diff --git a/.changeset/tidy-mcp-lifecycle.md b/.changeset/tidy-mcp-lifecycle.md
new file mode 100644
index 0000000..0714c86
--- /dev/null
+++ b/.changeset/tidy-mcp-lifecycle.md
@@ -0,0 +1,5 @@
+---
+"posthog-ruby": patch
+---
+
+Fix instance tracking for client subclasses so `PostHog::MCP::Client` can initialize outside test mode and shut down without raising. Clients that skip registration no longer remove another client's duplicate-instance warning on shutdown.
diff --git a/lib/posthog/client.rb b/lib/posthog/client.rb
index 73e21b7..17eae40 100644
--- a/lib/posthog/client.rb
+++ b/lib/posthog/client.rb
@@ -187,7 +187,8 @@ def initialize(opts = {})
 
       # Warn when multiple clients are created with the same API key (can cause dropped events)
       unless @disabled || opts[:test_mode] || opts[:disable_singleton_warning]
-        previous_count = self.class._increment_instance_count(@api_key)
+        previous_count = PostHog::Client._increment_instance_count(@api_key)
+        @instance_registered = true
         if previous_count >= 1
           logger.warn(
             'Multiple PostHog client instances detected for the same API key. ' \
@@ -947,7 +948,7 @@ def shutdown(timeout: nil)
 
       flushed = false
       begin
-        self.class._decrement_instance_count(@api_key) unless @disabled
+        PostHog::Client._decrement_instance_count(@api_key) if @instance_registered
         @feature_flags_poller&.shutdown_poller
         flushed =
           if @sync_mode
diff --git a/spec/posthog/mcp/client_spec.rb b/spec/posthog/mcp/client_spec.rb
index a371e3d..8f68f78 100644
--- a/spec/posthog/mcp/client_spec.rb
+++ b/spec/posthog/mcp/client_spec.rb
@@ -3,6 +3,45 @@
 require_relative 'spec_helper'
 
 RSpec.describe PostHog::MCP::Client do
+  describe 'lifecycle' do
+    [PostHog::Client, described_class].each do |client_class|
+      [{ test_mode: true }, { disable_singleton_warning: true }].each do |options|
+        it "preserves live-client warnings after #{client_class} shuts down with #{options.inspect}" do
+          clients = []
+          logger = PostHog::Logging.logger
+          allow(logger).to receive(:warn)
+          live = PostHog::Client.new(api_key: 'phc_shared_lifecycle')
+          clients << live
+          unregistered = client_class.new({ api_key: 'phc_shared_lifecycle' }.merge(options))
+          clients << unregistered
+
+          expect(unregistered.shutdown(timeout: 2)).to be(true)
+          expect(unregistered.shutdown(timeout: 2)).to be(true)
+          clients << PostHog::Client.new(api_key: 'phc_shared_lifecycle')
+          expect(logger).to have_received(:warn).with(a_string_including('Multiple PostHog client instances')).once
+
+          clients.each { |instance| instance.shutdown(timeout: 2) }
+          clients << PostHog::Client.new(api_key: 'phc_shared_lifecycle')
+          expect(logger).to have_received(:warn).with(a_string_including('Multiple PostHog client instances')).once
+        ensure
+          clients.each { |instance| instance.shutdown(timeout: 2) }
+        end
+      end
+    end
+
+    [{ test_mode: true }, { sync_mode: true }, {}].each do |options|
+      it "constructs and shuts down with #{options.inspect}" do
+        stub_request(:post, 'https://us.i.posthog.com/batch/').to_return(status: 200, body: '{}')
+        mcp_client = described_class.new({ api_key: 'phc_lifecycle' }.merge(options))
+        mcp_client.capture_tool_call('echo', distinct_id: 'user')
+
+        expect(mcp_client.shutdown(timeout: 2)).to be(true)
+        expect(mcp_client.shutdown(timeout: 2)).to be(true)
+        expect(mcp_client.capture(event: 'after shutdown', distinct_id: 'user')).to be(false)
+      end
+    end
+  end
+
   let(:client) { described_class.new(api_key: 'phc_test', test_mode: true) }
 
   it 'captures tool calls with $lib override, anonymous distinct id and error scalars' do