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GPT-5.1 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.

OpenAI
Text & Chat
vs
Anthropic
Text & Chat
Verdict

Choose GPT-5.1 for cost-sensitive workloads — it is roughly 3.2× cheaper on input tokens. Choose Claude Opus 5.5 when you need its broader capabilities or stronger benchmarks.

Side-by-side specs

SpecGPT-5.1Claude Opus 5.5
ProviderOpenAIAnthropic
CategoryText & ChatText & Chat
Input cost / 1M tokens$1.50$4.80
Output cost / 1M tokens$12.00$24.00
Context window400K tokens1.0M tokens
Max output tokens128,000128,000
Avg. latency——
Featured—Yes
New—Yes
Capabilities
text
image
text
image

Pricing example

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

GPT-5.1
$0.75

100K in × $1.50 + 50K out × $12.00

Claude Opus 5.5
$1.68

100K in × $4.80 + 50K out × $24.00

For this workload, GPT-5.1 is cheaper than Claude Opus 5.5 by $0.93 per request.

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 GPT-5.1
let r = await client.chat.completions.create({
  model: "gpt-5.1",
  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 GPT-5.1
r = client.chat.completions.create(
    model="gpt-5.1",
    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 GPT-5.1
curl https://railwail.com/v1/chat/completions \
  -H "Authorization: Bearer $RAILWAIL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-5.1",
    "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
Tie

Multimodal/vision support is required for image inputs.

Real-time chat
Tie

Lower average latency wins for interactive UX.

Cost-sensitive
GPT-5.1

The model with the lower input-token price wins.

Frequently asked questions

Which is cheaper, GPT-5.1 or Claude Opus 5.5?
GPT-5.1 is cheaper. On a 100K input + 50K output example, GPT-5.1 costs about $0.75 versus $1.68 for Claude Opus 5.5 — a saving of $0.93.
Which has more context, GPT-5.1 or Claude Opus 5.5?
Claude Opus 5.5 has the larger context window at 1.0M tokens, compared to 400K tokens for GPT-5.1.
Is GPT-5.1 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 GPT-5.1 and Claude Opus 5.5 via Railwail?
Yes. Both GPT-5.1 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 GPT-5.1 to Claude Opus 5.5?
Replace the model identifier "gpt-5.1" 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 GPT-5.1 and Claude Opus 5.5 side by side

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

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