diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..0fa5deb
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,49 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Development Commands
+
+- **Run tests**: `bundle exec rake test` or `bundle exec rspec`
+- **Run linting**: `bundle exec rubocop`
+- **Run both tests and linting**: `bundle exec rake` (default task)
+- **Build gem**: `bundle exec rake build`
+- **Install dependencies**: `bundle install`
+- **Generate documentation**: `bundle exec yard`
+- **Interactive console**: `./script/console`
+
+## Architecture Overview
+
+This is a Ruby gem that provides a client library for Instapaper's Full API. The architecture follows a modular design:
+
+### Core Components
+
+- **`Instapaper::Client`** - Main entry point that users instantiate with OAuth credentials
+- **API modules** - Separate modules for each API category (Accounts, Bookmarks, Folders, Highlights, OAuth) mixed into the Client
+- **HTTP layer** - Custom HTTP handling using the `http.rb` gem with OAuth signature generation
+- **Response objects** - Custom model classes (Bookmark, Folder, Highlight, etc.) using Virtus for attribute definition
+
+### Key Patterns
+
+- **Modular API design**: Each API endpoint category is in its own module (`lib/instapaper/api/`)
+- **HTTP abstraction**: All API calls go through `Instapaper::HTTP::Request` which handles OAuth signing and response parsing
+- **Custom response objects**: API responses are parsed into specific model classes rather than raw hashes
+- **Method naming convention**: API methods are descriptive (`star_bookmark`, `add_folder`) rather than generic (`star`, `add`)
+
+### Authentication Flow
+
+Uses OAuth 1.0a with xAuth for obtaining access tokens. The client requires consumer key/secret and access token/secret for API calls.
+
+## Testing
+
+- Uses RSpec with WebMock for stubbing HTTP requests
+- Fixtures in `spec/fixtures/` contain sample API responses
+- Test coverage tracked with SimpleCov (disabled in CI)
+- Tests are organized by API module structure
+
+## Dependencies
+
+- **http.rb** - HTTP client library (replaced Faraday in v1.0)
+- **virtus** - Attribute definitions for model objects
+- **simple_oauth** - OAuth signature generation
+- **addressable** - URI parsing and manipulation
\ No newline at end of file
diff --git a/Gemfile b/Gemfile
index 4eb11ab..f69d498 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,7 +2,6 @@ source 'https://rubygems.org'
 
 gem 'rake'
 gem 'yard'
-gem 'simple_oauth', git: 'https://github.com/samsonjs/simple_oauth.git', tag: 'v0.3.2'
 
 gem 'jruby-openssl', platforms: :jruby
 gem 'json', platforms: :mri_19
diff --git a/instapaper.gemspec b/instapaper.gemspec
index f06f352..e7766a8 100644
--- a/instapaper.gemspec
+++ b/instapaper.gemspec
@@ -9,7 +9,6 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'dry-types', '~> 1.0'
   spec.add_dependency 'http', '>= 2', '< 6'
   spec.add_dependency 'multi_json', '~> 1'
-  spec.add_dependency 'simple_oauth', '~> 0.3'
   spec.author        = 'Steve Agalloco'
   spec.description   = "Ruby Client for Instapaper's Full API"
   spec.email         = 'steve.agalloco@gmail.com'
diff --git a/lib/instapaper/http/headers.rb b/lib/instapaper/http/headers.rb
index 085f680..096834f 100644
--- a/lib/instapaper/http/headers.rb
+++ b/lib/instapaper/http/headers.rb
@@ -1,6 +1,6 @@
 require 'addressable/uri'
 require 'base64'
-require 'simple_oauth'
+require 'instapaper/oauth'
 
 module Instapaper
   module HTTP
@@ -22,7 +22,7 @@ module Instapaper
       private
 
       def oauth_header
-        SimpleOAuth::Header.new(@request_method, @uri, @options, credentials.merge(ignore_extra_keys: true))
+        Instapaper::OAuth::Header.new(@request_method, @uri, @options, credentials.merge(ignore_extra_keys: true))
       end
 
       # Authentication hash
diff --git a/lib/instapaper/oauth.rb b/lib/instapaper/oauth.rb
new file mode 100644
index 0000000..84c7fcf
--- /dev/null
+++ b/lib/instapaper/oauth.rb
@@ -0,0 +1,92 @@
+require 'openssl'
+require 'base64'
+require 'cgi'
+require 'securerandom'
+
+module Instapaper
+  module OAuth
+    class Header
+      OAUTH_VERSION = '1.0'
+      OAUTH_SIGNATURE_METHOD = 'HMAC-SHA1'
+
+      attr_reader :method, :url, :params, :credentials
+
+      def initialize(method, url, params, credentials)
+        @method = method.to_s.upcase
+        @url = url.to_s
+        @params = params || {}
+        @credentials = credentials
+        @credentials.delete(:ignore_extra_keys)
+      end
+
+      def to_s
+        "OAuth #{auth_header_params.map { |k, v| %(#{k}="#{escape(v)}") }.join(', ')}"
+      end
+
+      private
+
+      def auth_header_params
+        params = oauth_params.dup
+        params['oauth_signature'] = signature
+        params.sort.to_h
+      end
+
+      def oauth_params
+        {
+          'oauth_consumer_key' => credentials[:consumer_key],
+          'oauth_token' => credentials[:token],
+          'oauth_signature_method' => OAUTH_SIGNATURE_METHOD,
+          'oauth_timestamp' => timestamp,
+          'oauth_nonce' => nonce,
+          'oauth_version' => OAUTH_VERSION,
+        }.compact
+      end
+
+      def signature
+        Base64.strict_encode64(
+          OpenSSL::HMAC.digest(
+            OpenSSL::Digest.new('sha1'),
+            signing_key,
+            signature_base_string,
+          ),
+        )
+      end
+
+      def signing_key
+        "#{escape(credentials[:consumer_secret])}&#{escape(credentials[:token_secret] || '')}"
+      end
+
+      def signature_base_string
+        [
+          method,
+          escape(normalized_url),
+          escape(normalized_params),
+        ].join('&')
+      end
+
+      def normalized_url
+        uri = URI.parse(url)
+        port = uri.port
+        port = nil if (uri.scheme == 'http' && port == 80) || (uri.scheme == 'https' && port == 443)
+        "#{uri.scheme}://#{uri.host}#{":#{port}" if port}#{uri.path}"
+      end
+
+      def normalized_params
+        all_params = params.merge(oauth_params)
+        all_params.map { |k, v| "#{escape(k)}=#{escape(v)}" }.sort.join('&')
+      end
+
+      def escape(value)
+        CGI.escape(value.to_s).gsub('+', '%20')
+      end
+
+      def timestamp
+        @timestamp ||= Time.now.to_i.to_s
+      end
+
+      def nonce
+        @nonce ||= SecureRandom.hex(16)
+      end
+    end
+  end
+end
diff --git a/spec/instapaper/oauth_spec.rb b/spec/instapaper/oauth_spec.rb
new file mode 100644
index 0000000..f690614
--- /dev/null
+++ b/spec/instapaper/oauth_spec.rb
@@ -0,0 +1,154 @@
+require 'spec_helper'
+
+describe Instapaper::OAuth::Header do
+  let(:method) { 'POST' }
+  let(:url) { 'https://www.instapaper.com/api/1/bookmarks/list' }
+  let(:params) { {folder_id: '12345', limit: '10'} }
+  let(:credentials) do
+    {
+      consumer_key: 'test_consumer_key',
+      consumer_secret: 'test_consumer_secret',
+      token: 'test_token',
+      token_secret: 'test_token_secret',
+    }
+  end
+
+  subject { described_class.new(method, url, params, credentials) }
+
+  describe '#initialize' do
+    it 'upcases the HTTP method' do
+      header = described_class.new('get', url, params, credentials)
+      expect(header.method).to eq('GET')
+    end
+
+    it 'converts URL to string' do
+      uri = URI.parse(url)
+      header = described_class.new(method, uri, params, credentials)
+      expect(header.url).to eq(url)
+    end
+
+    it 'handles nil params' do
+      header = described_class.new(method, url, nil, credentials)
+      expect(header.params).to eq({})
+    end
+
+    it 'removes ignore_extra_keys from credentials' do
+      creds_with_extra = credentials.merge(ignore_extra_keys: true)
+      header = described_class.new(method, url, params, creds_with_extra)
+      expect(header.credentials).not_to have_key(:ignore_extra_keys)
+    end
+  end
+
+  describe '#to_s' do
+    it 'returns a properly formatted OAuth header' do
+      # Stub time and nonce for consistent output
+      allow(subject).to receive(:timestamp).and_return('1234567890')
+      allow(subject).to receive(:nonce).and_return('abcdef1234567890')
+
+      header_string = subject.to_s
+      expect(header_string).to start_with('OAuth ')
+      expect(header_string).to include('oauth_consumer_key="test_consumer_key"')
+      expect(header_string).to include('oauth_nonce="abcdef1234567890"')
+      expect(header_string).to include('oauth_signature=')
+      expect(header_string).to include('oauth_signature_method="HMAC-SHA1"')
+      expect(header_string).to include('oauth_timestamp="1234567890"')
+      expect(header_string).to include('oauth_token="test_token"')
+      expect(header_string).to include('oauth_version="1.0"')
+    end
+
+    it 'sorts OAuth parameters alphabetically' do
+      allow(subject).to receive(:timestamp).and_return('1234567890')
+      allow(subject).to receive(:nonce).and_return('abcdef1234567890')
+
+      header_string = subject.to_s
+      params_order = header_string.scan(/oauth_\w+(?==)/)
+      expect(params_order).to eq(params_order.sort)
+    end
+  end
+
+  describe 'signature generation' do
+    it 'generates consistent signatures for the same input' do
+      allow(subject).to receive(:timestamp).and_return('1234567890')
+      allow(subject).to receive(:nonce).and_return('abcdef1234567890')
+
+      header1 = subject.to_s
+      header2 = subject.to_s
+
+      sig1 = header1[/oauth_signature="([^"]+)"/, 1]
+      sig2 = header2[/oauth_signature="([^"]+)"/, 1]
+
+      expect(sig1).to eq(sig2)
+    end
+
+    it 'generates different signatures for different parameters' do
+      allow(subject).to receive(:timestamp).and_return('1234567890')
+      allow(subject).to receive(:nonce).and_return('abcdef1234567890')
+
+      header1 = subject.to_s
+
+      different_params = {folder_id: '67890', limit: '20'}
+      subject2 = described_class.new(method, url, different_params, credentials)
+      allow(subject2).to receive(:timestamp).and_return('1234567890')
+      allow(subject2).to receive(:nonce).and_return('abcdef1234567890')
+
+      header2 = subject2.to_s
+
+      sig1 = header1[/oauth_signature="([^"]+)"/, 1]
+      sig2 = header2[/oauth_signature="([^"]+)"/, 1]
+
+      expect(sig1).not_to eq(sig2)
+    end
+  end
+
+  describe 'URL normalization' do
+    it 'removes default HTTP port 80' do
+      header = described_class.new(method, 'http://example.com:80/path', params, credentials)
+      expect(header.send(:normalized_url)).to eq('http://example.com/path')
+    end
+
+    it 'removes default HTTPS port 443' do
+      header = described_class.new(method, 'https://example.com:443/path', params, credentials)
+      expect(header.send(:normalized_url)).to eq('https://example.com/path')
+    end
+
+    it 'keeps non-default ports' do
+      header = described_class.new(method, 'https://example.com:8080/path', params, credentials)
+      expect(header.send(:normalized_url)).to eq('https://example.com:8080/path')
+    end
+  end
+
+  describe 'parameter encoding' do
+    it 'properly encodes spaces as %20' do
+      params_with_spaces = {title: 'Hello World'}
+      header = described_class.new(method, url, params_with_spaces, credentials)
+      allow(header).to receive(:timestamp).and_return('1234567890')
+      allow(header).to receive(:nonce).and_return('abcdef1234567890')
+
+      normalized = header.send(:normalized_params)
+      expect(normalized).to include('title=Hello%20World')
+      expect(normalized).not_to include('+')
+    end
+
+    it 'properly encodes special characters' do
+      params_with_special = {title: 'Test & Co.'}
+      header = described_class.new(method, url, params_with_special, credentials)
+      allow(header).to receive(:timestamp).and_return('1234567890')
+      allow(header).to receive(:nonce).and_return('abcdef1234567890')
+
+      normalized = header.send(:normalized_params)
+      expect(normalized).to include('title=Test%20%26%20Co.')
+    end
+  end
+
+  describe 'compatibility' do
+    it 'handles missing token credentials (2-legged OAuth)' do
+      two_legged_creds = {
+        consumer_key: 'test_consumer_key',
+        consumer_secret: 'test_consumer_secret',
+      }
+      header = described_class.new(method, url, params, two_legged_creds)
+      expect { header.to_s }.not_to raise_error
+      expect(header.to_s).not_to include('oauth_token=')
+    end
+  end
+end