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GPT-4o vs GPT-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
OpenAI
Text & Chat
Verdict

Choose GPT-5.5 for long documents (400K tokens context). Choose GPT-4o for shorter prompts where the smaller window keeps latency and cost down.

Side-by-side specs

SpecGPT-4oGPT-5.5
ProviderOpenAIOpenAI
CategoryText & ChatText & Chat
Input cost / 1M tokens$3.00$6.00
Output cost / 1M tokens$12.00$36.00
Context window128K tokens400K tokens
Max output tokens16,384128,000
Avg. latency2.0s—
FeaturedYesYes
New—Yes
Capabilities—
text
image

Pricing example

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

GPT-4o
$0.90

100K in × $3.00 + 50K out × $12.00

GPT-5.5
$2.40

100K in × $6.00 + 50K out × $36.00

For this workload, GPT-4o is cheaper than GPT-5.5 by $1.50 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-4o
let r = await client.chat.completions.create({
  model: "gpt-4o",
  messages: [{ role: "user", content: "Hello" }],
});

// After — switched to GPT-5.5
r = await client.chat.completions.create({
  model: "gpt-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-4o
r = client.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "Hello"}],
)

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

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

Which one wins for...

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

Coding
GPT-5.5

Higher coding category match or larger context wins.

Writing
GPT-5.5

Bigger context window helps maintain long-form coherence.

Long documents
GPT-5.5

The larger context window is the deciding factor.

Vision
GPT-5.5

Multimodal/vision support is required for image inputs.

Real-time chat
GPT-4o

Lower average latency wins for interactive UX.

Cost-sensitive
GPT-4o

The model with the lower input-token price wins.

Frequently asked questions

Which is cheaper, GPT-4o or GPT-5.5?
GPT-4o is cheaper. On a 100K input + 50K output example, GPT-4o costs about $0.90 versus $2.40 for GPT-5.5 — a saving of $1.50.
Which has more context, GPT-4o or GPT-5.5?
GPT-5.5 has the larger context window at 400K tokens, compared to 128K tokens for GPT-4o.
Is GPT-4o better than GPT-5.5 for coding?
For coding-heavy workloads we lean toward GPT-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-4o and GPT-5.5 via Railwail?
Yes. Both GPT-4o and GPT-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-4o to GPT-5.5?
Replace the model identifier "gpt-4o" with "gpt-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-4o and GPT-5.5 side by side

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

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