Claude Sonnet 4 - Actually Decent AI for Code That Won't Bankrupt You

Claude Sonnet 4 launched on May 22, 2025, and it's the first AI model that doesn't make me want to throw my laptop out the window. After spending months debugging Claude 3.5's weird hallucinations and paying through the nose for Opus, Sonnet 4 actually delivers what they promised.

Here's the reality: it costs $3/$15 per million tokens, which is 5x cheaper than Opus while handling most of the same complex coding tasks. The big difference is the dual-mode setup - standard responses for when you just need to fix a stupid syntax error, and extended thinking when you're staring at a bug that's been haunting your codebase for weeks.

Actually Useful Context Window (With Caveats)

The 200K context window is legit - you can dump entire codebases without worrying about truncation. The 1M token beta works but performance gets weird past 500K tokens and costs spiral fast.

I've been testing it on a React app with 50+ components and it actually maintains context across files - no more "sorry, I forgot what we were doing" bullshit. But watch your API usage because extended thinking gets expensive fast - I've seen $50+ bills from single debugging sessions.

SWE-bench Reality Check

Sonnet 4 scores 72.7% on SWE-bench Verified, which sounds impressive until you realize SWE-bench tests are cherry-picked GitHub issues. In practice, it's way better than 3.5 for debugging React hydration errors and finding edge cases in async code, but it still hallucinates function names sometimes.

The vision support is actually solid - it can read error screenshots and suggest fixes. Parallel tool execution means it doesn't take 30 seconds to run multiple API calls anymore, which was driving me insane with previous models.

Training Data Actually Matters

March 2025 training cutoff means it knows about React 19, Next.js App Router patterns, and TypeScript 5.x quirks that older models completely miss. It can help with Vite 6.0 migration, Tailwind v4 changes, and other recent framework updates that would leave GPT-4 scratching its digital head.

Extended thinking is where Sonnet 4 actually shines - it thinks through problems instead of barfing out garbage. I used it to debug some recursive component re-render nightmare that had me stumped for like 2 days. Worth every extra token when you're dealing with complex React patterns.

The Platform Mess (Choose Your Poison)

You can run Sonnet 4 through Anthropic's direct API, AWS Bedrock, or Google Cloud Vertex AI. AWS has been solid for production but rate limits are annoying. The direct API works fine but you'll hit demand spikes during peak hours.

Claude Code is their VS Code extension and it's honestly pretty good once you get past the initial setup headaches. Just don't enable extended thinking by default or you'll get surprise bills like I did in week one - burned through like 200 bucks before I figured out what happened.

After using Sonnet 4 for 3 months on production code, here's what actually works and what'll drive you crazy. It's not perfect, but it's the first AI model that feels like having a competent junior developer who doesn't need constant hand-holding.

What Actually Works in Production

Code reviews are where Sonnet 4 shines - it catches the stupid bugs I miss after staring at code for 6 hours. Spotted a nasty race condition that was breaking production randomly - way better than GitHub Copilot which suggests outdated React patterns from 2020. It understands complex PR contexts and actually suggests meaningful improvements instead of nitpicking semicolons.

The SWE-bench score translates to real value: it solved a gnarly authentication bug that had our whole team stumped for days. But here's the catch - it works great on well-structured codebases and completely chokes on spaghetti legacy code.

Legacy maintenance is hit-or-miss. It handled a jQuery → React migration better than expected, understanding ancient JavaScript patterns. But throw it at enterprise Java circa 2008 and it starts hallucinating Spring annotations that don't exist.

Extended Thinking: Worth It or Wallet Killer?

Extended thinking is Sonnet 4's killer feature when you're debugging something that makes no sense. It'll actually work through a problem step-by-step instead of immediately vomiting a solution. I've watched it spend 45 seconds analyzing a memory leak in a Node.js app and come up with the actual root cause.

But here's the reality check: extended thinking can cost 3-10x more tokens than standard responses. Use it for the hairy problems - architectural decisions, security reviews, or that one bug that's been mocking you for days. Don't enable it by default unless you enjoy surprise bills.

The sweet spot is using standard mode for scaffolding and quick fixes, then switching to extended thinking when you hit something genuinely complex. Had this weird distributed caching issue that was driving everyone crazy. Extended thinking cost me a chunk of change but figured out the root cause - saved us way more time than it cost.

IDE Integration Reality Check

Claude Code is their VS Code extension and it's actually decent once you survive the setup process. It shows proposed changes inline which is way better than copying code from a chat interface. But the first time you see it rewriting 6 files simultaneously, you'll panic and hit undo.

Background operations are clutch - it can refactor your entire component library while you grab coffee. Just don't let it loose on critical production code without supervision. I've seen it rename variables to "more semantic" names that completely broke the build pipeline.

For CI/CD, Sonnet 4 can actually read GitHub Actions errors and suggest fixes. It helped me fix a Docker build that was failing due to something fucked with the Node version that I couldn't spot. The March 2025 training data means it knows about recent GitHub Actions syntax changes that older models miss.

Cost Monitoring or You'll Get Fired

If you're deploying Sonnet 4 at scale, set up usage alerts immediately or prepare for awkward conversations with your CFO. I learned this the hard way when our team racked up like 800 bucks in a week because some genius left extended thinking on for our automated code review bot.

Prompt engineering actually matters here - concise, clear instructions cost way less than rambling paragraphs. "Fix this React component's hydration error" works better than "Please analyze this component and identify any potential issues that might cause problems."

The 200K context limit sounds generous until you hit it with a large codebase. Performance degrades around 150K tokens, and anything over 180K takes forever to process. Parallel tool execution is solid - it can run multiple API calls simultaneously instead of taking 2 minutes to complete a simple workflow.

Security: Better Than Most Humans

Sonnet 4's security analysis is legitimately good - it caught a SQL injection vulnerability in our legacy PHP code that 3 security reviews missed. It understands OWASP top 10 patterns and can spot common mistakes like unvalidated inputs or improper authentication.

Running it through AWS Bedrock or Google Cloud gives you enterprise compliance features and audit logging. Required for most corporate environments, though the direct API is fine for smaller teams.

Just don't blindly trust its security recommendations. It suggested implementing JWT tokens for session management in a scenario where simple cookies would've been way more secure for our use case. Always verify security changes with a human who actually understands your threat model - our senior dev quit and took all the tribal knowledge with him, so we're extra careful now.