diff --git a/lib/async/container/group.rb b/lib/async/container/group.rb
index 1cb9140..81466e8 100644
--- a/lib/async/container/group.rb
+++ b/lib/async/container/group.rb
@@ -100,9 +100,14 @@ def wait
 				end
 			end
 			
+			private def each_running(&block)
+				# We create a copy of the values here, in case the block modifies the running set:
+				@running.values.each(&block)
+			end
+			
 			# Perform a health check on all running processes.
 			def health_check!
-				@running.each_value do |fiber|
+				each_running do |fiber|
 					fiber.resume(:health_check!)
 				end
 			end
@@ -111,7 +116,7 @@ def health_check!
 			# This resumes the controlling fiber with an instance of {Interrupt}.
 			def interrupt
 				Console.info(self, "Sending interrupt to #{@running.size} running processes...")
-				@running.each_value do |fiber|
+				each_running do |fiber|
 					fiber.resume(Interrupt)
 				end
 			end
@@ -120,7 +125,7 @@ def interrupt
 			# This resumes the controlling fiber with an instance of {Terminate}.
 			def terminate
 				Console.info(self, "Sending terminate to #{@running.size} running processes...")
-				@running.each_value do |fiber|
+				each_running do |fiber|
 					fiber.resume(Terminate)
 				end
 			end
@@ -129,7 +134,7 @@ def terminate
 			# This resumes the controlling fiber with an instance of {Kill}.
 			def kill
 				Console.info(self, "Sending kill to #{@running.size} running processes...")
-				@running.each_value do |fiber|
+				each_running do |fiber|
 					fiber.resume(Kill)
 				end
 			end
diff --git a/releases.md b/releases.md
index 7b049b1..8dc28f7 100644
--- a/releases.md
+++ b/releases.md
@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - Fix race condition where `wait_for` could modify `@running` while it was being iterated over (`each_value`) during health checks.
+
 ## v0.27.3
 
   - Add log for starting child, including container statistics.
diff --git a/test/async/container/group.rb b/test/async/container/group.rb
new file mode 100644
index 0000000..896f0a7
--- /dev/null
+++ b/test/async/container/group.rb
@@ -0,0 +1,275 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2025, by Samuel Williams.
+
+require "async/container/group"
+require "async/container/channel"
+
+describe Async::Container::Group do
+	let(:group) {Async::Container::Group.new}
+	
+	with "#size" do
+		it "returns zero for empty group" do
+			expect(group.size).to be == 0
+		end
+		
+		it "returns the number of running processes" do
+			channel1 = Async::Container::Channel.new
+			channel2 = Async::Container::Channel.new
+			
+			fiber1 = Fiber.new {Fiber.yield}
+			fiber2 = Fiber.new {Fiber.yield}
+			
+			fiber1.resume
+			fiber2.resume
+			
+			group.running[channel1.in] = fiber1
+			group.running[channel2.in] = fiber2
+			
+			expect(group.size).to be == 2
+		end
+	end
+	
+	with "#running?" do
+		it "returns false for empty group" do
+			expect(group).not.to be(:running?)
+		end
+		
+		it "returns true when processes are running" do
+			channel = Async::Container::Channel.new
+			fiber = Fiber.new {Fiber.yield}
+			fiber.resume
+			
+			group.running[channel.in] = fiber
+			
+			expect(group).to be(:running?)
+		end
+	end
+	
+	with "#any?" do
+		it "returns false for empty group" do
+			expect(group).not.to be(:any?)
+		end
+		
+		it "returns true when processes are running" do
+			channel = Async::Container::Channel.new
+			fiber = Fiber.new {Fiber.yield}
+			fiber.resume
+			
+			group.running[channel.in] = fiber
+			
+			expect(group).to be(:any?)
+		end
+	end
+	
+	with "#empty?" do
+		it "returns true for empty group" do
+			expect(group).to be(:empty?)
+		end
+		
+		it "returns false when processes are running" do
+			channel = Async::Container::Channel.new
+			fiber = Fiber.new {Fiber.yield}
+			fiber.resume
+			
+			group.running[channel.in] = fiber
+			
+			expect(group).not.to be(:empty?)
+		end
+	end
+	
+	with "#inspect" do
+		it "provides human-readable representation" do
+			expect(group.inspect).to be =~ /Async::Container::Group/
+			expect(group.inspect).to be =~ /running=0/
+		end
+		
+		it "shows the number of running processes" do
+			channel = Async::Container::Channel.new
+			fiber = Fiber.new {Fiber.yield}
+			fiber.resume
+			
+			group.running[channel.in] = fiber
+			
+			expect(group.inspect).to be =~ /running=1/
+		end
+	end
+	
+	with "#health_check!" do
+		it "resumes all fibers with :health_check! message" do
+			messages = []
+			
+			2.times do
+				channel = Async::Container::Channel.new
+				fiber = Fiber.new do
+					result = Fiber.yield
+					messages << result
+				end
+				
+				fiber.resume
+				group.running[channel.in] = fiber
+			end
+			
+			group.health_check!
+			
+			expect(messages).to be == [:health_check!, :health_check!]
+		end
+		
+		it "does nothing for empty group" do
+			expect do
+				group.health_check!
+			end.not.to raise_exception
+		end
+	end
+	
+	with "#interrupt" do
+		it "resumes all fibers with Interrupt" do
+			messages = []
+			
+			2.times do
+				channel = Async::Container::Channel.new
+				fiber = Fiber.new do
+					result = Fiber.yield
+					messages << result
+				end
+				
+				fiber.resume
+				group.running[channel.in] = fiber
+			end
+			
+			group.interrupt
+			
+			expect(messages).to be == [Async::Container::Interrupt, Async::Container::Interrupt]
+		end
+	end
+	
+	with "#terminate" do
+		it "resumes all fibers with Terminate" do
+			messages = []
+			
+			2.times do
+				channel = Async::Container::Channel.new
+				fiber = Fiber.new do
+					result = Fiber.yield
+					messages << result
+				end
+				
+				fiber.resume
+				group.running[channel.in] = fiber
+			end
+			
+			group.terminate
+			
+			expect(messages).to be == [Async::Container::Terminate, Async::Container::Terminate]
+		end
+	end
+	
+	with "#kill" do
+		it "resumes all fibers with Kill" do
+			messages = []
+			
+			2.times do
+				channel = Async::Container::Channel.new
+				fiber = Fiber.new do
+					result = Fiber.yield
+					messages << result
+				end
+				
+				fiber.resume
+				group.running[channel.in] = fiber
+			end
+			
+			group.kill
+			
+			expect(messages).to be == [Async::Container::Kill, Async::Container::Kill]
+		end
+	end
+	
+	# Regression test for a bug where restarting a child during health check caused
+	# "RuntimeError: can't add a new key into hash during iteration"
+	# 
+	# The scenario:
+	# - A container spawns children with `restart: true` and `health_check_timeout: N`
+	# - health_check! calls @running.each_value { |fiber| fiber.resume(:health_check!) }
+	# - A resumed fiber detects health check failure and kills the child
+	# - The spawn fiber's while loop continues (restart: true) and calls wait_for with a new child
+	# - wait_for tries to add to @running while health_check! is still iterating
+	# - This used to cause: RuntimeError: can't add a new key into hash during iteration
+	it "can restart child during health_check! iteration without error" do
+		channel1 = Async::Container::Channel.new
+		channel2 = Async::Container::Channel.new
+		
+		# Simulate the spawn fiber that restarts on health check failure
+		restart = true
+		fiber = Fiber.new do
+			while restart
+				result = Fiber.yield # Wait to be resumed
+				
+				if result == :health_check!
+					# Health check failed! Simulate the restart logic:
+					# The wait_for will return (simulated by breaking from this iteration)
+					# and the while loop continues, creating a new child
+					
+					# Simulate: child.kill! happens, wait_for returns
+					# Now the while loop continues and calls wait_for with new child
+					Fiber.new do
+						group.wait_for(channel2) do |msg|
+							# New child waiting
+						end
+					end.resume
+					
+					restart = false # Only do this once for the test
+				end
+			end
+		end
+		
+		# Start the fiber and add it to @running (simulating first wait_for call)
+		fiber.resume
+		group.running[channel1.in] = fiber
+		
+		# The fix ensures this doesn't raise RuntimeError during iteration
+		expect do
+			group.health_check!
+		end.not.to raise_exception
+	end
+	
+	# Regression test with multiple children where one restarts during health check
+	it "can handle one of multiple children restarting during health check" do
+		# Create two children, both with restart capability
+		2.times do |i|
+			channel = Async::Container::Channel.new
+			
+			fiber = Fiber.new do
+				iteration = 0
+				loop do
+					iteration += 1
+					result = Fiber.yield
+					
+					# First child fails health check on first iteration
+					if i == 0 && iteration == 1 && result == :health_check!
+						# Simulate health check failure and restart:
+						# Kill the old child, create new one
+						new_channel = Async::Container::Channel.new
+						
+						# This mimics what happens in spawn's while @running loop
+						# after wait_for returns due to child being killed
+						group.wait_for(new_channel) do |msg|
+							# New child process
+						end
+						
+						break # Exit this child's loop
+					end
+				end
+			end
+			
+			fiber.resume
+			group.running[channel.in] = fiber
+		end
+		
+		# The fix ensures this doesn't raise RuntimeError when the first fiber restarts
+		expect do
+			group.health_check!
+		end.not.to raise_exception
+	end
+end