diff --git a/Appraisals b/Appraisals
index 3a87121..022a2fb 100644
--- a/Appraisals
+++ b/Appraisals
@@ -1,5 +1,16 @@
 # frozen_string_literal: true
 
+# Additional dependencies needed for specific gems.
+# These work around upstream gems that haven't declared dependencies properly.
+# See docs/appraisal-ruby-version-support.md for future Ruby version support.
+GEM_DEPENDENCIES = {
+  "openai" => ["base64"] # openai uses base64 but doesn't declare it (needed for Ruby 3.4+)
+}
+
+def gem_dependencies_for(gem_name)
+  GEM_DEPENDENCIES.fetch(gem_name, [])
+end
+
 # Optional dependencies to test
 OPTIONAL_GEMS = {
   "openai" => {
@@ -26,10 +37,13 @@ OPTIONAL_GEMS = {
 
 # Generate appraisals for each optional gem
 OPTIONAL_GEMS.each do |gem_name, versions|
+  extra_deps = gem_dependencies_for(gem_name)
+
   versions.each do |name, constraint|
     suffix = (name == "latest") ? "" : "-#{name.tr(".", "-")}"
     appraise "#{gem_name}#{suffix}" do
       gem gem_name, constraint
+      extra_deps.each { |dep| gem dep }
     end
   end
 
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 5d11826..c1301c3 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -77,3 +77,28 @@ for more details.
 ```bash
 rake -T test:vcr
 ```
+
+### Testing with Different Ruby Versions
+
+CI tests against Ruby 3.2, 3.3, and 3.4. To test locally with a different Ruby version:
+
+```bash
+# Install a different Ruby version
+mise install ruby@3.4
+
+# Run tests with that version
+mise exec ruby@3.4 -- bundle install
+mise exec ruby@3.4 -- bundle exec rake test
+
+# Run appraisal tests with that version
+mise exec ruby@3.4 -- bundle exec appraisal install
+mise exec ruby@3.4 -- bundle exec appraisal openai-0-33 rake test
+```
+
+To temporarily switch your shell to a different Ruby:
+
+```bash
+mise use ruby@3.4
+ruby --version  # => 3.4.x
+bundle exec rake test
+```
diff --git a/Gemfile.lock b/Gemfile.lock
index 7a03578..b2e79fd 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -17,7 +17,6 @@ GEM
       rake
       thor (>= 0.14.0)
     ast (2.4.3)
-    base64 (0.3.0)
     bigdecimal (3.3.1)
     builder (3.3.0)
     crack (1.0.1)
@@ -122,8 +121,7 @@ GEM
     unicode-display_width (3.2.0)
       unicode-emoji (~> 4.1)
     unicode-emoji (4.1.0)
-    vcr (6.3.1)
-      base64
+    vcr (6.4.0)
     webmock (3.25.1)
       addressable (>= 2.8.0)
       crack (>= 0.3.2)
diff --git a/gemfiles/openai.gemfile b/gemfiles/openai.gemfile
index 8a73574..edfe9d3 100644
--- a/gemfiles/openai.gemfile
+++ b/gemfiles/openai.gemfile
@@ -4,5 +4,6 @@ source "https://rubygems.org"
 
 gem "minitest-reporters", "~> 1.6"
 gem "openai", ">= 0.34"
+gem "base64"
 
 gemspec path: "../"
diff --git a/gemfiles/openai_0_33.gemfile b/gemfiles/openai_0_33.gemfile
index af9be30..a9eff3e 100644
--- a/gemfiles/openai_0_33.gemfile
+++ b/gemfiles/openai_0_33.gemfile
@@ -4,5 +4,6 @@ source "https://rubygems.org"
 
 gem "minitest-reporters", "~> 1.6"
 gem "openai", "~> 0.33.0"
+gem "base64"
 
 gemspec path: "../"
diff --git a/gemfiles/openai_0_34.gemfile b/gemfiles/openai_0_34.gemfile
index 2b9b038..2e2d6ab 100644
--- a/gemfiles/openai_0_34.gemfile
+++ b/gemfiles/openai_0_34.gemfile
@@ -4,5 +4,6 @@ source "https://rubygems.org"
 
 gem "minitest-reporters", "~> 1.6"
 gem "openai", "~> 0.34.0"
+gem "base64"
 
 gemspec path: "../"
diff --git a/lib/braintrust/internal/encoding.rb b/lib/braintrust/internal/encoding.rb
new file mode 100644
index 0000000..29d7923
--- /dev/null
+++ b/lib/braintrust/internal/encoding.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+module Braintrust
+  module Internal
+    # Encoding utilities using Ruby's native pack/unpack methods.
+    # Avoids dependency on external gems that became bundled gems in Ruby 3.4.
+    module Encoding
+      # Base64 encoding/decoding using Ruby's native pack/unpack methods.
+      # Drop-in replacement for the base64 gem's strict methods.
+      #
+      # @example Encode binary data
+      #   Encoding::Base64.strict_encode64(image_bytes)
+      #   # => "iVBORw0KGgo..."
+      #
+      # @example Decode base64 string
+      #   Encoding::Base64.strict_decode64("iVBORw0KGgo...")
+      #   # => "\x89PNG..."
+      #
+      module Base64
+        module_function
+
+        # Encodes binary data to base64 without newlines (strict encoding).
+        #
+        # @param data [String] Binary data to encode
+        # @return [String] Base64-encoded string without newlines
+        def strict_encode64(data)
+          [data].pack("m0")
+        end
+
+        # Decodes a base64 string to binary data (strict decoding).
+        #
+        # @param str [String] Base64-encoded string
+        # @return [String] Decoded binary data
+        def strict_decode64(str)
+          str.unpack1("m0")
+        end
+      end
+    end
+  end
+end
diff --git a/lib/braintrust/trace/attachment.rb b/lib/braintrust/trace/attachment.rb
index 81cf770..51a98d4 100644
--- a/lib/braintrust/trace/attachment.rb
+++ b/lib/braintrust/trace/attachment.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-require "base64"
 require "net/http"
+require_relative "../internal/encoding"
 require "uri"
 
 module Braintrust
@@ -110,7 +110,7 @@ def self.from_url(url)
       #   att.to_data_url
       #   # => "data:image/png;base64,iVBORw0KGgo..."
       def to_data_url
-        encoded = Base64.strict_encode64(@data)
+        encoded = Internal::Encoding::Base64.strict_encode64(@data)
         "data:#{@content_type};base64,#{encoded}"
       end
 
diff --git a/test/braintrust/internal/encoding_test.rb b/test/braintrust/internal/encoding_test.rb
new file mode 100644
index 0000000..ffee0ff
--- /dev/null
+++ b/test/braintrust/internal/encoding_test.rb
@@ -0,0 +1,105 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "braintrust/internal/encoding"
+
+class Braintrust::Internal::Encoding::Base64Test < Minitest::Test
+  BASE64_STDLIB_AVAILABLE = begin
+    require "base64"
+    true
+  rescue LoadError
+    false
+  end
+
+  # strict_encode64
+
+  def test_strict_encode64_encodes_string
+    result = Braintrust::Internal::Encoding::Base64.strict_encode64("Hello, World!")
+    assert_equal "SGVsbG8sIFdvcmxkIQ==", result
+  end
+
+  def test_strict_encode64_encodes_binary_data
+    binary = [0x89, 0x50, 0x4E, 0x47].pack("C*")
+    result = Braintrust::Internal::Encoding::Base64.strict_encode64(binary)
+    assert_equal "iVBORw==", result
+  end
+
+  def test_strict_encode64_encodes_empty_string
+    result = Braintrust::Internal::Encoding::Base64.strict_encode64("")
+    assert_equal "", result
+  end
+
+  def test_strict_encode64_produces_no_newlines
+    long_string = "x" * 1000
+    result = Braintrust::Internal::Encoding::Base64.strict_encode64(long_string)
+    refute_includes result, "\n"
+  end
+
+  def test_strict_encode64_matches_stdlib
+    skip "base64 stdlib not available" unless BASE64_STDLIB_AVAILABLE
+
+    test_cases = [
+      "Hello, World!",
+      "",
+      "x" * 1000,
+      [0x00, 0xFF, 0x89, 0x50].pack("C*")
+    ]
+
+    test_cases.each do |input|
+      expected = ::Base64.strict_encode64(input)
+      actual = Braintrust::Internal::Encoding::Base64.strict_encode64(input)
+      assert_equal expected, actual, "Mismatch for input: #{input.inspect}"
+    end
+  end
+
+  # strict_decode64
+
+  def test_strict_decode64_decodes_string
+    result = Braintrust::Internal::Encoding::Base64.strict_decode64("SGVsbG8sIFdvcmxkIQ==")
+    assert_equal "Hello, World!", result
+  end
+
+  def test_strict_decode64_decodes_binary_data
+    result = Braintrust::Internal::Encoding::Base64.strict_decode64("iVBORw==")
+    expected = [0x89, 0x50, 0x4E, 0x47].pack("C*")
+    assert_equal expected, result
+  end
+
+  def test_strict_decode64_decodes_empty_string
+    result = Braintrust::Internal::Encoding::Base64.strict_decode64("")
+    assert_equal "", result
+  end
+
+  def test_strict_decode64_matches_stdlib
+    skip "base64 stdlib not available" unless BASE64_STDLIB_AVAILABLE
+
+    test_cases = [
+      "SGVsbG8sIFdvcmxkIQ==",
+      "",
+      "eHh4eHh4eHg=",
+      "AP+JUA=="
+    ]
+
+    test_cases.each do |input|
+      expected = ::Base64.strict_decode64(input)
+      actual = Braintrust::Internal::Encoding::Base64.strict_decode64(input)
+      assert_equal expected, actual, "Mismatch for input: #{input.inspect}"
+    end
+  end
+
+  # round-trip
+
+  def test_round_trip_preserves_string_data
+    original = "The quick brown fox jumps over the lazy dog"
+    encoded = Braintrust::Internal::Encoding::Base64.strict_encode64(original)
+    decoded = Braintrust::Internal::Encoding::Base64.strict_decode64(encoded)
+    assert_equal original, decoded
+  end
+
+  def test_round_trip_preserves_binary_data
+    original = [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A, 0x00, 0xFF].pack("C*")
+    encoded = Braintrust::Internal::Encoding::Base64.strict_encode64(original)
+    decoded = Braintrust::Internal::Encoding::Base64.strict_decode64(encoded)
+    assert_equal original, decoded
+  end
+end
diff --git a/test/braintrust/trace/attachment_test.rb b/test/braintrust/trace/attachment_test.rb
index 51b0031..8674c37 100644
--- a/test/braintrust/trace/attachment_test.rb
+++ b/test/braintrust/trace/attachment_test.rb
@@ -2,6 +2,7 @@
 
 require "test_helper"
 require "braintrust/trace/attachment"
+require "braintrust/internal/encoding"
 
 class Braintrust::Trace::AttachmentTest < Minitest::Test
   def setup
@@ -58,7 +59,7 @@ def test_to_data_url_returns_correct_format
 
     # Verify it's valid base64 by extracting and decoding
     base64_part = data_url.sub(/^data:image\/png;base64,/, "")
-    decoded = Base64.strict_decode64(base64_part)
+    decoded = Braintrust::Internal::Encoding::Base64.strict_decode64(base64_part)
     assert_equal @test_png_data, decoded
   end