context.rb
@@ -12,10 +12,11 @@
 module Cadence
   class Workflow
     class Context
-      def initialize(state_manager, dispatcher, metadata)
+      def initialize(state_manager, dispatcher, metadata, history)
         @state_manager = state_manager
         @dispatcher = dispatcher
         @metadata = metadata
+        @history = history
       end
 
       def logger
@@ -33,9 +34,16 @@ def execute_activity(activity_class, *input, **args)
         input << args unless args.empty?
 
         execution_options = ExecutionOptions.new(activity_class, options)
+        
+        # Creates a unique activity id that is idempotent for this activity call in the current
+        # sequence of decision tasks
+        idem_activity_id = UUID.v5(
+            metadata.run_id,
+            activity_class.to_s + input.to_s + args.to_s + @history.events.length.to_s
+        )
 
         decision = Decision::ScheduleActivity.new(
-          activity_id: options[:activity_id],
+          activity_id: idem_activity_id,
           activity_type: execution_options.name,
           input: input,
           domain: execution_options.domain,
diff --git a/lib/cadence/workflow/executor.rb b/lib/cadence/workflow/executor.rb
index e7182203..665e3f1d 100644
--- a/lib/cadence/workflow/executor.rb
+++ b/lib/cadence/workflow/executor.rb
@@ -35,7 +35,7 @@ def run
       attr_reader :workflow_class, :dispatcher, :state_manager, :history, :new_decisions
 
       def execute_workflow(input, metadata)
-        context = Workflow::Context.new(state_manager, dispatcher, metadata)
+        context = Workflow::Context.new(state_manager, dispatcher, metadata, history)
 
         Fiber.new do
           workflow_class.execute_in_context(context, input)