diff --git a/app/controllers/reviews_controller.rb b/app/controllers/reviews_controller.rb
index b8bdc1c..1b4486b 100644
--- a/app/controllers/reviews_controller.rb
+++ b/app/controllers/reviews_controller.rb
@@ -3,6 +3,11 @@ def index
   end
 
   def create
+    unless Harness.configuration&.api_key.present?
+      return render json: { error: "OPENAI_API_KEY is not configured on the server." },
+                    status: :service_unavailable
+    end
+
     review = Review.create!(pr_url: params[:pr_url], status: Review::PENDING)
     ReviewJob.perform_later(review.id)
     render json: { id: review.id, status: review.status }
@@ -51,8 +56,8 @@ def review_json(review)
         {
           id: s.id, filename: s.filename, language: s.language,
           priority: s.priority, walkthrough: s.walkthrough,
-          findings: s.findings, human_comments: s.human_comments,
-          status: s.status
+          patch_text: s.patch_text, findings: s.findings,
+          human_comments: s.human_comments, status: s.status
         }
       end
     }
diff --git a/app/javascript/components/review/DiffPanel.jsx b/app/javascript/components/review/DiffPanel.jsx
index 909372e..0905d1e 100644
--- a/app/javascript/components/review/DiffPanel.jsx
+++ b/app/javascript/components/review/DiffPanel.jsx
@@ -1,6 +1,13 @@
 import React, { useState, useRef, useEffect } from 'react'
 
-const DiffPanel = ({ sections, highlightFile, highlightRange }) => {
+const lineStyle = (line) => {
+  if (line.startsWith('+')) return 'bg-green-50 text-green-800'
+  if (line.startsWith('-')) return 'bg-red-50 text-red-800'
+  if (line.startsWith('@@')) return 'bg-blue-50 text-blue-600 font-medium'
+  return 'text-gray-700'
+}
+
+const DiffPanel = ({ sections, status, highlightFile, highlightRange }) => {
   const [collapsed, setCollapsed] = useState({})
   const fileRefs = useRef({})
 
@@ -15,30 +22,52 @@ const DiffPanel = ({ sections, highlightFile, highlightRange }) => {
   }
 
   if (!sections || sections.length === 0) {
-    return <div className="text-sm text-gray-400 p-4">No diff data available</div>
+    if (status === 'reviewing' || status === 'submitting') {
+      return (
+        <div className="flex items-center gap-2 text-sm text-gray-400 p-4">
+          <span className="animate-spin inline-block w-4 h-4 border-2 border-gray-300 border-t-transparent rounded-full" />
+          Fetching diff...
+        </div>
+      )
+    }
+    return (
+      <div className="text-sm text-gray-400 p-4 text-center">
+        Submit a PR URL to see the diff here.
+      </div>
+    )
   }
 
   return (
-    <div className="space-y-2 overflow-y-auto">
+    <div className="space-y-2 overflow-y-auto max-h-[80vh]">
       {sections.map((section) => (
         <div
           key={section.filename}
           ref={el => fileRefs.current[section.filename] = el}
-          className="border border-gray-200 rounded"
+          className="border border-gray-200 rounded overflow-hidden"
         >
           <button
             onClick={() => toggleFile(section.filename)}
             className="w-full flex items-center justify-between px-3 py-2 bg-gray-50 hover:bg-gray-100 text-left text-sm"
           >
-            <span className="font-mono text-xs">{section.filename}</span>
+            <div className="flex items-center gap-2">
+              <span className="font-mono text-xs">{section.filename}</span>
+              {section.language && section.language !== 'text' && (
+                <span className="text-xs bg-gray-200 text-gray-600 px-1.5 py-0.5 rounded">{section.language}</span>
+              )}
+            </div>
             <span className="text-gray-400 text-xs">{collapsed[section.filename] ? '▸' : '▾'}</span>
           </button>
 
-          {!collapsed[section.filename] && section.findings && (
-            <div className="p-2 text-xs text-gray-500">
-              {section.findings.length} finding{section.findings.length !== 1 ? 's' : ''}
-              {section.findings.some(f => f.severity === 'red_flag') && (
-                <span className="ml-2 text-red-600 font-medium">contains red flags</span>
+          {!collapsed[section.filename] && (
+            <div className="border-t border-gray-200">
+              {section.patch_text ? (
+                <pre className="text-xs leading-5 overflow-x-auto p-0 m-0">
+                  {section.patch_text.split('\n').map((line, i) => (
+                    <div key={i} className={`px-3 ${lineStyle(line)}`}>{line || ' '}</div>
+                  ))}
+                </pre>
+              ) : (
+                <div className="p-3 text-xs text-gray-400 italic">No diff content available</div>
               )}
             </div>
           )}
diff --git a/app/javascript/components/review/ReviewApp.jsx b/app/javascript/components/review/ReviewApp.jsx
index 8fbbf22..a7352f3 100644
--- a/app/javascript/components/review/ReviewApp.jsx
+++ b/app/javascript/components/review/ReviewApp.jsx
@@ -121,17 +121,30 @@ const ReviewApp = () => {
       <PrInput onSubmit={handleSubmit} disabled={isReviewing} />
 
       {error && (
-        <div className="bg-red-50 border border-red-200 text-red-700 rounded-lg px-4 py-2 mb-4 text-sm">
+        <div className="bg-red-50 border border-red-200 text-red-700 rounded-lg px-4 py-3 mb-4 text-sm">
           {error}
         </div>
       )}
 
+      {status === 'failed' && !error && (
+        <div className="bg-red-50 border border-red-200 text-red-700 rounded-lg px-4 py-3 mb-4 text-sm">
+          Review failed. This could be a GitHub API issue or an invalid PR URL. Try again.
+        </div>
+      )}
+
+      {!reviewId && status === 'idle' && !error && (
+        <div className="text-center py-12 text-gray-400 text-sm">
+          Paste a public GitHub PR URL above to start an AI-powered code review.
+        </div>
+      )}
+
       {reviewId && (
         <div className="grid grid-cols-1 lg:grid-cols-2 gap-6">
           <div>
-            <h2 className="text-sm font-medium text-gray-500 mb-3 uppercase tracking-wide">Files</h2>
+            <h2 className="text-sm font-medium text-gray-500 mb-3 uppercase tracking-wide">Diff</h2>
             <DiffPanel
               sections={sections}
+              status={status}
               highlightFile={highlightFile}
               highlightRange={highlightRange}
             />
diff --git a/app/javascript/components/review/ReviewPanel.jsx b/app/javascript/components/review/ReviewPanel.jsx
index c280c30..58c3bd5 100644
--- a/app/javascript/components/review/ReviewPanel.jsx
+++ b/app/javascript/components/review/ReviewPanel.jsx
@@ -7,17 +7,30 @@ const ReviewPanel = ({ sections, synthesis, reviewId, status, onSynthesize, onFi
   const canSynthesize = status === 'complete' && hasSections && !synthesis?.verdict
 
   return (
-    <div className="space-y-4 overflow-y-auto">
+    <div className="space-y-4 overflow-y-auto max-h-[80vh]">
       {!hasSections && status === 'reviewing' && (
         <div className="flex items-center gap-2 text-sm text-gray-500 p-4">
           <span className="animate-spin inline-block w-4 h-4 border-2 border-purple-500 border-t-transparent rounded-full" />
-          Reviewing pull request...
+          Triaging files and starting review...
+        </div>
+      )}
+
+      {hasSections && status === 'reviewing' && (
+        <div className="flex items-center gap-2 text-xs text-gray-400 px-1">
+          <span className="animate-spin inline-block w-3 h-3 border-2 border-purple-400 border-t-transparent rounded-full" />
+          Reviewing more files...
+        </div>
+      )}
+
+      {status === 'complete' && !hasSections && (
+        <div className="text-sm text-gray-400 p-4 text-center">
+          No files were flagged for review.
         </div>
       )}
 
       {sections.map((section) => (
         <SectionCard
-          key={section.id}
+          key={section.id || section.filename}
           section={section}
           reviewId={reviewId}
           onFindingClick={onFindingClick}
diff --git a/app/jobs/review_job.rb b/app/jobs/review_job.rb
index efc61b7..098cb51 100644
--- a/app/jobs/review_job.rb
+++ b/app/jobs/review_job.rb
@@ -25,6 +25,7 @@ def build_callback(review)
       section = review.review_sections.create!(
         filename: file.filename,
         language: file.language,
+        patch_text: file.patch_text,
         walkthrough: "",
         findings: findings.map { |f| finding_to_hash(f) },
         status: ReviewSection::COMPLETE
diff --git a/app/jobs/synthesize_job.rb b/app/jobs/synthesize_job.rb
index 65202d1..7117646 100644
--- a/app/jobs/synthesize_job.rb
+++ b/app/jobs/synthesize_job.rb
@@ -7,7 +7,8 @@ def perform(review_id)
     human_comments = collect_comments(review)
 
     config = Harness.configuration
-    llm = Harness::LLM::AnthropicClient.new(
+    client_class = config.provider == :openai ? Harness::LLM::OpenAiClient : Harness::LLM::AnthropicClient
+    llm = client_class.new(
       api_key: config.api_key,
       model: config.model,
       max_tokens: config.max_tokens_per_call
diff --git a/app/lib/harness/configuration.rb b/app/lib/harness/configuration.rb
index adcede4..dca458c 100644
--- a/app/lib/harness/configuration.rb
+++ b/app/lib/harness/configuration.rb
@@ -4,8 +4,8 @@ class Configuration
                   :on_section_complete
 
     def initialize
-      @provider = :anthropic
-      @model = "claude-sonnet-4-20250514"
+      @provider = :openai
+      @model = "gpt-4o-mini"
       @max_tokens_per_call = 4096
       @on_section_complete = nil
     end
diff --git a/app/lib/harness/llm/open_ai_client.rb b/app/lib/harness/llm/open_ai_client.rb
new file mode 100644
index 0000000..9cc0055
--- /dev/null
+++ b/app/lib/harness/llm/open_ai_client.rb
@@ -0,0 +1,57 @@
+module Harness
+  module LLM
+    class OpenAiClient < Client
+      API_URL = "https://api.openai.com/v1/chat/completions"
+
+      def initialize(api_key:, model:, max_tokens: 4096)
+        @api_key = api_key
+        @model = model
+        @max_tokens = max_tokens
+      end
+
+      def complete(messages:, system: nil)
+        body = build_body(messages, system)
+        raw = post(body)
+        parse_response(raw)
+      end
+
+      private
+
+      def build_body(messages, system)
+        msgs = []
+        msgs << { role: "system", content: system } if system
+        msgs.concat(messages.map(&:to_h))
+
+        {
+          model: @model,
+          max_tokens: @max_tokens,
+          messages: msgs,
+          response_format: { type: "json_object" }
+        }.to_json
+      end
+
+      def post(body)
+        uri = URI(API_URL)
+        request = Net::HTTP::Post.new(uri)
+        request["Content-Type"] = "application/json"
+        request["Authorization"] = "Bearer #{@api_key}"
+        request.body = body
+
+        response = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) do |http|
+          http.request(request)
+        end
+
+        JSON.parse(response.body, symbolize_names: true)
+      end
+
+      def parse_response(raw)
+        Response.new(
+          content: raw.dig(:choices, 0, :message, :content) || "",
+          model: raw[:model] || @model,
+          input_tokens: raw.dig(:usage, :prompt_tokens) || 0,
+          output_tokens: raw.dig(:usage, :completion_tokens) || 0
+        )
+      end
+    end
+  end
+end
diff --git a/app/lib/harness/review/pipeline.rb b/app/lib/harness/review/pipeline.rb
index 85df5c8..7aa9840 100644
--- a/app/lib/harness/review/pipeline.rb
+++ b/app/lib/harness/review/pipeline.rb
@@ -20,7 +20,8 @@ def call(pr_url:, pr_description: "")
       private
 
       def build_client
-        LLM::AnthropicClient.new(
+        client_class = @config.provider == :openai ? LLM::OpenAiClient : LLM::AnthropicClient
+        client_class.new(
           api_key: @config.api_key,
           model: @config.model,
           max_tokens: @config.max_tokens_per_call
diff --git a/config/initializers/harness.rb b/config/initializers/harness.rb
index 10bb94a..f212b6f 100644
--- a/config/initializers/harness.rb
+++ b/config/initializers/harness.rb
@@ -1,6 +1,7 @@
 Rails.application.config.after_initialize do
   Harness.configure do |config|
-    config.api_key = ENV["ANTHROPIC_API_KEY"]
-    config.model = "claude-sonnet-4-20250514"
+    config.provider = :openai
+    config.api_key = ENV["OPENAI_API_KEY"]
+    config.model = "gpt-4o-mini"
   end
 end
diff --git a/db/migrate/20260311120000_add_patch_text_to_review_sections.rb b/db/migrate/20260311120000_add_patch_text_to_review_sections.rb
new file mode 100644
index 0000000..d3119c5
--- /dev/null
+++ b/db/migrate/20260311120000_add_patch_text_to_review_sections.rb
@@ -0,0 +1,5 @@
+class AddPatchTextToReviewSections < ActiveRecord::Migration[7.1]
+  def change
+    add_column :review_sections, :patch_text, :text
+  end
+end