diff --git a/CHANGELOG.md b/CHANGELOG.md
index e12f2ef..92d5d69 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,17 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.4.0] - 2026-07-11
+
+### Added
+
+- `RubricLLM.evaluate_batch` validates every sample upfront (must be a Hash with non-nil `:question` and `:answer`, string or symbol keys) and raises `ArgumentError` with the offending index before any LLM call, so sequential and concurrent modes fail identically and without API spend
+
+### Changed
+
+- Add `csv` as a runtime dependency; `csv` moved from a default gem to a bundled gem in Ruby 3.4, so consumers previously hit a `LoadError` on `Report#export_csv`
+- Require `ruby_llm ~> 1.16`
+
 ## [0.3.0] - 2026-07-11
 
 ### Changed
diff --git a/Gemfile b/Gemfile
index 7b45873..98f5a3f 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,7 +4,6 @@ source "https://rubygems.org"
 
 gemspec
 
-gem "csv"
 gem "minitest"
 gem "rake"
 gem "rdoc"
diff --git a/lib/rubric_llm.rb b/lib/rubric_llm.rb
index 17af220..446e8de 100644
--- a/lib/rubric_llm.rb
+++ b/lib/rubric_llm.rb
@@ -56,6 +56,7 @@ def evaluate(question:, answer:, context: [], ground_truth: nil, metrics: nil,
     #   report = RubricLLM.evaluate_batch(dataset)
     #   report = RubricLLM.evaluate_batch(dataset, concurrency: 4)
     def evaluate_batch(dataset, metrics: nil, config: self.config, custom_prompt: nil, concurrency: nil)
+      validate_dataset!(dataset)
       config = apply_custom_prompt(config, custom_prompt)
       pool_size = concurrency || config.concurrency
       start_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
@@ -87,6 +88,22 @@ def evaluate_retrieval(retrieved:, relevant:)
 
     private
 
+    def validate_dataset!(dataset)
+      dataset.each_with_index do |sample, index|
+        raise ArgumentError, "sample at index #{index} is not a Hash" unless sample.is_a?(Hash)
+
+        question_present = sample.key?(:question) || sample.key?("question")
+        answer_present = sample.key?(:answer) || sample.key?("answer")
+        raise ArgumentError, "sample at index #{index} is missing :question" unless question_present
+        raise ArgumentError, "sample at index #{index} is missing :answer" unless answer_present
+
+        question_provided = !sample[:question].nil? || !sample["question"].nil?
+        answer_provided = !sample[:answer].nil? || !sample["answer"].nil?
+        raise ArgumentError, "sample at index #{index} has nil :question" unless question_provided
+        raise ArgumentError, "sample at index #{index} has nil :answer" unless answer_provided
+      end
+    end
+
     def evaluate_sample(evaluator, sample)
       sample = normalize_sample(sample)
       evaluator.call(
diff --git a/lib/rubric_llm/report.rb b/lib/rubric_llm/report.rb
index 214752e..544d11d 100644
--- a/lib/rubric_llm/report.rb
+++ b/lib/rubric_llm/report.rb
@@ -42,7 +42,7 @@ def summary
     end
 
     def export_csv(path)
-      require "csv" # optional dependency — add `gem "csv"` to your Gemfile if missing
+      require "csv"
       metrics = all_metric_names
       CSV.open(path, "w") do |csv|
         csv << ["question", "answer", "overall", *metrics]
diff --git a/lib/rubric_llm/version.rb b/lib/rubric_llm/version.rb
index 5e614f9..7f3eede 100644
--- a/lib/rubric_llm/version.rb
+++ b/lib/rubric_llm/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module RubricLLM
-  VERSION = "0.3.0"
+  VERSION = "0.4.0"
 end
diff --git a/rubric_llm.gemspec b/rubric_llm.gemspec
index b4d4b5c..0e9603c 100644
--- a/rubric_llm.gemspec
+++ b/rubric_llm.gemspec
@@ -37,5 +37,6 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
   spec.extra_rdoc_files = Dir["README.md", "CHANGELOG.md", "LICENSE.txt"]
 
-  spec.add_dependency "ruby_llm", "~> 1.13"
+  spec.add_dependency "csv"
+  spec.add_dependency "ruby_llm", "~> 1.16"
 end
diff --git a/test/test_batch_validation.rb b/test/test_batch_validation.rb
new file mode 100644
index 0000000..03199ba
--- /dev/null
+++ b/test/test_batch_validation.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class TestBatchValidation < Minitest::Test
+  include TestSetup
+
+  def test_evaluate_batch_rejects_non_hash_samples_before_judge_calls
+    chat = RubyLLMStub::FakeChat.new
+    RubyLLMStub.fake_chat = chat
+
+    error = assert_raises(ArgumentError) do
+      RubricLLM.evaluate_batch(
+        [{ question: "q1", answer: "a1" }, "not a sample"],
+        metrics: [RubricLLM::Metrics::Relevance]
+      )
+    end
+
+    assert_match "sample at index 1 is not a Hash", error.message
+    assert_equal 0, chat.call_count
+  end
+
+  def test_evaluate_batch_rejects_samples_missing_required_keys
+    [{ question: "q1" }, { "answer" => "a1" }].each_with_index do |sample, index|
+      error = assert_raises(ArgumentError) do
+        RubricLLM.evaluate_batch([sample], metrics: [RubricLLM::Metrics::Relevance])
+      end
+
+      assert_match "sample at index 0", error.message
+      assert_match(index.zero? ? ":answer" : ":question", error.message)
+    end
+  end
+
+  def test_evaluate_batch_rejects_samples_with_nil_required_values_before_judge_calls
+    [{ question: nil, answer: "a1" }, { "question" => "q2", "answer" => nil }].each do |sample|
+      chat = RubyLLMStub::FakeChat.new
+      RubyLLMStub.fake_chat = chat
+
+      error = assert_raises(ArgumentError) do
+        RubricLLM.evaluate_batch([sample], metrics: [RubricLLM::Metrics::Relevance])
+      end
+
+      assert_match "sample at index 0 has nil", error.message
+      assert_equal 0, chat.call_count
+    end
+  end
+
+  def test_evaluate_batch_accepts_complete_symbol_and_string_keyed_samples
+    stub_judge_response('{"score": 0.9, "reasoning": "ok"}')
+
+    report = RubricLLM.evaluate_batch(
+      [{ question: "symbol question", answer: "symbol answer" },
+       { "question" => "string question", "answer" => "string answer" }],
+      metrics: [RubricLLM::Metrics::Relevance]
+    )
+
+    assert_equal 2, report.results.size
+  end
+
+  def test_evaluate_batch_validates_before_starting_concurrent_work
+    chat = RubyLLMStub::FakeChat.new
+    RubyLLMStub.fake_chat = chat
+
+    error = assert_raises(ArgumentError) do
+      RubricLLM.evaluate_batch(
+        [{ question: "q1", answer: "a1" }, { question: "q2" }],
+        metrics: [RubricLLM::Metrics::Relevance],
+        concurrency: 2
+      )
+    end
+
+    assert_match "sample at index 1 is missing :answer", error.message
+    assert_equal 0, chat.call_count
+  end
+end