Railwail
Log ind

Claude Sonnet 4 vs Claude Opus 5.5: Which AI Model Should You Choose?

Pricing, context windows, latency, capabilities, and a one-line code switch — everything you need to pick the right model.

Anthropic
Text & Chat
vs
Anthropic
Text & Chat
Verdict

Choose Claude Opus 5.5 for long documents (1.0M tokens context). Choose Claude Sonnet 4 for shorter prompts where the smaller window keeps latency and cost down.

Side-by-side specs

SpecClaude Sonnet 4Claude Opus 5.5
ProviderAnthropicAnthropic
CategoryText & ChatText & Chat
Input cost / 1M tokensn/a$4.80
Output cost / 1M tokensn/a$24.00
Context window200K tokens1.0M tokens
Max output tokens8,192128,000
Avg. latency3.0s—
FeaturedYesYes
New—Yes
Capabilities—
text
image

Pricing example

A typical chat workload of 100,000 input tokens plus 50,000 output tokens.

Claude Sonnet 4
n/a

100K in × n/a + 50K out × n/a

Claude Opus 5.5
$1.68

100K in × $4.80 + 50K out × $24.00

Switch in one line

Both models live behind Railwail's OpenAI-compatible endpoint. Replace the model string and you are done.

JavaScript / TypeScript
import OpenAI from "openai";

const client = new OpenAI({
  apiKey: process.env.RAILWAIL_API_KEY,
  baseURL: "https://railwail.com/v1",
});

// Before — using Claude Sonnet 4
let r = await client.chat.completions.create({
  model: "claude-sonnet-4-20250514",
  messages: [{ role: "user", content: "Hello" }],
});

// After — switched to Claude Opus 5.5
r = await client.chat.completions.create({
  model: "claude-opus-5-5",
  messages: [{ role: "user", content: "Hello" }],
});
Python
from openai import OpenAI

client = OpenAI(
    api_key=os.environ["RAILWAIL_API_KEY"],
    base_url="https://railwail.com/v1",
)

# Before — using Claude Sonnet 4
r = client.chat.completions.create(
    model="claude-sonnet-4-20250514",
    messages=[{"role": "user", "content": "Hello"}],
)

# After — switched to Claude Opus 5.5
r = client.chat.completions.create(
    model="claude-opus-5-5",
    messages=[{"role": "user", "content": "Hello"}],
)
cURL
# Before — using Claude Sonnet 4
curl https://railwail.com/v1/chat/completions \
  -H "Authorization: Bearer $RAILWAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4-20250514",
    "messages": [{"role": "user", "content": "Hello"}]
  }'

# After — switched to Claude Opus 5.5
curl https://railwail.com/v1/chat/completions \
  -H "Authorization: Bearer $RAILWAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-opus-5-5",
    "messages": [{"role": "user", "content": "Hello"}]
  }'

Which one wins for...

Quick verdicts derived from public specs. Always validate on your own workload.

Coding
Claude Opus 5.5

Higher coding category match or larger context wins.

Writing
Claude Opus 5.5

Bigger context window helps maintain long-form coherence.

Long documents
Claude Opus 5.5

The larger context window is the deciding factor.

Vision
Claude Opus 5.5

Multimodal/vision support is required for image inputs.

Real-time chat
Claude Sonnet 4

Lower average latency wins for interactive UX.

Cost-sensitive
Tie

The model with the lower input-token price wins.

Frequently asked questions

Which is cheaper, Claude Sonnet 4 or Claude Opus 5.5?
Claude Sonnet 4 and Claude Opus 5.5 cannot be compared on per-token price: at least one of them has no per-token price listed (it is priced per run or not yet priced). See each model page for its current price.
Which has more context, Claude Sonnet 4 or Claude Opus 5.5?
Claude Opus 5.5 has the larger context window at 1.0M tokens, compared to 200K tokens for Claude Sonnet 4.
Is Claude Sonnet 4 better than Claude Opus 5.5 for coding?
For coding-heavy workloads we lean toward Claude Opus 5.5 on this comparison — it scores higher on the relevant heuristics (category, tags, or context window). Both models are usable for code via Railwail's OpenAI-compatible endpoint, so the safest path is to A/B test on your own prompts.
Can I use both Claude Sonnet 4 and Claude Opus 5.5 via Railwail?
Yes. Both Claude Sonnet 4 and Claude Opus 5.5 are accessible through a single Railwail API key and the OpenAI-compatible /v1/chat/completions endpoint. You only change the "model" parameter to switch between them — no SDK swap, no separate billing.
How do I switch from Claude Sonnet 4 to Claude Opus 5.5?
Replace the model identifier "claude-sonnet-4-20250514" with "claude-opus-5-5" in your request payload. Everything else — API key, base URL, request shape — stays the same. See the code example on this page for the exact one-line change.

Try Claude Sonnet 4 and Claude Opus 5.5 side by side

One API key, one endpoint, both models. Start free — no credit card required.

Get Started FreeView Claude Sonnet 4View Claude Opus 5.5