Claude Opus 5 API: Model ID, Setup, and Context Window Guide

TL;DR

The exact model ID is claude-opus-5. Anthropic positions Opus 5 for complex agentic coding and enterprise work, with a 1M-token context window and 128K-token synchronous output limit. To use it in Claude Code, launch with claude --model claude-opus-5 or switch in-session with /model claude-opus-5. For the Anthropic Messages API through llmapi.pro, set ANTHROPIC_BASE_URL=https://llmapi.pro and ANTHROPIC_API_KEY=sk-relay-..., then POST to /v1/messages with the header anthropic-version: 2023-06-01.

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The Model ID and Context Window

The canonical identifier is claude-opus-5 (no dots, no suffix). Anthropic describes this model as suited for complex agentic coding tasks and enterprise-grade work.

Key specifications from Anthropic's official model overview:

• Context window: 1M tokens
• Synchronous Messages API maximum output: 128K tokens
• Adaptive thinking: Supported; effort defaults to high on the Claude API and in Claude Code. If you need a different level, set effort explicitly in your request.

These are the upstream specifications. For current llmapi.pro availability, check the live model catalogue at GET /v1/models or browse /all-models on the web dashboard.

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Using Opus 5 in Claude Code

Launch with the Model Flag

claude --model claude-opus-5

This starts a session with Opus 5 as the active model from the first turn.

Switch Models Mid-Session

If you launched Claude Code with a different model or the default, you can switch at any time:

/model claude-opus-5

The next request uses Opus 5. All subsequent turns remain on Opus 5 until you switch again or exit.

Pointing Claude Code at a Relay

Claude Code respects the standard Anthropic environment variables. To route through llmapi.pro instead of the official endpoint:

export ANTHROPIC_BASE_URL=https://llmapi.pro
export ANTHROPIC_API_KEY=sk-relay-your-key-here
claude --model claude-opus-5

On Windows (PowerShell):

$env:ANTHROPIC_BASE_URL = "https://llmapi.pro"
$env:ANTHROPIC_API_KEY = "sk-relay-your-key-here"
claude --model claude-opus-5

For persistent setup across sessions, add these to your shell profile or use the Claude Code config file. A complete walkthrough of environment setup and key registration is in the Claude Code API Key Setup Guide.

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Messages API Smoke Test

Here's a minimal curl request that exercises the Anthropic Messages API protocol through llmapi.pro:

curl -X POST https://llmapi.pro/v1/messages \
  -H "x-api-key: sk-relay-your-key-here" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-5",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "Echo: test"}
    ]
  }'

A successful response returns HTTP 200 with a JSON body containing id, type: "message", role: "assistant", and a content array. If you see a connection timeout or 502, check your network path and proxy settings; the Claude Code Connection Error and Timeout Fix covers common failure modes and diagnostic steps.

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Adaptive Thinking and Effort Levels

Opus 5 supports adaptive thinking. On the Claude API and in Claude Code, the default effort is high. If you want to override this—either to lower cost on simpler tasks or to request maximum reasoning depth—set the effort parameter explicitly in your request:

{
  "model": "claude-opus-5",
  "max_tokens": 2048,
  "messages": [{"role": "user", "content": "Analyze this codebase structure."}],
  "effort": "medium"
}

Supported effort levels depend on the model and provider. Consult the Anthropic documentation for the current set.

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When to Use Opus 5

Good fit:

• Multi-file refactors that span thousands of lines
• Architecturally complex decisions (framework migrations, system-design trade-offs)
• Long-context workflows where you need to keep an entire repository or a deep conversation thread in working memory
• Tasks where the 128K synchronous output limit is useful (generating full modules, exhaustive test suites, or detailed architecture documents in a single response)

Not always necessary:

• Quick one-liners, syntax fixes, or straightforward queries often finish faster and cheaper on Sonnet or Haiku.
• If your task rarely uses more than a few thousand tokens of context, a smaller model may be more economical.

For budget-conscious workflows and cost comparisons across the model lineup, see Claude Code Token Cost and Context Management.

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Checking Live Availability and Pricing

llmapi.pro's model catalogue and pricing are subject to change. To see what's currently available:

• API: curl https://llmapi.pro/v1/models -H "x-api-key: sk-relay-..."
• Web dashboard: /all-models after login

Do not hard-code relay prices or assume universal availability. The live endpoints are the authority.

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Summary

Aspect	Value
Model ID	claude-opus-5
Context window	1M tokens
Synchronous output limit	128K tokens
Adaptive thinking default	high (override with effort)
Claude Code launch	claude --model claude-opus-5
Claude Code switch	/model claude-opus-5
Claude Code base URL	https://llmapi.pro
Required headers	x-api-key, anthropic-version: 2023-06-01

Opus 5 is the current top-tier model in Anthropic's lineup. Use the exact ID, point your environment at the relay if you're routing through llmapi.pro, and check the live model list for current availability. The setup is two environment variables and one model flag—no elaborate config, no hidden steps.

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llmapi.pro is an independent, Claude-compatible API relay; we are not affiliated with Anthropic. Claude, Claude Code, and the Anthropic API are used for identification only.