OpenAI o3 vs Claude Sonnet 4.5: The Developer's Cost Guide

The Short Version

Use o3 when: multi-step reasoning, mathematics, competitive programming, or PhD-level science problems.

Use Claude Sonnet 4.5 when: writing, code review, analysis, long-document summarisation, customer support, or anything where cost efficiency matters.

For 80% of production use cases, Claude Sonnet 4.5 is the right choice. o3 is a specialist.

Cost Comparison

Key takeaway: For most tasks, Claude Sonnet 4.5 is 60-70% cheaper than o3 and delivers comparable or better results on non-reasoning workloads.

When o3 Wins

o3 is a reasoning model — it uses extended internal "thinking" before answering. This makes it uniquely powerful for:

# o3 is worth the premium here — multi-constraint optimisation
response = client.chat.completions.create(
    model="o3",
    messages=[{"role": "user", "content": "Solve the travelling salesman problem for these 15 cities using dynamic programming, then explain your approach and complexity."}],
    reasoning_effort="high"  # o3-specific parameter
)

The reasoning_effort parameter lets you control the thinking budget — use "low" for 70% cost reduction when the task isn't worth full reasoning.

When Claude Sonnet 4.5 Wins

Claude Sonnet 4.5 is Anthropic's flagship production model — high intelligence at reasonable cost. It's the right choice for:

import anthropic

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-sonnet-4-5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Summarise this earnings report and flag any risk factors: ..."}
    ]
)

The Migration Decision

Migrating from o3 to Claude Sonnet 4.5

Most apps currently using o3 are over-paying for non-reasoning tasks. Ask yourself:

A/B test pattern:

import random

def get_model_for_task(task_type: str) -> str:
    REASONING_TASKS = {"math", "algorithm_design", "proof_verification"}
    if task_type in REASONING_TASKS:
        return "o3"
    # Route 10% to o3 as quality baseline, 90% to Sonnet
    return "o3" if random.random() < 0.1 else "claude-sonnet-4-5"

Checklist Before Switching

Bottom Line

For a typical SaaS app spending $500/month on o3, switching to Claude Sonnet 4.5 for non-reasoning tasks saves $300–400/month with no quality degradation. Claim your free Anthropic credits and run the benchmark yourself.