Claude 4.5 vs GPT-5.1: I Tested Both on 5 Real Projects — Here's What I Found

Last Updated: November 26, 2025

The battle for AI coding supremacy just got real. Claude 4.5 Sonnet dropped in September with a record 77.2% on SWE-bench Verified. Then GPT-5.1 launched in November, hitting 76.3% and claiming to be faster and cheaper.

But benchmarks don't tell the whole story. I spent two weeks testing both models on actual coding tasks—the kind developers face every day. Here's what actually matters.

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The Testing Setup

I gave both models the same 5 real-world coding challenges:

1. React Component Refactor: Modernize a legacy class component to hooks
2. Python Data Pipeline: Build ETL pipeline with error handling
3. Database Migration: Add full-text search to existing PostgreSQL schema
4. Bug Hunt: Fix a race condition in concurrent code
5. Code Review: Identify security issues in authentication middleware

All tests used default settings. No prompt engineering tricks. Just: "Here's the task, write the code."

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Test 1: React Component Refactoring

Task: Convert a 200-line class component with lifecycle methods to modern React hooks.

Claude 4.5

// Output was nearly perfect
- Correctly identified 3 lifecycle methods to convert
- Preserved all edge case handling
- Added proper TypeScript types
- Suggested useMemo for expensive calculations I hadn't thought of

Issues: None. Code ran on first try.

GPT-5.1

// Good but had subtle issues
- Converted 2/3 lifecycle methods correctly
- Missed componentDidUpdate edge case
- Types were less strict (used 'any' in 2 places)
- Faster response (3 seconds vs Claude's 7)

Issues: Tests failed until I manually fixed the missing lifecycle logic.

Winner: Claude 4.5 — More thorough, caught edge cases GPT missed.

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Test 2: Python Data Pipeline

Task: Build an ETL pipeline to process CSV files, validate data, and load to Postgres.

Claude 4.5

# Strengths
- Excellent error handling (specific exceptions for each failure mode)
- Added logging I didn't ask for (but actually needed)
- Proper connection pooling
- Type hints throughout

# Weaknesses
- Slower to generate (12 seconds)
- Used pandas where pure Python would've been lighter

GPT-5.1

# Strengths
- Lightning fast response (5 seconds)
- Cleaner, more concise code
- Better variable names

# Weaknesses
- Basic error handling (generic try/except)
- No logging
- Missed SQL injection protection

Winner: Claude 4.5 — Production-ready code vs. prototype-quality code.

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Test 3: Database Schema Migration

Task: Add full-text search to a PostgreSQL database with 4 existing tables.

Claude 4.5

-- Comprehensive approach
CREATE INDEX CONCURRENTLY idx_posts_fts
  ON posts USING GIN(to_tsvector('english', title || ' ' || body));

-- Also provided:
- Rollback migration
- Index monitoring query
- Performance comparison before/after
- Tips on when to use pg_trgm vs tsquery

GPT-5.1

-- Functional but basic
CREATE INDEX idx_posts_search ON posts(title, body);

-- Also provided:
- Basic rollback
- No performance guidance
- Used B-tree instead of GIN (less optimal for full-text)

Winner: Claude 4.5 — Showed understanding of PostgreSQL internals.

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Test 4: Concurrent Code Debugging

Task: Find and fix a race condition in Go code with goroutines.

Claude 4.5

Identified the bug immediately:

// Issue: shared map access without mutex
// Provided 2 solutions:
1. Add sync.RWMutex
2. Use channels (idiomatic Go)

// Explained trade-offs of each approach

GPT-5.1

Took 2 attempts:

// First response missed the actual race condition
// After I said "tests still fail", it found the real issue
// Provided only mutex solution (didn't suggest channels)

Winner: Claude 4.5 — First-time accuracy matters when debugging.

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Test 5: Security Code Review

Task: Review authentication middleware and identify vulnerabilities.

Claude 4.5 Found:

✅ JWT secret hardcoded (critical) ✅ No rate limiting on login endpoint ✅ Timing attack vulnerability in string comparison ✅ Missing CSRF protection ✅ Weak password requirements

Confidence: All findings were real issues.

GPT-5.1 Found:

✅ JWT secret hardcoded ✅ No rate limiting ❌ False positive: Claimed SQL injection (we weren't using SQL here) ❌ Missed timing attack ❌ Missed CSRF issue

Winner: Claude 4.5 — Higher accuracy, no false alarms.

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Performance & Cost Breakdown

Metric	Claude 4.5	GPT-5.1	Verdict
Avg Response Time	8.2 seconds	4.1 seconds	GPT wins
Code Quality	9.2/10	7.8/10	Claude wins
First-Try Success Rate	4/5 (80%)	2/5 (40%)	Claude wins
API Cost (5 tests)	$0.32	$0.21	GPT wins
Debug Time Saved	~2 hours	~30 min	Claude wins

Reality Check: Claude costs 50% more per token, but I spent 75% less time fixing its code. Net savings favored Claude.

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When to Use Each Model

Choose Claude 4.5 if:

• Code quality > speed (production systems, critical infrastructure)
• Complex refactoring (large codebases, architectural changes)
• Security matters (authentication, payments, PII handling)
• You want fewer revisions (higher upfront cost, less debugging time)

Choose GPT-5.1 if:

• Speed matters (prototyping, MVPs, experiments)
• Budget-conscious (startups, hobby projects)
• Simple tasks (CRUD endpoints, basic scripts)
• Non-critical code (internal tools, one-off scripts)

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The Uncomfortable Truth

Claude 4.5 writes better code. It's not close.

But GPT-5.1 is faster and cheaper. For many use cases—especially early-stage development—that trade-off makes sense.

The SWE-bench gap (77.2% vs 76.3%) seems small. But in practice, that 0.9 percentage point translates to:

• Fewer bugs in production
• Less time spent debugging AI output
• More confidence in generated code

My recommendation: Use Claude for production code, GPT for everything else. Or better yet, use both—Claude for the hard stuff, GPT for the grunt work.

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What About Claude 5?

If Claude 4.5 already leads, what will Claude 5 bring?

Anthropic hasn't announced specifics, but based on the trajectory from Claude 3.5 (49% SWE-bench) to 4.5 (77.2%), we might see:

• 85%+ SWE-bench score (approaching human-level)
• Multi-file refactoring (change 10+ files in one go)
• Real-time collaboration (pair programming mode)

Expected release: Q2-Q3 2026. Track the countdown here.

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Data Sources & Methodology

Testing Environment:

• Both models tested via API (November 2025 versions)
• Same prompts, no cherry-picking
• All code ran in isolated environments
• Timing measured wall-clock time (includes API latency)

Benchmark Sources:

• SWE-bench Verified: Anthropic (Sep 2025), OpenAI (Nov 2025)
• Pricing: Official API docs (as of Nov 26, 2025)
• Personal testing: Original research for this article

Conflicts of Interest: None. I pay for both Claude Pro and ChatGPT Plus.

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Conclusion: Claude 4.5 is the better coder. GPT-5.1 is the better bargain. Choose based on what you're building—but if code quality matters, Claude is worth the premium.

Want more AI comparisons? Check out SWE-bench explained or track Claude 5 release.