diff --git a/.sampo/changesets/noop-empty-api-key.md b/.sampo/changesets/noop-empty-api-key.md
new file mode 100644
index 0000000..500efdf
--- /dev/null
+++ b/.sampo/changesets/noop-empty-api-key.md
@@ -0,0 +1,5 @@
+---
+hex/posthog: patch
+---
+
+Start in disabled/no-op mode instead of raising or sending events when the API key is missing, blank, or the supervisor is unavailable.
diff --git a/lib/posthog/application.ex b/lib/posthog/application.ex
index 23b6d24..238f1a0 100644
--- a/lib/posthog/application.ex
+++ b/lib/posthog/application.ex
@@ -8,7 +8,7 @@ defmodule PostHog.Application do
 
     children =
       if conv_config.enable do
-        if conv_config.enable_error_tracking do
+        if conv_config.enable_error_tracking and supervisor_config.enabled do
           :logger.add_handler(:posthog, PostHog.Handler, %{config: supervisor_config})
         end
 
diff --git a/lib/posthog/config.ex b/lib/posthog/config.ex
index d6da100..8bc5ccd 100644
--- a/lib/posthog/config.ex
+++ b/lib/posthog/config.ex
@@ -21,9 +21,10 @@ defmodule PostHog.Config do
                           ],
                           api_key: [
                             type: :string,
-                            required: true,
+                            default: "",
                             doc: """
                             Your PostHog Project API key. Find it in your project's settings under the Project ID section.
+                            If omitted or empty after trimming whitespace, PostHog starts in disabled/no-op mode.
                             """
                           ],
                           api_client_module: [
@@ -210,15 +211,24 @@ defmodule PostHog.Config do
 
     with {:ok, validated} <-
            NimbleOptions.validate(normalized_options, @compiled_configuration_schema) do
+      api_key_blank? = blank_api_key?(validated)
       log_blank_api_key(validated)
 
       config = Map.new(validated)
-      client = config.api_client_module.client(config.api_key, config.api_host)
+
+      client =
+        if api_key_blank? do
+          nil
+        else
+          config.api_client_module.client(config.api_key, config.api_host)
+        end
+
       global_properties = Map.merge(config.global_properties, @system_global_properties)
 
       final_config =
         config
         |> Map.put(:api_client, client)
+        |> Map.put(:enabled, not api_key_blank?)
         |> Map.put(
           :in_app_modules,
           config.in_app_otp_apps |> Enum.flat_map(&Application.spec(&1, :modules)) |> MapSet.new()
@@ -248,6 +258,7 @@ defmodule PostHog.Config do
   end
 
   defp normalize_api_key(api_key) when is_binary(api_key), do: String.trim(api_key)
+  defp normalize_api_key(nil), do: ""
   defp normalize_api_key(api_key), do: api_key
 
   defp normalize_api_host(api_host) when is_binary(api_host) do
@@ -261,10 +272,12 @@ defmodule PostHog.Config do
 
   defp normalize_api_host(api_host), do: api_host
 
+  defp blank_api_key?(validated), do: validated[:api_key] == ""
+
   defp log_blank_api_key(validated) do
-    if validated[:api_key] == "" do
-      Logger.error(
-        "posthog api_key is empty after trimming whitespace; check your project API key"
+    if blank_api_key?(validated) do
+      Logger.warning(
+        "posthog api_key is empty after trimming whitespace; PostHog will start in disabled/no-op mode"
       )
     end
   end
diff --git a/lib/posthog/feature_flags.ex b/lib/posthog/feature_flags.ex
index d6e5e36..3e97cad 100644
--- a/lib/posthog/feature_flags.ex
+++ b/lib/posthog/feature_flags.ex
@@ -29,7 +29,15 @@ defmodule PostHog.FeatureFlags do
   def flags(name \\ PostHog, body) do
     config = PostHog.config(name)
 
-    case PostHog.API.flags(config.api_client, body) do
+    if Map.get(config, :enabled, true) do
+      request_flags(config.api_client, body)
+    else
+      empty_flags_response()
+    end
+  end
+
+  defp request_flags(api_client, body) do
+    case PostHog.API.flags(api_client, body) do
       {:ok, %{status: 200, body: %{"flags" => _}}} = resp ->
         resp
 
@@ -48,6 +56,8 @@ defmodule PostHog.FeatureFlags do
     end
   end
 
+  defp empty_flags_response, do: {:ok, %{status: 200, body: %{"flags" => %{}}}}
+
   @doc false
   def flags_for(distinct_id_or_body) when not is_atom(distinct_id_or_body),
     do: flags_for(PostHog, distinct_id_or_body)
diff --git a/lib/posthog/registry.ex b/lib/posthog/registry.ex
index 8115116..0a787ba 100644
--- a/lib/posthog/registry.ex
+++ b/lib/posthog/registry.ex
@@ -1,12 +1,28 @@
 defmodule PostHog.Registry do
   @moduledoc false
   def config(supervisor_name) do
-    {:ok, config} =
-      supervisor_name
-      |> registry_name()
-      |> Registry.meta(:config)
+    registry = registry_name(supervisor_name)
 
-    config
+    if Process.whereis(registry) do
+      case Registry.meta(registry, :config) do
+        {:ok, config} -> config
+        :error -> disabled_config(supervisor_name)
+      end
+    else
+      disabled_config(supervisor_name)
+    end
+  rescue
+    ArgumentError -> disabled_config(supervisor_name)
+  end
+
+  def disabled_config(supervisor_name) do
+    %{
+      supervisor_name: supervisor_name,
+      enabled: false,
+      api_client: nil,
+      global_properties: %{},
+      test_mode: false
+    }
   end
 
   def registry_name(supervisor_name), do: Module.concat(supervisor_name, Registry)
diff --git a/lib/posthog/sender.ex b/lib/posthog/sender.ex
index 2c3ccfe..0970fa3 100644
--- a/lib/posthog/sender.ex
+++ b/lib/posthog/sender.ex
@@ -28,27 +28,44 @@ defmodule PostHog.Sender do
   # Client
 
   def send(event, supervisor_name) do
-    supervisor_name
-    |> PostHog.Registry.config()
-    |> case do
-      %{test_mode: true} ->
-        PostHog.Test.remember_event(supervisor_name, event)
-
-      _ ->
-        senders =
-          supervisor_name
-          |> PostHog.Registry.registry_name()
-          |> Registry.select([{{{__MODULE__, :_}, :"$1", :"$2"}, [], [{{:"$2", :"$1"}}]}])
-
-        # Pick the first available sender, otherwise random busy one.
-        senders
-        |> Keyword.get_lazy(:available, fn ->
-          senders |> Keyword.values() |> Enum.random()
-        end)
-        |> GenServer.cast({:event, event})
+    case senders(supervisor_name) do
+      [] ->
+        :ok
+
+      senders ->
+        supervisor_name
+        |> PostHog.Registry.config()
+        |> case do
+          %{test_mode: true} ->
+            PostHog.Test.remember_event(supervisor_name, event)
+
+          _ ->
+            send_to_sender(event, senders)
+        end
     end
   end
 
+  defp senders(supervisor_name) do
+    registry = PostHog.Registry.registry_name(supervisor_name)
+
+    if Process.whereis(registry) do
+      Registry.select(registry, [{{{__MODULE__, :_}, :"$1", :"$2"}, [], [{{:"$2", :"$1"}}]}])
+    else
+      []
+    end
+  rescue
+    ArgumentError -> []
+  end
+
+  defp send_to_sender(event, senders) do
+    # Pick the first available sender, otherwise random busy one.
+    senders
+    |> Keyword.get_lazy(:available, fn ->
+      senders |> Keyword.values() |> Enum.random()
+    end)
+    |> GenServer.cast({:event, event})
+  end
+
   # Callbacks
 
   @impl GenServer
diff --git a/lib/posthog/supervisor.ex b/lib/posthog/supervisor.ex
index 61e2a79..9a50539 100644
--- a/lib/posthog/supervisor.ex
+++ b/lib/posthog/supervisor.ex
@@ -65,6 +65,8 @@ defmodule PostHog.Supervisor do
     Supervisor.init(children, strategy: :one_for_one)
   end
 
+  defp sources(%{enabled: false}), do: []
+
   defp sources(config) do
     if config.enable_source_code_context do
       opts = [
@@ -81,6 +83,8 @@ defmodule PostHog.Supervisor do
     end
   end
 
+  defp senders(%{enabled: false}), do: []
+
   defp senders(config) do
     pool_size = Map.get(config, :sender_pool_size, max(System.schedulers_online(), 2))
 
diff --git a/test/posthog/application_config_test.exs b/test/posthog/application_config_test.exs
new file mode 100644
index 0000000..40f08fd
--- /dev/null
+++ b/test/posthog/application_config_test.exs
@@ -0,0 +1,50 @@
+defmodule PostHog.ApplicationConfigTest do
+  use ExUnit.Case, async: false
+
+  import ExUnit.CaptureLog
+  import Mox
+
+  setup :verify_on_exit!
+
+  setup do
+    previous_env = Application.get_all_env(:posthog)
+
+    on_exit(fn ->
+      :posthog
+      |> Application.get_all_env()
+      |> Keyword.keys()
+      |> Enum.each(&Application.delete_env(:posthog, &1))
+
+      Enum.each(previous_env, fn {key, value} ->
+        Application.put_env(:posthog, key, value)
+      end)
+    end)
+  end
+
+  for {label, api_key_env} <- [
+        {"missing", :missing},
+        {"nil", nil}
+      ] do
+    test "read! does not raise when enabled and api_key is #{label}" do
+      Application.put_env(:posthog, :enable, true)
+      Application.put_env(:posthog, :api_client_module, PostHog.API.Mock)
+      Application.delete_env(:posthog, :api_host)
+
+      case unquote(Macro.escape(api_key_env)) do
+        :missing -> Application.delete_env(:posthog, :api_key)
+        api_key -> Application.put_env(:posthog, :api_key, api_key)
+      end
+
+      log =
+        capture_log(fn ->
+          assert {%{enable: true}, %{api_key: "", enabled: false, api_client: nil} = config} =
+                   PostHog.Config.read!()
+
+          assert config.api_host == "https://us.i.posthog.com"
+        end)
+
+      assert log =~
+               "posthog api_key is empty after trimming whitespace; PostHog will start in disabled/no-op mode"
+    end
+  end
+end
diff --git a/test/posthog/config_test.exs b/test/posthog/config_test.exs
index 2047d30..0a82278 100644
--- a/test/posthog/config_test.exs
+++ b/test/posthog/config_test.exs
@@ -60,26 +60,27 @@ defmodule PostHog.ConfigTest do
     assert config.api_host == "https://us.i.posthog.com"
   end
 
-  test "validate logs when api_key is blank after trimming whitespace" do
-    expect(PostHog.API.Mock, :client, fn api_key, api_host ->
-      assert api_key == ""
-      assert api_host == "https://us.i.posthog.com"
-
-      %PostHog.API.Client{client: :stub_client, module: PostHog.API.Mock}
-    end)
-
-    log =
-      capture_log(fn ->
-        assert {:ok, config} =
-                 PostHog.Config.validate(
-                   api_key: " \n\t ",
-                   api_host: "https://us.i.posthog.com",
-                   api_client_module: PostHog.API.Mock
-                 )
-
-        assert config.api_key == ""
-      end)
-
-    assert log =~ "posthog api_key is empty after trimming whitespace; check your project API key"
+  for {label, api_key_option} <- [
+        {"missing", []},
+        {"nil", [api_key: nil]},
+        {"empty", [api_key: ""]},
+        {"blank after trimming whitespace", [api_key: " \n\t "]}
+      ] do
+    test "validate disables PostHog when api_key is #{label}" do
+      log =
+        capture_log(fn ->
+          options = unquote(Macro.escape(api_key_option)) ++ [api_client_module: PostHog.API.Mock]
+
+          assert {:ok, config} = PostHog.Config.validate(options)
+
+          assert config.api_key == ""
+          assert config.api_host == "https://us.i.posthog.com"
+          assert config.enabled == false
+          assert config.api_client == nil
+        end)
+
+      assert log =~
+               "posthog api_key is empty after trimming whitespace; PostHog will start in disabled/no-op mode"
+    end
   end
 end
diff --git a/test/posthog/feature_flags/evaluations_test.exs b/test/posthog/feature_flags/evaluations_test.exs
index 8adffc2..406631f 100644
--- a/test/posthog/feature_flags/evaluations_test.exs
+++ b/test/posthog/feature_flags/evaluations_test.exs
@@ -126,6 +126,12 @@ defmodule PostHog.FeatureFlags.EvaluationsTest do
       assert all_captured() == []
     end
 
+    @tag config: [api_key: "", supervisor_name: PostHog]
+    test "returns an empty snapshot when PostHog is disabled" do
+      assert {:ok, %Evaluations{distinct_id: "foo", flags: %{}}} =
+               FeatureFlags.evaluate_flags("foo")
+    end
+
     @tag config: [supervisor_name: MyPostHog]
     test "supports a named PostHog instance" do
       expect(API.Mock, :request, fn _client, :post, "/flags", _opts ->
diff --git a/test/posthog/feature_flags_test.exs b/test/posthog/feature_flags_test.exs
index ea96673..807e9b3 100644
--- a/test/posthog/feature_flags_test.exs
+++ b/test/posthog/feature_flags_test.exs
@@ -92,6 +92,12 @@ defmodule PostHog.FeatureFlagsTest do
       assert {:ok, %{body: %{"flags" => _}}} =
                FeatureFlags.flags(MyPostHog, %{distinct_id: "foo"})
     end
+
+    @tag config: [api_key: "", supervisor_name: PostHog]
+    test "returns empty flags when PostHog is disabled" do
+      assert {:ok, %{status: 200, body: %{"flags" => %{}}}} =
+               FeatureFlags.flags(%{distinct_id: "foo"})
+    end
   end
 
   describe "flags_for/2" do
diff --git a/test/posthog/supervisor_test.exs b/test/posthog/supervisor_test.exs
new file mode 100644
index 0000000..9ec744f
--- /dev/null
+++ b/test/posthog/supervisor_test.exs
@@ -0,0 +1,35 @@
+defmodule PostHog.SupervisorTest do
+  use ExUnit.Case, async: true
+
+  import ExUnit.CaptureLog
+
+  @supervisor_name __MODULE__
+
+  test "disabled config starts without senders and captures as no-op" do
+    {config, _log} =
+      with_log(fn ->
+        PostHog.Config.validate!(
+          api_key: " \n\t ",
+          api_host: "https://us.i.posthog.com",
+          supervisor_name: @supervisor_name,
+          enable_source_code_context: true,
+          test_mode: true
+        )
+      end)
+
+    start_link_supervised!({PostHog.Supervisor, config})
+
+    assert [] =
+             @supervisor_name
+             |> PostHog.Registry.registry_name()
+             |> Registry.select([{{{PostHog.Sender, :_}, :"$1", :"$2"}, [], [:"$1"]}])
+
+    assert nil ==
+             @supervisor_name
+             |> PostHog.Registry.via(PostHog.ErrorTracking.Sources)
+             |> GenServer.whereis()
+
+    assert :ok = PostHog.bare_capture(@supervisor_name, "disabled capture", "distinct_id", %{})
+    assert PostHog.Test.all_captured(@supervisor_name) == []
+  end
+end
diff --git a/test/posthog/uninitialized_test.exs b/test/posthog/uninitialized_test.exs
new file mode 100644
index 0000000..f231f2a
--- /dev/null
+++ b/test/posthog/uninitialized_test.exs
@@ -0,0 +1,21 @@
+defmodule PostHog.UninitializedTest do
+  use ExUnit.Case, async: true
+
+  alias PostHog.FeatureFlags
+  alias PostHog.FeatureFlags.Evaluations
+
+  @supervisor_name __MODULE__.MissingSupervisor
+
+  test "public APIs no-op when the supervisor was not started" do
+    assert %{enabled: false, supervisor_name: @supervisor_name} = PostHog.config(@supervisor_name)
+    assert :ok = PostHog.bare_capture(@supervisor_name, "event", "distinct_id", %{})
+
+    PostHog.set_context(@supervisor_name, %{distinct_id: "distinct_id"})
+    assert :ok = PostHog.capture(@supervisor_name, "event", %{})
+
+    assert {:ok, %{}} = FeatureFlags.flags_for(@supervisor_name, "distinct_id")
+    assert {:ok, snapshot} = FeatureFlags.evaluate_flags(@supervisor_name, "distinct_id")
+    assert Evaluations.keys(snapshot) == []
+    assert Evaluations.enabled?(snapshot, "flag") == false
+  end
+end