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Nemotron 3 Ultra 550B (Free) API

nvidia/nemotron-3-ultra-550b

nvidia

NVIDIA Nemotron 3 Ultra 550B-A55B, free. 550B total / 55B active MoE with a 1M-token context — the largest free model in the catalog.

Code Examples

from blockrun_llm import LLMClient

client = LLMClient()  # Uses BLOCKRUN_WALLET_KEY (never sent to server)
response = client.chat("nvidia/nemotron-3-ultra-550b", "Hello!")
import { LLMClient } from '@blockrun/llm';

const client = new LLMClient();  // Uses BLOCKRUN_WALLET_KEY (never sent to server)
const response = await client.chat('nvidia/nemotron-3-ultra-550b', 'Hello!');
curl -X POST https://blockrun.ai/api/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "PAYMENT-SIGNATURE: <payment_header>" \
  -d '{
    "model": "nvidia/nemotron-3-ultra-550b",
    "messages": [{"role": "user", "content": "Hello!"}],
    "max_tokens": 1024
  }'
4.81 s
median latency
72.4 s
p95 latency
86.8%
success rate
1,000+
calls, last 7 days

On BlockRun since 2026-08-30. Measured on BlockRun over the last 7 days, end to end, successful calls only. Not a vendor benchmark.

Pricing

InputFree / 1M tokens
OutputFree / 1M tokens
Context1M tokens
Max Output16K tokens

Free model — no payment or wallet required.

Payment

Network
Base
Currency
USDC
Protocol
x402

Pay per request with USDC on Base. No subscription required.

Try It

Send a message to try Nemotron 3 Ultra 550B (Free)

Connect your wallet to enable payments

About NVIDIA Nemotron 3 Ultra 550B (Free)

Nemotron 3 Ultra 550B (Free) is a reasoning model from NVIDIA with a 1M-token context window and up to 16K tokens of output per call. It is free to call here — no payment header and no wallet. It is served through BlockRun's OpenAI-compatible API, so it can be called without an account, an API key, or a subscription.

Use Nemotron 3 Ultra 550B (Free) for

  • Multi-step reasoning: planning, analysis and math that need thinking before an answer
  • Long documents and whole codebases in a single prompt
  • Chat assistants and agents that answer, summarise and draft
  • Prototyping and evaluation at no cost

What it costs

Nemotron 3 Ultra 550B (Free) is free to call on BlockRun. No payment header is required, no wallet needs funding, and no API key is issued — requests are rate limited per IP rather than billed. It is a practical way to develop against the API surface before moving production traffic onto a paid model.

Specifications

Context window
1,000,000 tokens
Maximum output
16,384 tokens
API compatibility
OpenAI-compatible
Payment
Free — no payment required
Categories
chat, reasoning

Calling it from your code

Pass nvidia/nemotron-3-ultra-550b as the model field. Because the endpoint mirrors the OpenAI chat completions schema, any existing OpenAI client works by changing the base URL — streaming, tool use, and multi-turn messages all behave the same way.

The first request returns a 402 carrying the exact price; a signed retry runs it, and client libraries fold the two into one call — how the payment works. See the documentation for request and response shapes, the LLM API page for every model on this endpoint, or browse the full catalog of 90 chat models to compare alternatives.

Nemotron 3 Ultra 550B (Free) or another model

The closest alternatives by price that do the same kind of work. Each is called on the same endpoint with the same request, so comparing two is a one-line change.

ModelPriceContext
NVIDIA Nemotron 3 Ultra 550B (Free) (this page)Free1M context
NVIDIA Muse Glimmer 30B (Free)Free131K context
NVIDIA Nemotron 3 Nano Omni (Free)Free256K context
NVIDIA Nemotron 3.5 Lightning (Free)Free1M context

Nemotron 3 Ultra 550B (Free) API questions

How much does the Nemotron 3 Ultra 550B (Free) API cost?

Nemotron 3 Ultra 550B (Free) is free to call here, with no payment header and no wallet.

What is Nemotron 3 Ultra 550B (Free)'s context window?

1,000,000 tokens of context, with up to 16,384 tokens of output per call.

Is the Nemotron 3 Ultra 550B (Free) API OpenAI-compatible?

Yes. Send an OpenAI chat-completions request with "model": "nvidia/nemotron-3-ultra-550b" to the BlockRun endpoint. An existing OpenAI client works by changing the base URL; streaming and multi-turn messages behave the same way.

Do I need an API key to use Nemotron 3 Ultra 550B (Free)?

No. Each request carries its own payment, so an agent or script can call Nemotron 3 Ultra 550B (Free) with no account and no key. Teams that prefer one can use an API key with card or invoice billing instead.

What happens if Nemotron 3 Ultra 550B (Free) is unavailable?

A request that fails is not charged. If you set "allow_fallback": true, a failed call can be retried on NVIDIA Nemotron 3.5 Lightning (Free) instead, never at a higher price than you were quoted.

Related models

Every model on BlockRun is reached through the same endpoint and the same payment flow, so switching is a one-line change to the model field. These are the closest alternatives to Nemotron 3 Ultra 550B (Free):

Two ways to run this

Get an API key

Sign in, get a key, and call the same endpoints with usage billed to your account. For teams that want one invoice instead of one wallet.

Get an API key 

Or let your agent pay per call, in USDC

No account, no API key. Three steps, and the last one is the request this page is about.

  1. 01
    Point your agent at BlockRun

    Install ClawRouter, or add the BlockRun MCP server to Claude Code, Cursor, Codex or OpenClaw. Any OpenAI-compatible client works too — change the base URL, nothing else.

    curl -fsSL https://blockrun.ai/ClawRouter-update | bash
  2. 02
    Fund a wallet with USDC

    Send USDC to a wallet on Base, Solana or Arc. There is no account to create and no API key to issue: the wallet is the account, and it pays per call.

  3. 03
    Call Nemotron 3 Ultra 550B (Free)

    The first call returns HTTP 402 with the exact price, your agent signs it, and the answer comes back. Payment settles on-chain; nothing is charged if the call fails.

    curl -X POST https://blockrun.ai/api/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{"model": "nvidia/nemotron-3-ultra-550b", "messages": [{"role": "user", "content": "Hello!"}]}'
ClawRouter setup →How paying per call works →

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