diff --git a/CHANGELOG.md b/CHANGELOG.md
index b1bd4c9..c1510f6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # main [(unreleased)](https://github.com/fastruby/next_rails/compare/v1.6.0...main)
 
 - [BUGFIX: example](https://github.com/fastruby/next_rails/pull/<number>)
+- [FEATURE: Validate the DeprecationTracker mode at initialization, treating a blank mode as the default `save`](https://github.com/fastruby/next_rails/pull/186)
 
 * Your changes/patches go here.
 
diff --git a/exe/deprecations b/exe/deprecations
index db824d1..64c5bb7 100755
--- a/exe/deprecations
+++ b/exe/deprecations
@@ -3,10 +3,14 @@ require "json"
 require "rainbow"
 require "optparse"
 require "set"
+require_relative "../lib/deprecation_tracker/valid_modes"
 require_relative "../lib/deprecation_tracker/shard_merger"
 
 def run_tests(deprecation_warnings, opts = {})
   tracker_mode = opts[:tracker_mode]
+  unless DeprecationTracker.valid_mode?(tracker_mode)
+    abort "Invalid --tracker-mode #{tracker_mode.inspect}. Must be one of: #{DeprecationTracker.valid_modes_display}."
+  end
   next_mode = opts[:next_mode]
   rspec_command = if next_mode
     "bin/next rspec"
@@ -69,7 +73,7 @@ option_parser = OptionParser.new do |opts|
     options[:next] = next_mode
   end
 
-  opts.on("--tracker-mode MODE", "Set DEPRECATION_TRACKER in test mode. Options: save or compare") do |tracker_mode|
+  opts.on("--tracker-mode MODE", "Set DEPRECATION_TRACKER in test mode. Options: #{DeprecationTracker.valid_modes_display}") do |tracker_mode|
     options[:tracker_mode] = tracker_mode
   end
 
diff --git a/lib/deprecation_tracker.rb b/lib/deprecation_tracker.rb
index 2c357be..9c4cefc 100644
--- a/lib/deprecation_tracker.rb
+++ b/lib/deprecation_tracker.rb
@@ -10,6 +10,8 @@
 # Tracks deprecation warnings, grouped by spec file. After the test run, compare against shitlist of expected
 # deprecation warnings. If anything is added or removed, raise an error with a diff of the changes.
 #
+require_relative "deprecation_tracker/valid_modes"
+
 class DeprecationTracker
   UnexpectedDeprecations = Class.new(StandardError)
 
@@ -75,9 +77,19 @@ def Kernel.warn(*args, &block)
 
   DEFAULT_PATH = "spec/support/deprecation_warning.shitlist.json"
 
+  # Returns the mode as-is, or nil when it is blank. A blank DEPRECATION_TRACKER
+  # (e.g. `DEPRECATION_TRACKER= rspec`) is truthy in Ruby, so callers can use this
+  # to treat an empty value as unset and fall back to the default mode.
+  def self.sanitize_mode(mode)
+    return if mode.nil?
+
+    stripped = mode.to_s.strip
+    stripped.empty? ? nil : stripped
+  end
+
   def self.init_tracker(opts = {})
     shitlist_path = opts[:shitlist_path] || DEFAULT_PATH
-    mode = opts[:mode] || ENV["DEPRECATION_TRACKER"] || :save
+    mode = sanitize_mode(opts[:mode] || ENV["DEPRECATION_TRACKER"]) || :save
     transform_message = opts[:transform_message]
     node_index = opts[:node_index]
     deprecation_tracker = DeprecationTracker.new(shitlist_path, transform_message, mode, node_index: node_index)
@@ -141,6 +153,9 @@ def initialize(shitlist_path, transform_message = nil, mode = :save, node_index:
     @transform_message = transform_message || -> (message) { message }
     @deprecation_messages = {}
     @mode = mode ? mode.to_sym : :save
+    unless self.class.valid_mode?(@mode)
+      raise ArgumentError, "mode must be one of: #{self.class.valid_modes_display}. Got: #{mode.inspect}"
+    end
     if @mode == :compare && node_index
       raise ArgumentError, "node_index cannot be used with compare mode"
     end
diff --git a/lib/deprecation_tracker/valid_modes.rb b/lib/deprecation_tracker/valid_modes.rb
new file mode 100644
index 0000000..699257f
--- /dev/null
+++ b/lib/deprecation_tracker/valid_modes.rb
@@ -0,0 +1,12 @@
+class DeprecationTracker
+  VALID_MODES = %i[save compare].freeze
+
+  # Human-readable list of valid modes for error messages and CLI help text.
+  def self.valid_modes_display
+    @valid_modes_display ||= VALID_MODES.map(&:to_s).join(", ")
+  end
+
+  def self.valid_mode?(mode)
+    mode && VALID_MODES.include?(mode.to_sym)
+  end
+end
diff --git a/spec/deprecation_tracker_spec.rb b/spec/deprecation_tracker_spec.rb
index 67bb311..ad1a734 100644
--- a/spec/deprecation_tracker_spec.rb
+++ b/spec/deprecation_tracker_spec.rb
@@ -263,13 +263,6 @@
       tracker = DeprecationTracker.new(shitlist_path, nil, nil)
       expect(tracker.mode).to eq(:save)
     end
-
-    it "does not save nor compare if mode is invalid" do
-      tracker = DeprecationTracker.new(shitlist_path, nil, "random_stuff")
-      expect(tracker).not_to receive(:save)
-      expect(tracker).not_to receive(:compare)
-      tracker.after_run
-    end
   end
 
   describe "#init_tracker" do
@@ -426,6 +419,61 @@ def self.behavior
       tracker = DeprecationTracker.init_tracker({})
       expect(tracker.mode).to eq(:compare)
     end
+
+    it "raises ArgumentError naming the invalid ENV['DEPRECATION_TRACKER'] value" do
+      stub_const("ENV", ENV.to_h.merge("DEPRECATION_TRACKER" => "bogus"))
+      expect {
+        DeprecationTracker.init_tracker({})
+      }.to raise_error(ArgumentError, /mode must be one of: save, compare\..*bogus/)
+    end
+
+    it "treats a blank ENV['DEPRECATION_TRACKER'] as unset and defaults to save" do
+      stub_const("ENV", ENV.to_h.merge("DEPRECATION_TRACKER" => ""))
+      tracker = DeprecationTracker.init_tracker({})
+      expect(tracker.mode).to eq(:save)
+    end
+
+    it "treats a blank opts[:mode] as unset and defaults to save" do
+      tracker = DeprecationTracker.init_tracker(mode: "")
+      expect(tracker.mode).to eq(:save)
+    end
+  end
+
+  describe "VALID_MODES" do
+    it "accepts all valid modes without error" do
+      DeprecationTracker::VALID_MODES.each do |mode|
+        expect {
+          DeprecationTracker.new(shitlist_path, nil, mode)
+        }.not_to raise_error
+      end
+    end
+
+    it "rejects an invalid mode at construction, naming the bad value" do
+      expect {
+        DeprecationTracker.new(shitlist_path, nil, "random_stuff")
+      }.to raise_error(ArgumentError, /mode must be one of: save, compare\..*random_stuff/)
+    end
+  end
+
+  describe ".valid_mode?" do
+    it "returns true for valid modes, as symbol or string" do
+      expect(DeprecationTracker.valid_mode?(:save)).to be_truthy
+      expect(DeprecationTracker.valid_mode?("compare")).to be_truthy
+    end
+
+    it "returns a falsey value for an unknown mode" do
+      expect(DeprecationTracker.valid_mode?("random_stuff")).to be_falsey
+    end
+
+    it "returns a falsey value for nil" do
+      expect(DeprecationTracker.valid_mode?(nil)).to be_falsey
+    end
+  end
+
+  describe ".valid_modes_display" do
+    it "renders the valid modes as a comma-separated string" do
+      expect(DeprecationTracker.valid_modes_display).to eq("save, compare")
+    end
   end
 
   describe DeprecationTracker::KernelWarnTracker do